DN81

DN81
Lighting handbook
Introduction
Light Emitting Diodes (LEDs) are becoming more and more popular in general illumination. They
offer benefits in energy efficiency, long life and ruggedness. LEDs are low voltage devices and
both safe and easy to use.
However, LEDs are ‘current driven’ devices and simply applying a voltage to drive them is not a
good method of control. Current control schemes are essential to maintain constant brightness.
Additionally, LEDs offer longevity which means that they can often work in excess of 50,000
hours. LED drivers play a key role in achieving maximum working life. In particular, switching
regulators maximize electrical and thermal efficiency.
Zetex provides a comprehensive range of high brightness LED drivers to suit a wide range of
applications. These high efficiency drivers meet all these stringent requirements.. In this
handbook, a broad range of design notes are included for customers to select the right device and
application circuits. Test results and bill of materials are also included to provide a convenient
means to achieve optimum solutions.
Individual datasheets for all the devices mentioned in these these devices can be found on
www.zetex.com. All the designs have been built and evaluated. However, users should satisfy
themselves of the suitability for their specific application.
Issue 7 - March 2008
© Zetex Semiconductors plc 2006
www.zetex.com
Table of contents, ordered by application
Flashlight, portable lighting
DN61 Dual cell powered ZXSC310 solution for a 1W high power white LED. . . . . . . . . . . . . . . . 3
DN64 ZXSC310 Solution flashlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
DN67 ZXSC400 solution for 1W high powered LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
DN68 ZXSC310 High power torch reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
DN70 ZXSC400 Driving 2 serial high power LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
DN71 ZXSC400 Solution for Luxeon® V Star high powered LED . . . . . . . . . . . . . . . . . . . . . . . . 37
DN73 ZXSC300 Step down converter for 3W LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
DN78 ZXSC310 with reverse polarity protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
DN79 ZXSC400 1W LED driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
DN84 ZXSC400 Driving 3W high power LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
DN85 ZXSC400 1W/3W buck LED drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
General lighting, illumination, signage
DN62 ZXSC310 Solution to drive 3 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
DN63 ZXSC310 Solution to drive 8 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
DN65 ZXSC310 Solution for emergency light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
DN69 ZXSC310 Garden light reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
DN75 ZXSC310 Solar powered garden light reference design . . . . . . . . . . . . . . . . . . . . . . . . . . 45
DN86 Reduced component count and compact reference design for MR16
replacement lamps using multiple 1W LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
DN83 LED MR16 Lamp solution using the ZXLD1350 LED driver . . . . . . . . . . . . . . . . . . . . . . . . 57
AN44 A high power LED driver for low voltage halogen replacement . . . . . . . . . . . . . . . . . . . . 81
Backlight
DN62 ZXSC310 Solution to drive 3 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
DN63 ZXSC310 Solution to drive 8 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
DN66 An OLED bias supply for a clamshell handset sub display . . . . . . . . . . . . . . . . . . . . . . . . 15
DN72 ZXLD1101 Driving 8 series LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
DN76 ZXLD1100 and ZXLD1101 driving from 3 to 6 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Photoflash
DN74 ZXSC400 Photoflash LED reference design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Miscellaneous
AN47 Getting more out of the ZXLD1350 - dimming techniques . . . . . . . . . . . . . . . . . . . . . . . . 87
DN48 Getting more out of the ZXLD1350 - high output current . . . . . . . . . . . . . . . . . . . . . . . . . 95
AN50 Feed forward compensation for ZXSC300 LED driver . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Issue 7 - March 2008
© Zetex Semiconductors plc 2008
1
www.zetex.com
Intentionally left blank
www.zetex.com
2
Issue 7 - March 2008
© Zetex Semiconductors plc 2008
DN61
Dual cell powered ZXSC310 solution for a 1W high
power white LED
Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors
Description
High power LEDs are increasingly being used in lighting applications (general illumination,
portable, signage/security, traffic, automotive, architectural) as lumens, and efficacy of high
power LEDs are increasing while the cost per lumens is decreasing.
Low cost, small and simple solutions are important in applications such as flashlight, signage and
illuminations where 1W high power LED is powered from a low voltage supply as in single and
dual cell batteries.
Figure 1 shows a typical simple low cost solution with a ZXSC310 driving a 1W LED with a typical
forward voltage of 3.4V at 300mA from a dual cell battery. A dual cell supply will have a voltage
range of 1.8V to 2.5V for NiCd and NiMH type batteries and up to 3V for alkaline type batteries.
The component values are tabulated (see Tables 1 and 2), depending on the range of voltage
which is defined by the battery chemistry.
VIN
1.5V to 2.5/3V
C2
Stdn
ZXSC310
RSENSE
GND
Figure 1
Typical dual cell battery powered 1W LED drive circuit
ZXSC310 is a constant current boost converter in a small SOT23-5 package. It has a typical drive
current of 2.3mA at 1.8V. The drive current at 25°C is 1.5mA minimum at 1.5V supply.
The bipolar transistor switch, Q1, should have adequate voltage and peak switching current
ratings, a very high transistor gain (hfe), a very low saturation voltage (VCE) and a small device
package size with an adequate thermal capability. The transistor, Q1 in this application, is a low
saturation voltage transistor, ZXTN25012EFL, with a very high gain of 700 at 1A collector current
at 25°C to match the drive current from the Drive pin of the ZXSC310.
Note: If transistors with lower gain are used, then at lower temperatures, it may not support a full
switching current and therefore proper operation may not start or may take few seconds to start.
The Schottky diode should have an adequate peak switching current rating and a very low
forward voltage. The Zetex ZXSC1000 Schottky diode, SD1, has a low forward voltage. If
operation at higher temperature is required then the low leakage, low forward voltage, Zetex
ZLLS1000 can be used.
The choice of inductor, L1, depends on the desired switching frequency, the LED current, the input
voltage, forward voltage of the Schottky diode, SD1, and the LED forward voltage.
Note: The LED current output is dependent on the input voltage, the LED forward voltage, the
sense resistor and the inductor value.
Issue 5 - August 2007
© Zetex Semiconductors plc 2006
3
www.zetex.com
DN61
Dual cell NiCd/NiMH battery solution
A dual cell NiCd/NiMH battery voltage range is 1.8V to 2.5V. Table 1 shows the component values
for a dual cell NICd/NiMH battery powered ZXSC310 solution for a 1W high power white LED. The
efficiency and the LED current versus the input voltage performance are shown in Figures 2 and 3.
Efficiency vs Input voltage
LED Current vs Supply voltage
100
0.4
90
Luxion
LED Current (A)
Efficiency (%)
80
70
With low im pedance pow er supply:
Voltage ram ping dow n
60
50
With dual AA size
1300m Ahr NiMH battery
40
30
20
Luxion
TM
With dual AA size
1300mAhr NiMH battery
0.1
With low impedance power supply:
voltage ramping down
White LED
0
2.5
2.25
2
1.75
1.5
White LED
With low impedance power supply:
voltage ramping up
0.2
10
0
2.5
TM
0.3
2.25
1.25
2
1.75
1.5
1.25
Input voltage (V)
Input voltage (V)
Figure 2
Figure 3
Efficency vs. input supply voltage
LED current vs. input supply voltage
Reference Part no.
Value
Manufacturer Contact details
U1
ZXSC310E5
LED driver
Zetex
www.zetex.com
Q1
ZXTN25012EFH
high gain, low VCE(sat)
Zetex
www.zetex.com
SD1
ZHCS1000 or ZLLS1000 low forward voltage VF Zetex
www.zetex.com
L1
DO3316P-103
10␮H, 2A
Coilcraft
www.coilcraft.com
RSENSE
Generic
33m⍀
Generic
NA
R1
Generic
10k⍀
Generic
NA
C1
Generic
1␮F, 6.3V, X7R
Generic
NA
C2
Generic
6.8␮F, 6.3V
Generic
NA
LED1
LXHL-NW98
White LED; 3.4V
Lumileds
www.lumileds.com
Table 1
Bill of materials for dual cell NiCd/NiMH battery powered single 1W LED driver
www.zetex.com
4
Issue 5 - August 2007
© Zetex Semiconductors plc 2006
DN61
Dual cell alkaline battery solution
The dual cell alkaline battery has a voltage range of up to 3V. Table 2 shows the component values
for a dual cell alkaline battery powered ZXSC310 solution for a 1W high power white LED. The
efficiency and the LED current versus the input voltage performance are shown in Figures 4 and 5.
Efficiency vs Input voltage
LED Current vs Supply voltage
100
0.4
90
Luxion
TM
White LED
70
LED Current (A)
Efficiency (%)
80
With dual AA size
1300mAhr NiMH battery
60
50
With low impedance power supply:
Voltage ramping down
40
30
20
Luxion
TM
White LED
0.3
With low impedance power supply:
voltage ramping down
0.2
With dual AA size
Alkaline battery
0.1
With low impedance power supply:
voltage ramping up
10
0
3
2.75
2.5
2.25
2
1.75
1.5
1.25
0
1
3
Input voltage (V)
Figure 4
2.75
2.5
2.25
2
1.75
1.5
1.25
Efficiency vs. input signal voltage
Figure 5
LED current vs. input supply voltage
Reference
Part no.
Value
Manufacturer
Contact details
U1
ZXSC310E5
LED driver
Zetex
www.zetex.com
Q1
ZXTN25012EFL
high gain,
low VCE(sat)
Zetex
www.zetex.com
SD1
ZHCS1000 or
ZLLS1000
low forward voltage Zetex
VF
www.zetex.com
L1
DO3316P-103
10uH, 2A
Coilcraft
www.coilcraft.com
RSENSE
Generic
50m⍀
Generic
NA
R1
Generic
10k⍀
Generic
NA
C1
Generic
1␮F, 6.3V, X7R
Generic
NA
C2
Generic
6.8␮F, 6.3V
Generic
NA
LED1
LXHL-NW98
White LED
Lumileds
www.lumileds.com
Table 2
1
Input voltage (V)
Bill of materials for dual cell alkaline battery powered 1W LED driver
Dimming and shutdown
In Figure 1, the shutdown pin, Stdn, can be tied to VCC pin for normal operation. If the shutdown
pin is taken to ground, the ZXSC310 enters standby mode with a low quiescent current of 5␮A.
The shutdown pin can also be used for PWM dimming by connecting a PWM signal. The LED
current is then dependent on PWM duty ratio.
Thermal management
The LED junction temperature should be maintained within the specified maximum or dederating curve, whichever is lower, by use of proper thermal management for lumens
maintenance and LED protection. Size 0805 for the sense resistor is adequate.
Issue 5 - August 2007
© Zetex Semiconductors plc 2006
5
www.zetex.com
DN61
Boot-strap operation
In boot-strap mode, the supply to the VCC is from the output stage (cathode of SD1) to maintain
the supply to the ZXSC310 at a reasonably constant voltage even when the battery voltage
reduces. This improves the ZXSC310 drive pin current capability due to the reasonably constant
voltage of 3.4V typical (or the forward voltage of the LED) at the VCC pin, even though the battery
voltage may drop below 1.5V.
The boot-strap allows the ZXSC310 to continue driving the LED even with battery supply drops
below 0.8V after the initial successful start-up. The boot-strap mode is recommended for a single
cell alkaline/NiMH/NiCd battery. The boot-strap mode can also be used in throw-away (single use)
dual cell alkaline batteries to draw as much energy as possible before discarding the battery.
Figures 6 and 7 show the efficiency and LED current versus battery voltage for a boot-strap mode
of operation with an AA size dual cell alkaline battery.
Efficiency vs Input voltage
in boot-strap mode
LED Current vs Supply voltage
in boot-strap mode
0.4
100
Luxion
90
70
LED Current (A)
Efficiency (%)
80
With
dual
size
With
dual
AAAA
size
Alkaline
batteryin in
boot-strap m
mode
Alkaline
battery
boost-starp
ode
60
50
With
pedance pow
er supply:
supply: voltage
voltage
Withlow
lowimimpedance
power
ram
ping dow
n ininboost-strap
m ode
ramping
down
boot-strap mode
40
30
Luxion
10
White LED
With low impedance power supply: voltage
ramping down in boot-strap mode
0.2
With dual AA size
Alkaline battery in
boot-strap mode
0.1
20
TM
TM
0.3
With low impedance power supply:
voltage ramping up in boot-strap mode
White LED
0
0
3
2.75
2.5
2.25
2
1.75
1.5
1.25
1
0.75
3
0.5
Figure 6
Efficiency vs. input supply voltage
2.75
2.5
2.25
2
1.75
1.5
1.25
1
0.75
0.5
Input voltage (V)
Input voltage (V)
Figure 7
LED current vs. input supply voltage
Note: To prevent rechargeable batteries entering a deep discharge state, ZXSC310 devices can be
shut down (by pulling the shutdown pin low to the ground) by an external circuit when the
rechargeable battery voltage falls below its recommended minimum voltage. The boot-strap
mode is not recommended with a ZXSC310 for dual/three cell NiCd/NiMH rechargeable batteries
without a under voltage protection.
www.zetex.com
6
Issue 5 - August 2007
© Zetex Semiconductors plc 2006
DN62
ZXSC310 Solution to drive 3 LEDs connected in series
Description
This solution is optimized for an input voltage range of 4.3V to 3V. The LED current is set to 15mA
VIN = 4.3V and 8mA at VIN = 3V.
ZXSC310
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
Figure 1
Schematic diagram
Figure 2
Performance graphs
7
www.zetex.com
DN62
Reference
Part no.
Value
Manufacturer
Contact details
U1
ZXSC310E5
NA
Zetex
www.zetex.com
Q1
FMMT618
NA
Zetex
www.zetex.com
D1
ZHCS1000
1A
Zetex
www.zetex.com
R1
Generic
510m⍀
Generic
NA
R2
Generic
510␮F
Generic
NA
C1
Generic
2.2␮F
Generic
NA
L1
DO1608P-103
10␮H
Coilcraft
www.coilcraft.com
LED1-3
NSPMW500BS
White LED
Nichia
www.nichia.com
Table 1
www.zetex.com
Bill of materials
8
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
DN63
ZXSC310 Solution to drive 8 LEDs connected in series
Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors
Description
Low cost, small simple and low power multi-LED drive solutions are important in applications
including LCD backlight, key illuminations and effects for handheld devices (e.g. cell phones),
signage and indicators. The LED current is generally between 10mA to 30mA and is powered from
a single cell Li-Ion or three cell alkaline/NiMH/NiCad batteries. For battery powered applications
low shutdown quiescent current is important to conserve battery life.
Figure 1 shows a simple low cost boost convertor, ZXSC310, driving eight series connected LEDs.
ZXSC310 is in a small SOT23-5 package. The design solution is for an application with an input
voltage range of 4.5V to 2.5V (e.g. a single cell Li-Ion can have a voltage range of 4.3V to 2.6V)
with LED current optimized at 20mA typical, at 4.0V supply. The LED current at 4V is chosen to
match the 20mA typical forward current of the LED used.
D1
ZHCS1000
VIN
4.5V to 2.5V
L1
68␮H
LED1
U1
Q1
ZXTN25040DFH
VCC
LED2
Drv
Stdn
Stdn
L1
68␮H
ISENSE
GND
C1
100nF
LED7
ZXSC310
R1
100k⍀
RSENSE
200m⍀
LED8
GND
Figure 1
Schematic diagram
With a single cell Li-Ion battery, the circuit in Figure 1 can drive 3 or more series connected LEDs,
the maximum number of LEDs limited by the breakdown voltage of the bipolar transistor Q1.
Depending on the number of LEDs connected in series, the sense resistor, RSENSE, will have to be
adjusted to obtain the required LED current at a certain supply voltage.
The ZXSC310 can be shutdown by pulling the Stdn pin low. The quiescent current in the
shutdown mode is typically 5␮A. If shutdown feature is not required tie the Stdn pin to the VCC
pin.
Figure 2 shows the efficiency and the LED current against supply voltage. The LED current
decreases with the supply voltage. This helps to draw less current from a discharged battery.
The bill of materials for the circuit in Figure 1 is shown in Table 1.
Issue 5 - August 2007
© Zetex Semiconductors plc 2007
9
www.zetex.com
DN63
Efficiency vs input voltage
LED current vs supply voltage
100
20
90
LED Current (mA)
Efficiency (%)
80
70
60
50
40
30
20
3 LEDs in Series
10
0
4.5
4
3.5
3
15
10
5
3 LEDs in Series
0
4.5
2.5
4
Input voltage (V)
3.5
3
2.5
Input voltage (V)
Figure 2
Performance graphs
Ref.
Part no.
Value
Manufacturer Contact details
U1
Q1
ZXSC310E5
NA
ZXTN25040DFH NPN, VCEO = 40V
D1
ZHCS1000
Zetex
Zetex
www.zetex.com
www.zetex.com
1A, low forward voltage VF Zetex
www.zetex.com
RSENSE Generic
200m⍀
Generic
NA
R1
C1
C2
L1
LED1-8
100k⍀
100nF, 6.3V, X7R
2.2␮F, 35V
68␮H
White LED
Generic
Generic
Generic
Coilcraft
Nichia
NA
NA
NA
www.coilcraft.com
www.nichia.com
Generic
Generic
Generic
DO1608P-683
NSPMW500BS
Table 1
www.zetex.com
Bill of materials
10
Issue 5 - August 2007
© Zetex Semiconductors plc 2007
DN64
ZXSC310 Solution flashlight
Description
A solution is provided for flashlight driving 4 white LEDs connected in series from a 2 alkaline cell
input.
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
Figure 1
Schematic diagram
Figure 2
Performance graphs
11
www.zetex.com
DN64
Reference
Part no.
Value
Manufacturer
Contact details
U1
ZXSC310E5
LED driver
Zetex
www.zetex.com
Q1
FMMT618
2.5A, low VCE(sat)
Zetex
www.zetex.com
D1
ZHCS1000
1A, low VF
Zetex
www.zetex.com
L1
LPO2506OB-683
68␮H, 0.4A
Coilcraft
www.coilcraft.com
R1
Generic
130m⍀
Generic
NA
C1
Generic
2.2␮F
Generic
NA
LED1
Learn-4753A
White LED
LG Innotek
www.iginnotek.com
Table 1
www.zetex.com
Bill of materials
12
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
DN65
ZXSC310 Solution for emergency light
Description
This solution is provided for an emergency light driving 8 white LEDs connected in series from a
4 cell input.
ZXSC310
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
Figure 1
Schematic diagram
Figure 2
Performance graphs
13
www.zetex.com
DN65
Reference
Part no.
Value
Manufacturer
Contact details
U1
ZXSC310E5
LED driver
Zetex
www.zetex.com
Q1
FMMT619
2A, low VCE(sat)
Zetex
www.zetex.com
D1
ZHCS1000
1A, low VF
Zetex
www.zetex.com
L1
LPO2506OB-683
68␮H, 0.4A
Coilcraft
www.coilcraft.com
R1
Generic
82m⍀
Generic
NA
C1
Generic
2.2␮F
Generic
NA
LED1
NSPW500BS
White LED
Nichia
www.nichia.com
Table 1
www.zetex.com
Bill of matrials
14
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
DN66
An OLED bias supply for a clamshell handset sub display
Author - Kit Latham, Applications Engineer, Zetex Semiconductors
Description
Portable applications such as cell phones are becoming increasingly complex with more and
more features designed into every generation. One popular feature is to replace the STN sub
display with an OLED sub display. OLED displays have infinite contrast ratio and are selfilluminating. This gives the handset manufacturer two key advantages, the first is lower power
consumption and the second is a slimmer display. One disadvantage with OLED sub displays over
LCD sub displays is the higher leakage current when not in use, which is the majority of the time.
The way to overcome this issue is to disconnect the OLED sub display when the handset is
dormant.
The ZXLB1600 is a boost converter that can provide the power requirements for OLED sub display
with the additional feature of a fully integrated isolation switch which disconnects the input from
output when the ZXLB1600 is shutdown, making it ideally suited to OLED biasing.
The schematic diagram in Figure 1 shows a full color OLED bias supply for clamshell handset sub
display.
U2
BAT54S
L1
Output
U1
ZXLB1600
VBATT
C3
SW
C1
N/C
LBF
SENSE
C2
N/C
FB
EN
N/C
R1
LX
VIN
ADJ
LBT
N/C
GND
Figure 1
R2
Schematic diagram
Note:
For applications where OLED leakage is not an issue and the ZXLB1600 isolation switch is not
needed, the SW pin can be shorted to the VIN pin, giving a further 3% to 5% improvement in
efficiency.
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
15
www.zetex.com
DN66
The materials list and associated performance characteristics provide an OLED biasing solution
for the following sub display specification:
•
Input voltage:
4.2V to 3.0V
•
Output voltage:
12V
•
Output current:
20mA (max.)
•
Output ripple:
50mVpk-pk (max.)
Reference Value
Part number
Manufacturer Contact details
Comments
U1
ZXLB1600X10
Zetex
www.zetex.com
OLED bias IC
U2
BAT54S
Zetex
www.zetex.com
Dual Schottky
diode
L1
22␮H
NPIS32Q220MTRF
NIC
www.niccomp.com Low profile
R1
715k⍀
Generic
Generic
NA
0603 size
R2
82k⍀
Generic
Generic
NA
0603 size
C1
10␮F/6V3 NMC0805X7R106M16 NIC
www.niccomp.com 0805 size
C2(1)
10␮F/16V NMC1206X7R106M16 NIC
www.niccomp.com 1206 size
C3
82pF/16V NMC0603NPO820J50 NIC
www.niccomp.com 0603 size
Table 1
Bill of materials
NOTES:
(1) For a lower profile, two 4.7␮F 0805 capacitors can be used by connecting in parallel.
www.zetex.com
16
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
DN66
Typical operating characteristics
(For typical application circuit where VIN = 3V, VOUT = 12V, IOUT = 20mA unless otherwise stated)
Series1
Series1
140
12
120
10
8
V output
mA
100
80
60
6
40
4
20
2
0
3
3.2
3.4
3.6
Volts
3.8
4
0
4.2
3
3.2
3.4
3.6
V inout
3.8
4
4.2
Output voltage v. input voltage
Input I v. input voltage
Series1
Series1
80
20
70
60
15
mA
%
50
40
10
30
20
5
10
0
0
3
3.2
3.4
3.6
Vin
3.8
4
4.2
Figure 2
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
3
3.2
3.4
3.6
Vin
3.8
4
4.2
Efficiency v. input voltage
Output current v. input current voltage
Performance graphs
17
www.zetex.com
DN66
Typical operating waveforms
(For typical application circuit where VIN = 3V, VOUT = 12V, IOUT = 20mA unless otherwise stated)
Figure 3
www.zetex.com
Typical operating waveforms
18
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
DN66
1.6V input with a 2mA load with a 25V DC output.
LX drive
1.6V input with a 6mA load.
LX drive, output is now unregulated.
5.5V input with an 18mA load producing 28V DC output
LX drive
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
19
www.zetex.com
DN66
5.5V output with no load producing 28V output
This shows the fixed output LX drive waveform which can be as wide as 10␮sec.
At 5.5V input and an output of 28V, this graph shows the typical output regulation to be <10% and
ripple < 1V from no load to full load.
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
20
www.zetex.com
DN66
Additional notes
Adjusting output voltage
1) R1 and R2
When connected without external resistors R1 and R1, the ZXLB1600 will produce a nominal
output voltage of 28V. This is because the chip has an internal high value resistor divider which
is shunted by R1 and R2 externally if low value resistors are used.
The relationship between R1 and R2 and VOUT is:
VOUT(DC) = (R1+R2)/R1 x 1.23V
The following table gives suggested E24/E96 resistor values for various output voltages.
Required output voltage
5V
12V
18V
20V
22V
25V
Table 2
External resistor across R2
280k
715k
1M
1.15M
1.15M
1.2 M
External resistor across R1
91k
82k
75k
75k
68.1k
62k
Resistor values for output voltages
2) Output adjustment by external voltage
The internal voltage reference (Pin ADJ) may be overdriven by an external control voltage to set
the output voltage. The relationship between applied voltage (VADJ) and output voltage (VOUT) is:
VOUT = 22.86 x VADJ
Note that the output can be set to any value between the input voltage and the maximum
operating voltage in this way. However, some non-linearity in the above expression may occur at
values of VADJ below approximately 0.5V.
Also note that when driving the ADJ pin, the control voltage must have sufficiently low
impedance to sink the bias current of the internal reference (10␮A max).
3) PWM output adjustment
A Pulse Width Modulated (PWM) signal can be applied to the EN pin in order to adjust the output
voltage to a value below the value set in 1) or 2). This method of adjustment permits the device
to be turned on and the output voltage set by a single logic signal applied to the EN pin. No
external resistors or capacitors are required and the amplitude of the control signal is not critical,
providing it conforms to the limits defined in the electrical characteristics.
Two modes of adjustment are possible as described below:
Filtered 'DC' mode
If a PWM signal of 10kHz or higher is applied to the EN pin, the device will remain active when the
EN pin is low. However, the input to the internal low pass filter will be switched alternately from
VREF to ground, with a duty cycle (D) corresponding to that of the PWM signal. This will present
a filtered dc voltage equal to the duty cycle multiplied by VREF to the control loop and will produce
a dc output voltage lower than the maximum set value. This voltage is given by:
VOUT = 28 x D
A square wave signal applied to the EN pin, for example, will turn the device on and produce a
nominal regulated output of 14V.
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
21
www.zetex.com
DN66
Gated mode
The ZXLB1600 contains a timing circuit that switches the device on a few microseconds after the
application of a rising edge to EN and turns it back off again nominally 120␮s after the falling edge
of EN. So, if a lower frequency of 1kHz or less is applied to the EN pin, the device will be gated on
and off at a duty cycle (D) corresponding to that of the input signal. The average output voltage
is then given by:
VOUT(avg) ~ 28 x D
Output voltage can be adjusted all the way down to the input voltage by means of PWM control,
but for best results, the duty cycle range should be kept within the specified range of 0.4 to 1.
Lower duty cycles may result in increased output ripple and non-linearity in the relationship
between duty cycle and output voltage. If a greater control range, or reduced ripple is required,
the nominal output can be adjusted by one of the other methods before the PWM signal is
applied.
Negative output
The ZXLB1600 can be used to provide a negative output voltage (in addition to the normal
positive output) as shown in the application circuit below. In this circuit, the external resistors R3
an R4 are used to set the output voltage to 22V as described in the previous section. These
resistors and output capacitor C2 have relatively low values in this circuit in order to give a short
time constant. This improves the regulation of the negative voltage.
BAT54S
L1
22␮H
D1
1
2
D2
3
R3
210k
VBATT (3V nom.)
on
C1
10␮F
6V
C2
10nF
35V
ZXLB1600
off
R4
12k
GND
C4
100nF
50V
2
C3
1␮F
35V
BAT54S
D4
1
Figure 4
www.zetex.com
D3
3
Negative output
-22V @ 5mA
Title???????
22
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
DN66
Capacitor selection
A low ESR ceramic capacitor grounded close to the GND pin of the package is recommended at
the output of the device. Surface mount types offer the best performance due to their lower
inductance. A minimum value of 1␮F is advised, although higher values will lower switching
frequency and improve efficiency especially at lower load currents. A higher value will also
minimize ripple when using the device to provide an adjustable dc output voltage.
A good quality, low ESR capacitor should also be used for input decoupling, as the ESR of this
capacitor is effectively in series with the source impedance and lowers overall efficiency. This
capacitor has to supply the relatively high peak current to the coil and smooth the current ripple
on the input supply. A minimum value of 3.3␮F is acceptable if the input source is close to the
device, but higher values are recommended at lower input voltages, when the source impedance
is high. The input capacitor should be mounted as close as possible to the IC.
For maximum stability over temperature, capacitors with X7R dielectric are recommended, as
these have a much smaller temperature coefficient than other types.
Inductor selection
The choice of inductor will depend on available board space as well as required performance.
Small value inductors have the advantage of smaller physical size and may offer lower series
resistance and higher saturation current compared to larger values. A disadvantage of smaller
inductors is that they result in higher frequency switching, which in turn causes reduced
efficiency due to switch losses. Higher inductor values can provide better performance at lower
supply voltages. However, if the inductance is too high, the output power will be limited by the
internal oscillator, which will prevent the coil current from reaching its peak value. This condition
will arise whenever the ramp time ILX(peak) x L/VIN exceeds the preset 10␮s maximum 'on' time
limit for the LX output.
The ZXLB1600 has been optimized for use with inductor values in the range 10␮H to 100␮H. The
typical characteristics show how efficiency and available output current vary with input voltage
and inductance. The inductor should be mounted as close to the device as possible with low
resistance connections to the LX and SW pins.
Suitable coils for use with the ZXLB1600 are those in the NPIS range listed from NIC components
or LP02506 and DO1608 series, made by Coilcraft if preferred.
Diode selection
The rectifier diode (D1) should be a fast low capacitance switching type with low reverse leakage
at the working voltage. It should also have a peak current rating above the peak coil current and
a continuous current rating higher than the maximum output load current. Small Schottky diodes
such as the BAT54 are suitable for use with the ZXLB1600 and this diode will give good all round
performance over the output voltage and current range. At lower output voltages, a larger
Schottky diode such as the ZHCS500 or MBR0540 will provide a smaller forward drop and higher
efficiency. At higher output voltages, where forward drop is less important, a silicon switching
diode such as the 1N4148 can be used, however this will give lower efficiency but will have better
leakage characteristics than a Schottky device.
The BAT54S device specified in the application circuit contains a second diode (D2) as one half of
a series connected pair. This second diode is used here to clamp possible negative excursions
(due to coil ringing) from driving the drain of the output transistor below -0.5V. This prevents
internal coupling effects, which might otherwise affect output regulation. The table below gives
some typical characteristics for various diodes.
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
23
www.zetex.com
DN66
Diode
Forward voltage
at 100mA (V)
Peak current
(mA)
Continuous
current (mA)
Reverse leakage
(µA)
BAT54
530
300
200
2
ZHCS500
300
1000
500
15
MBR0540
390
1000
500
1
1N4148
950
450
200
0.025
Table 3
Typical diode characteristics
Increased efficiency
If isolation of the coil from the supply is not needed, the high side of this can be connected directly
to VIN to improve efficiency. This prevents power loss in the internal PMOS switch and typical
efficiency gains of 5% can be achieved. (See efficiency vs. load curves). Some applications may
require the coil to be fed from a separate supply with a different voltage to VIN. In this case, the
SW pin should be left floating.
Layout considerations
PCB tracks should be kept as short as possible to minimize ground bounce and the ground pin of
the device should be soldered directly to the ground plane. It is particularly important to mount
the coil and the input/output capacitors close to the device to minimize parasitic resistance and
inductance, which will degrade efficiency and increase output ripple. The FB and LBT pins are
high impedance inputs, so PCB track lengths to these should also be kept as short as possible to
reduce noise pickup. Output ripple is typically only 50mV p-p, but a small feed-forward capacitor
(~100pF) connected from the FB pin to the output may help to reduce this further. Capacitance
from the FB pin to ground should be avoided, but a capacitor can be connected from the LBT pin
to ground to reduce noise pickup into the low battery comparator if required.
Low Battery Detection Circuit (LBDC)
The device contains an independent low battery detection circuit that remains powered when the
device is shutdown. The detection threshold is set internally to a default value of 1.98V, but can
be adjusted by means of external resistors as described below.
Low Battery Threshold adjustment (LBT)
The internal potential divider network R3/R4 sets the detection threshold. This is accessible at the
LBT pin and can be shunted by means of external resistors to set different nominal threshold
voltages. The potential divider defines threshold voltage according to the relationship:
VLBT = (R3+R4)/R4 x 1.21V
When using external resistors, these should be chosen with lower values than the internal
resistors to minimize errors caused by the +/-25% absolute value variation of the internal
resistors. The internal resistors have high values in order to minimize these errors.
Low Battery Flag output (LBF)
This is an open drain output that switches low when the battery voltage falls below the detection
threshold. An external pull-up resistor can be connected to this pin to allow it to interface to any
voltage up to a maximum of 29V. Current in the pull-up resistor should be limited to a value below
IBLOL.
www.zetex.com
24
Issue 5 - July 2007
© Zetex Semiconductors plc 2007
DN67
ZXSC400 solution for 1W high powered LED
Mike Farley, Field Applications Engineer. December 2003
Description
The ZXSC400, although designed for small LEDs in LCD backlighting, is sufficiently flexible to
provide an efficient 1W solution producing a nominal 350mA constant current source from 2
NiMH or NiCd cells.
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
Figure 1
Schematic diagram
Figure 2
Performance graphs
25
www.zetex.com
DN67
Reference
Part number
U1
Value
Manufacturer
Contact details
ZXSC400E6
Zetex
www.zetex.com
Q1
FMMT617
Zetex
www.zetex.com
D1
ZHCS2000
Zetex
www.zetex.com
D2
LXHL-NW98
Lumileds
www.lumileds.com
L1
DO1608C-332
3.3␮H
Coilcraft
www.coilcraft.com
C1
GRM42-6X5R226K6.3
22␮F
Murata
www.murata.com
C2
GRM42-6X5R226K6.3
22␮F
Murata
www.murata.com
R1(1)
17m⍀
Generic
NA
R2
0.82⍀
Generic
NA
Table 1
Bill of materials
NOTES:
(1) Actual in-circuit value, see notes overleaf
Figure 3
Open circuit protection
Additional BoM
AD1 - 5V6
R3 - 1K⍀
www.zetex.com
26
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
DN67
VIN
L1
Q1
+
C1
D1
U1
R1
GND
R2
Figure 4
K’ D2
+
A’ D2
C2
Layout suggestion
Note
For these approximate layout dimensions, R1 is 15m⍀. See note 3.
Notes:
1. D1 can be exchanged with a SOT23 4. Open circuit protection can be added as
ZHCS1000 with a loss of 5% efficiency.
shown below. The voltage rating of the
small signal Zener diode ZD1 is not critical.
2. Inductor DCR (DC resistance) strongly
It must be greater than the maximum
influences efficiency, keep below 0.1⍀.
forward voltage of the LED and less than
3. R1 is small and it is strongly advised to take
the maximum VCE rating of the switching
track resistance into account. A proven
transistor, 15V in the case of the FMMT617.
method is to source a 1A current from the
The supply current in the open circuit
Sense pin to the GND pin and check for 16condition is around 2mA.
17mV. This resistor can be made from a
22m⍀ in parallel with a 47m⍀ (or a single
15m⍀ resistor if available) with the PCB
trace contributing the difference.
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
27
www.zetex.com
DN67
Intentionally left blank
www.zetex.com
28
Issue 3 - July 2006
© Zetex Semiconductors plc 2006
DN68
ZXSC310 High power torch reference design
Description
This design note shows a typical ZXSC310 LED driver circuit for a high powered LED torch. The
input voltage ranges from 0.7V to 1.6V with a maximum output current of 335mA at 1.4V input.
A typical schematic diagram is shown in Figure 1.
D1
L1
VIN
U1
Q1
Vcc
Vdrive
C1
C2
Stdn
C3
Isense
Gnd
ZXSC310
Figure 1
R1
Schematic diagram
Reference Value
Part number
Manufacturer Contact details
Comments
U1
ZXSC310E5
Zetex
www.zetex.com
LED driver in SOT23-5
Q1
FMMT617
Zetex
www.zetex.com
Low sat. NPN in SOT23
Zetex
www.zetex.com
2A Schottky in SOT23
D1
2A
ZHCS2000
L1
7.5␮H
DO3316P-153x2 Coilcraft
R1
19.5m⍀ Generic
Generic
NA
C1
1␮F
Generic
Generic
NA
C2
220␮F
Generic
Generic
NA
C3
100␮F
Generic
Generic
NA
Table 1
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
www.coilcraft.com ISAT = 3A
0805 size
Bill of materials
29
www.zetex.com
DN68
Figure 2
www.zetex.com
Performance graphs
30
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN69
ZXSC310 Garden light reference design
Description
This design note shows a typical ZXSC310 LED driver circuit for a solar power garden light. The
input voltage ranges from1.7V to 2.5V with a maximum output current of 160mA at 2.4V input.
A typical schematic diagram is shown in Figure 1.
L1
1.7V to 2.5V
D1
U1
Q1
VCC
VDRIVE
C1
Stdn
ISENSE
Gnd
ZXSC310
R1
Figure 1
Ref
Value
Schematic diagram
Part number
Manufacturer Contact details
Comments
U1
ZXSC310E5
Zetex
www.zetex.com
LED driver in SOT23-5
Q1
FMMT617
Zetex
www.zetex.com
Low sat NPN in SOT23
Zetex
www.zetex.com
0.5A Schottky in SOT23
D1
500mA ZHCS500
L1
15␮H
DO3316P-153 Coilcraft
www.coilcraft.com ISAT =3A
R1
70m⍀
Generic
Generic
NA
C1
100␮F
Generic
Generic
NA
Table 1
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
0805 size
Bill of materials
31
www.zetex.com
DN69
Total output current
Table 2 shows the maximum available output current and the current per LED for a given number
of LEDs. An LED forward voltage of 3.5V is assumed.
Total LED Current (mA)
4 LEDs
5 LEDs
6 LEDs
176
44
35
29
163
41
33
27
153
38
31
25
141
35
28
23
131
33
26
22
119
30
24
20
110
27
22
18
97
24
19
16
89
22
18
15
80
20
16
13
70
18
14
12
61
15
12
10
Table 2
www.zetex.com
Total output current
32
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN69
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Figure 2
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
Performance graphs
33
www.zetex.com
DN69
Intentionally left blank
www.zetex.com
34
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN70
ZXSC400 Driving 2 serial high power LEDs
Description
This design note shows the ZXSC400 driving 2 serial LEDs. The input voltage ranges from 2V to
3.6V with a maximum output current of 360mA from 2.6V input.
Figure 1 shows a typical constant current solution with the ZXSC400 driving two 1W LEDs in
series. The wide input voltage range allows the use of different battery cell combinations. This
could be dual alkaline cells with voltage starting from 3V down to 2V or triple NiCad/NiMH cells
with voltage starting from 3.6V down to 2.7V.
D1
L1
VIN=1.8V to 3.6V
Q1
C2
U1
C1
Vcc
Vdrive
Stdn
Isense
Gnd
VFB
ZXSC400
R1
Figure 1
Ref.
Value
R2
Schematic diagram
Part number
Manufacturer
Comments
U1
ZXSC400E6
Zetex
LED driver in SOT23-6
Q1
ZXTN25012EFH Zetex
Low sat. NPN transistor in SOT23
D1
2A
ZHCS2000
Zetex
2A Schottky in SOT23
L1
22␮H
Generic
Generic
ISAT = 2A
R1
18m⍀
Generic
Generic
0805 size
R2
820m⍀
Generic
Generic
0805 size
R3
1K⍀
Generic
Generic
0805 size
C1
22uF/10V
Generic
Generic
C2
100uF/10V
Generic
Generic
C3
220nF/10V
Generic
Generic
Table 1
Issue 3 - July 2007
© Zetex Semiconductors plc 2007
0805 size
Bill of materials
35
www.zetex.com
DN70
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
100
0.4
Output Current (A)
Efficiency (%)
90
80
70
60
50
0.2
0.0
3.6
3.4
3.2
3
2.8
2.6
2.4
2.2
2
3.6
3.4
3.2
Input Voltage (V)
1.6
8
1.2
6
0.8
0.4
0.0
3.4
3.2
3
2.8
2.6
2.6
2.4
2.2
2
2.4
2.2
2.2
2
4
2
0
2
3.6
3.4
3.2
Input Voltage (V)
3
2.8
2.6
2.4
Input Voltage (V)
Input Voltage vs Input Current
Figure 2
www.zetex.com
2.8
Input Voltage vs Output Current
Output Voltage (V)
Input Current (A)
Input Voltage vs Efficiency
3.6
3
Input Voltage (V)
Input Voltage vs Output Voltage
Performance graphs
36
Issue 3 - July 2007
© Zetex Semiconductors plc 2007
DN71
ZXSC400 Solution for Luxeon® V Star high powered LED
Description
This design note shows the ZXSC400 driving a Luxeon® V Star LED. The input voltage ranges
from 4.2V to 5.4 V with a maximum output current of 700mA at 5V input.
A typical schematic diagram is shown in Figure 1.
D1
L1
VIN = 4.4V to 5.4V
Q1
C2
U1
C1
VCC
VDRIVE
Stdn
ISENSE
GND
VFB
R4
ZXSC400
C3
R1
Figure 1
R2
R3
Schematic diagram
#
Ref.
Value
Part number
Manufacturer
Comments
U1
ZXSC400E6
Zetex
LED driver in SOT23-6
Q1
ZXTN25012EFH
Zetex
Low sat. NPN in SOT23
D1
2A
ZHCS2000
Zetex
2A Schottky in SOT23
L1
22␮H
Generic
Generic
ISAT = 2A
R1
18m⍀
Generic
Generic
0805 size
R2, R3
820m⍀
Generic
Generic
0805 size
R4
1k⍀
Generic
Generic
0805 size
C1
22␮F/10V
Generic
Generic
C2
100␮F/10V
Generic
Generic
C3
100nF/10V
Generic
Generic
Table 1
Issue 6 - July 2007
© Zetex Semiconductors plc 2007
0805 size
Bill of materials
37
www.zetex.com
DN71
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
100
0.4
Output Current (A)
Efficiency (%)
90
80
70
60
50
0.2
0.0
3.6
3.4
3.2
3
2.8
2.6
2.4
2.2
2
3.6
3.4
3.2
Input Voltage (V)
1.6
8
1.2
6
0.8
0.4
0.0
3.4
3.2
3
2.8
2.6
2.6
2.4
2.2
2
2.4
2.2
2.2
2
4
2
0
2
3.6
3.4
3.2
Input Voltage (V)
3
2.8
2.6
2.4
Input Voltage (V)
Input Voltage vs Input Current
Figure 2
www.zetex.com
2.8
Input Voltage vs Output Current
Output Voltage (V)
Input Current (A)
Input Voltage vs Efficiency
3.6
3
Input Voltage (V)
Input Voltage vs Output Voltage
Performance graphs
38
Issue 6 - July 2007
© Zetex Semiconductors plc 2007
DN72
ZXLD1101 Driving 8 series LEDs
Description
This design note shows the ZXLD1101 driving 8 series connected LEDs. The input voltage ranges
from 4.2V to 5.2V with a maximum output current of 24mA at 5V input.
A typical schematic diagram is shown in Figure 1.
L1
D1
VIN
C1
x8 LEDs
C2
VIN
LX
EN
ZXLD1101
C3
FB
GND
Figure 1
Ref.
Value
U1
R1
Schematic diagram
Part number
Manufacturer
Comments
ZXLD1101E6
Zetex
LED Driver in SOT23-6
1A Schottky in SOT23
D1
1A
ZHCS1000
Zetex
L1
33␮H
Generic
Generic
R1(1)
0⍀
Generic
Generic
C1
100␮F
Generic
Generic
C2
1␮F
Generic
Generic
C3
10␮F
Generic
Generic
Table 1
0805 size
Bill of materials
NOTES:
(1) R1 is set to zero. It shows the maximum output power characteristic of the LED driver. A regulated LED
current below the maximum value can be set by: ILED = VFB/R1, where VFB = 0.1V.
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
39
www.zetex.com
DN72
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Figure 2
www.zetex.com
Performance graphs
40
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN73
ZXSC300 Step down converter for 3W LED
Description
This design note shows the ZXSC300 or ZXSC310 driving a 3W LED. The input voltage ranges
from 6.2V to 3.8V with a maximum output current of 1.11A at 6V input.
A typical schematic diagram is shown in Figure 1.
Figure 1
Ref.
Value
Schematic diagram
Part number
Manufacturer
Comments
U1
ZXSC300/310
Zetex
LED Driver in SOT23-5
Q1
ZXMN2A01F
Zetex
SOT23 MOSFET
D1
1A
ZHCS1000
Zetex
1A Schottky in SOT23
L1
22␮H
Generic
Generic
ISAT = 3A
R1
20m⍀
Generic
Generic
0805 size
C1
100␮F
Generic
Generic
C2
100␮F
Generic
Generic
Table 1
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
Bill of materials
41
www.zetex.com
DN73
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Figure 2
www.zetex.com
Performance graphs
42
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN74
ZXSC400 Photoflash LED reference design
Description
This design note shows the ZXSC400 driving a Photoflash LED. The input voltage is 3V with a
maximum pulsed output current of 1A for 2ms.
A typical schematic diagram is shown in Figure 1.
C3
U2
L1
SW1
VBATT
U1
VCC
VDRIVE
STDN
ISENSE
GND
C1
SW2
C2
R2
VFB
ZXSC400
R3
R1
R4
Charging mode: SW1 closed, SW2 open
Discharging mode: SW1 open, SW2 closed
Figure 1
Schematic diagram
Operation
In charging mode, SW1 is closed and SW2 is open the ZXSC400 is configured as a typical boost
converter, charging capacitor C2 up the regulated output voltage set by the ratio of R1 and R2.
This is typically 16V. The peak current of the converter (current drawn from the battery) is
controlled by R3 plus R4, and is typically 280mA for this application. When C2 is charged to 16V
the SW1 is opened and SW2 is closed, converting the ZXSC400 to a step down converter to
provide a 1A constant current for 2ms to the photoflash LED. During step down operation, current
flows from C2, through the photoflash LED, L1, U2 and is returned to C2 through R3. This means
that the peak current is set at a higher value than in charging mode, typically 1A. When the current
reaches it's peak value, U2 is switched off and current flows from L1 through the Schottky diode
in U2, to the photoflash LED. This cyclic process is repeated until C2 is discharged.
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
43
www.zetex.com
DN74
Ref
Value
Part number
Manufacturer
Comments
U1
ZXSC400E6
Zetex
LED Driver in SOT23-6
U2
ZX3CDBS1M832
Zetex
Dual NPN and Schottky
L1
12␮H
Generic
Generic
ISAT=1A
R1
10k⍀
Generic
Generic
0805 size
R2
510k⍀
Generic
Generic
0805 size
R3
22m⍀
Generic
Generic
0805 size
R4
100m⍀
Generic
Generic
0805 size
C1
1␮F
Generic
Generic
C2
150␮F
Generic
Generic
C3
1␮F
Generic
Generic
Table 1
Bill of materials
Typical operating waveforms
(For typical application circuit where VBATT = 3V and Tamb = 25°C unless otherwise stated)
Figure 2
www.zetex.com
Performance graphs
44
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN75
ZXSC310 Solar powered garden light reference design
Description
This design note shows a typical ZXSC310 LED driver circuit for a solar powered garden light. The
input voltage ranges from 0.4V to 1.6V with a maximum output current of 43mA at 1V input.
A typical schematic diagram is shown in Figure 1.
D1
L1
VIN
U1
C3
Q1
VCC
VDRIVE
C1
C2
Stdn
ISENSE
Gnd
ZXSC310
R1
Figure 1
Ref.
Value
Schematic diagram
Part Number
Manufacturer
Comments
U1
ZXSC310E5
Zetex
LED Driver in SOT23-5
Q1
FMMT617
Zetex
Low sat NPN in SOT23
ZHCS1000
Zetex
1A Schottky in SOT23
0805 size
D1
1A
L1
37␮H
R1
100m⍀
Generic
Generic
C1
1␮F
Generic
Generic
C2
22␮F
Generic
Generic
C3
10␮F
Generic
Generic
Table 1
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
Bill of materials
45
www.zetex.com
DN75
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Figure 2
www.zetex.com
Performance graphs
46
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN76
ZXLD1100 and ZXLD1101 driving from 3 to 6 LEDs
Description
This design note shows the ZXLD1100 and ZXLD1101 driving series connected LEDs. The input
voltage range is 2.5V to 5.5V. The same circuit can be used for up to 6 LEDs.
The ZXLD1100 contains onchip open circuit LED protection. This function would require an
additional Zener and resistor with the ZXLD1101.
Note: LED current is set to 15mA
Figure 1
Ref.
Value
Package
Part number
Schematic diagrams
Manufacturer Contact details
Notes
U1
TSOT23-5 ZXLD1101ET5
Zetex
www.zetex.com
LED driver IC
U1
SC706
ZXLD1100H6
Zetex
www.zetex.com
LED driver IC
Zetex
www.zetex.com
400mA Schottky
diode
D1
400mA SOD323
ZHCS400
L1
10␮H
CMD4D11-100MC Sumida
R1
6.8⍀
0603
Generic
Generic
R21
100k⍀
0603
Generic
Generic
C1
1␮F
0603
Generic
Generic
C2
1␮F
0603
Generic
Generic
NSCW215
Nichia
LEDs
Table 1
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
www.sumida.com 1mm height profile
www.nichia.com 6pcs per board
Bill of materials
47
www.zetex.com
DN76
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Figure 2
www.zetex.com
Performance graphs
48
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
DN78
ZXSC310 with reverse polarity protection
Ray Liu - Applications Engineer, Zetex Semiconductors
Description
The schematic diagram shown in Figure 1 is a typical example of the ZXSC310 used in a LED
flashlight application. The input voltage can either be one or two alkaline cells. If the battery is put
in the flashlight the wrong way, the reverse polarity can damage the ZXSC310 and switching
transistor, Q1. Implementing a mechanical reverse protection method can be expensive, and not
always reliable. This paper describes methods of electronic reverse protection, without efficiency
loss, for the ZXSC series ICs and related LED flashlight application circuits.
Circuit problems caused by the reverse polarity battery
If a negative voltage appears at the input terminal of Figure 1 then reverse current will flow from
the ground pin of the ZXSC310 to the VCC terminal and back to the battery. This current is high
and will damage the ZXSC310. Some of this reverse current will also flow through the VDRIVE
terminal of the ZXSC310 and into Q1 base-collector completing the circuit to the battery.
The reverse current through base-collector of Q1 turns the transistor on in the reverse direction
and causes high current to flow from ground, through emitter-collector to the battery, resulting
in battery drainage and possible damage to the switching transistor, Q1.
A common method of reverse polarity protection
A common method of reverse protection is to add a Schottky diode in series with the battery
positive. The problem with this method of reverse protection is that there is a loss of efficiency
due to the forward voltage drop of the diode, typically 5% to 10% depending upon input voltage,
reducing the usable battery life. The proposed method of reverse protection for the ZXSC series
IC's gives full protection with no loss of efficiency.
D1
L1
VIN
U1
Q2
VCC
C2
VDRIVE
Stdn
ISENSE
GND
ZXSC310
Figure 1
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
R1
Schematic diagram
49
www.zetex.com
DN78
Reverse protection without efficiency loss
By adding current limiting resistor and Schottky diode, the reverse current flow can be eliminated
without a loss of efficiency.
Flashlight circuit with bootstrap
For the bootstrap circuit in Figure 2, the current through the ZXSC310 is blocked by the reversed
biased Schottky diode, D1.
The current from VDRIVE, which turns on Q1 in the reverse direction, is diverted via D2 back to the
battery so that Q1 does not turn on. R2 is a current limiting resistor to control this VDRIVE current.
This value is typically set to 100⍀ to 500⍀ to minimize battery current drain without affecting the
normal operation of the circuit.
D1
L1
VIN
D2
U1
VCC
C1
Q1
R2
VDRIVE
Stdn
ISENSE
GND
C2
R1
ZXSC310
Figure 2
Ref
Part number
Manufacturer
Comments
U1
ZXSC310E5
Zetex
LED driver in SOT23-5
Q1
ZXTN25012EFL
Zetex
Low sat. NPN in SOT23
D1
Value
750mA
BAT750
Zetex
750mA Schottky in SOT23
200mA
BAT54
Zetex
200mA Schottky in SOT23
68␮H
Generic
Generic
ISAT>0.4A, R<0.8⍀
270m⍀
Generic
Generic
0805 size
100⍀
Generic
Generic
0805 size
C1
10␮F/6.3V
Generic
Generic
C2
22␮F/6.3V
Generic
Generic
D2
(1)
L1
R1
R2
(1)
Table 1
Bill of materials
NOTES:
(1) Add for reverse protection
www.zetex.com
50
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
DN78
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Without Protection
With Protection
Without Protection
100
50
Output Current (mA)
90
Efficiency (%)
With Protection
60
80
70
60
40
30
20
10
50
0
3
2.6
2.2
1.8
1.4
1
3
2.6
Input Voltage (V)
2.2
1.8
1.4
1
Input Voltage (V
Input Voltage vs Output Curren
Input Voltage vs Efficiency
Without Protection
With Protection
Without Protection
100
With Protection
4
80
Output Voltage (V)
Input Current (mA)
60
40
20
0
-20
-40
-60
3
2
1
-80
-100
3
2
1
0
-1
-2
0
2.8
-3
2.6
2.4
2.2
2
1.8
1.6
1.4
Input Voltage (V)
Input Voltage (V)
Input Voltage vs Input Current
Input Voltage vs Output Voltage
Figure 3
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
1.2
1
Performance graphs
51
www.zetex.com
DN78
Other circuit examples using reverse polarity protection
Flashlight circuit without bootstrap
The circuit shown in Figure 4 is for an LED flashlight application without bootstrap. As described
previously, reverse current can flow from the GND terminal to VCC and back to the battery. To
block this current path an extra diode, D2b, is added. It is recommended that a Schottky diode be
used for this application to maximize the start-up input voltage from VCC(MAX) to VCC(MIN) + D2b
VF, 3V to 1V. The Schottky diode, D2a, and resistor, R2, work in the same way as described in the
bootstrap circuit in Figure 2. A dual Schottky diode, BAT54S, is recommended for D2 in order to
achieve low component count.
L1
D1
VIN
U1
VCC
C1
R2
C2
Stdn
GND
ISENSE
ZXSC310
Figure 4
www.zetex.com
Q1
VDRIVE
R1
LED flashlight application without bootstrap
52
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
DN78
Other circuit examples using reverse polarity protection
Flashlight circuit without bootstrap
Figure 5 is a step down converter with reverse polarity protection. The main application for this
circuit is a four alkaline cell flashlight driving a high powered LED. Again the protection circuit
operates as described above. A dual Schottky diode, BAT54S, is recommended for D2 in order to
achieve low component count.
C2
L1
D1
VIN
D2a
D2b
R2
VCC
VDRIVE
Q1
Stdn
C1
ISENSE
GND
ZXSC310
R1
Figure 5
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
Step down converter with reverse polarity protection
53
www.zetex.com
DN78
Intentionally left blank
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
54
www.zetex.com
DN79
ZXSC400 1W LED driver
Neil Wolstenholme, Applications Engineer, Zetex Semiconductors plc
Description
ZXSC400 is configured to the reference design below. The target application is a 1W white LED
driven from a two cell NiCd/NiMH or alkaline battery input for flashlights and high powered LED
driving.
The supply voltage for ZXSC400 reference design is: VIN = 1.8V ~ 3V.
L1
D1
22uH
ZHCS1000
VIN
Q1
LED1
FMMT617
U1
ZXSC400
C1
100uF/6V3
VCC
VDRIVE
STDN
ISENSE
GND
LXHL-PW01
C2
VFB
100uF/6V3
R1
R2
22mohm
0.82ohm
GND
Figure 1
Schematic diagram
Ref.
Value
Package Part number
Manufacturer Contact details
Notes
U1
N/A
SOT23-6 ZXSC400E6
Zetex
www.zetex.com
Boost converter
Q1
N/A
SOT23 FMMT617
Zetex
www.zetex.com
Low sat NPN
transistor
D1
40V/1A
SOT23 ZHCS1000
Zetex
www.zetex.com
40V/1A Schottky
diode
L1
22uH/2.5A N/A
Coilcraft
www.coilcraft.com 22␮H/2.5A SMT
inductor
R1
22m⍀
0805
Generic
NA
0805 5% tolerance
R2
0.82⍀
0805
Generic
NA
0805 5% tolerance
C1
100␮F/6V3 1812
18126D107KAT2A AVX
www.avx.com
100␮F/6V3/X5R/1812
C2
100␮F/6V3 1812
18126D107KAT2A AVX
www.avx.com
100␮F/6V3/X5R/1812
LXHL-PW01
www.lumileds.com 1W white LED
emitter
LED1 1W
N/A
DO3316P-223
Table 1
Issue 1 - May 2005
© Zetex Semiconductors plc 2005
Lumileds
Bill of materials
55
www.zetex.com
DN79
Performance
Increasing efficiency
On ZXSC400 reference design, R1 is set to 22mΩ to ensure that the LED current is regulated over
the full input voltage range of 3V ~ 1.8V. For improved efficiency R1 can be changed to a 33mΩ
resistor but LED current will not be regulated below 2V. See Figure 2, Performance graphs.
400
VSTDN=VIN
ILED=350mA
90
LED Current (mA)
Efficiency (%)
100
R1=33mohms
80
70
R1=22mohms
60
50
3.0
2.8
2.6
2.4
2.2
2.0
VSTDN=VIN
380
R1=22mohms
360
340
R1=33mohms
320
300
3.0
1.8
2.8
2.6
2.4
2.2
2.0
1.8
Input voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage
LED Current vs Input Voltage
VSTDN=VIN
800
VSTDN=VIN
R1=22mohms
600
400
3.0
LED Voltage (V)
Input Current (mA)
4.0
1000
R1=33mohms
2.8
2.6
2.4
2.2
2.0
3.8
R1=33mohms
3.6
3.4
R1=22mohms
3.2
3.0
3.0
1.8
2.8
Input Voltage (V)
Input Current vs Input Voltage
Figure 2
www.zetex.com
2.6
2.4
2.2
2.0
1.8
Input Voltage (V)
LED Voltage vs Input Voltage
Performance graphs
56
Issue 1 - May 2005
© Zetex Semiconductors plc 2005
DN83
LED MR16 Lamp solution using the ZXLD1350 LED driver
Neil Chadderton, Colin Davies, Roger Heap, Zetex Semiconductors
Introduction
Lighting class LEDs are now available that deliver the brightness, efficacy, lifetime, color
temperature, and white point stability required for general illumination. As a result, these LEDs
are being adopted into most general lighting applications including roadway, parking area and
indoor directional lighting. LED-based luminaires reduce Total-Cost-of-Ownership (TCO) in these
applications through maintenance avoidance and reduced energy costs.
MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that have traditionally
employed a halogen filament capsule as the light source. They are used in many retail and
consumer lighting applications where their size, configurability, spot-lighting capability and
aesthetics provide utility and creativity. Their low efficiency, heat generation (an issue for
illuminating heat sensitive subjects and materials) and halogen capsule handling issues are
typically cited among the disadvantages of the technology. They typically operate from 12V AC
or 12V DC, though designs for 6V to 24V are also popular and as such require a step-down
transformer to allow use from offline supplies. This is usually effected with conventional
electromagnetic or electronic transformers.
With the advancement of HB (High Brightness) LED technologies, MR16 lamps can now be
realized with an alternate light source. This hybrid solution can yield a cost effective, long-life,
maintenance free, cooler operating unit which has not been previously possible.
Figure 1
MR16 Lamps (Incandescent A-lamp on far right shown for size comparison)
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
57
www.zetex.com
DN83
Description
This design note describes a driver solution developed using the Zetex ZXLD1350 LED driver IC
to drive three CREE® XLamp XR-E High Brightness (HB) LEDs.
The ZXLD1350 features can be summarized as:
•
Wide input voltage range
•
7V to 30V; internal 30V NDMOS switch
•
Up to 350mA output current (the ZXLD1360 can
provide up to 1A output current)
•
Capable of driving up to 8 series connected
1 Watt LEDs
•
High efficiency (see datasheet - but >90%
with 8 LEDs)
•
Low quiescent current: (100␮A typical)
•
1A max shutdown current
•
Brightness control using DC voltage or PWM (low or high frequency)
•
Internal PWM filter for high frequency PWM signals
•
Optional soft-start; up to 1MHz switching frequency
The Cree XR-E LED is a lighting class device that provides energy
savings for many traditional technologies such as the MR16 halogen
lamp. The XR-E LED is capable of operating at forward currents of up
to 1A without any noticeable shifts in chromaticity. The XR-E is ideally
suited for direct replacement of MR16 when used in clusters of three
at a forward current of 300mA—1000mA. They are specified at 80
lumens and 70 lumens per watt at 350mA (136 lumens at 700mA).
These lighting class LEDs offer efficient, directional light that offers a
lumen maintenance of 70% at 50,000 hours, in addition to significantly
reducing power consumption.
The circuit diagram of the ZXLD1350 effected MR16 lamp solution is shown in Figure 2. Table 1
provides the bill of materials. A full bridge (D1—D4) is employed using 1A DC rated, low leakage
Schottky diodes to allow AC or DC input supplies. A thermistor circuit is incorporated to reduce
the output current of the circuit to provide thermal feedback control, which allows the circuit to a)
match the thermal de-rating requirements of the LEDs to ensure lumen maintenance expectations
are achieved and b) prevent overheating. The thermistor must be thermally coupled to the LEDs
to ensure accurate and responsive tracking. Adjustment of the thermal feedback circuit can be
accomplished by the choice of the thermistor R3 - which sets the slope of the current vs.
temperature response, and resistor R2 - which determines the temperature threshold point for the
control circuit. R1 and D5 provide a reference voltage for the thermal control circuit. Q1 is a low
VCE(sat) PNP transistor. Schottky diode D9 is again a low leakage 1A rated device in a SOT23
package - its low forward voltage and low reverse current ensure high efficiency and thermal
stability in the main switching circuit. C3 may be added to reduce the amplitude of the current
ramp waveform experienced by the LED string but in many applications this isn't required as the
integrating nature of human sight cannot perceive quickly changing light levels. Depending on
layout intricacies and EMC dictates, it may be necessary to exchange the positions of the inductor
and LED string - this isn't always possible mechanically but does give a lower EMI signature.
www.zetex.com
58
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
DN83
Figure 3 shows the measured response of the LED current drive with respect to temperature, with
the values given in Table 2. The selection of components for the thermal feedback circuit is not
only dependent on the choice of R2 and R3, but also on the amount of heat sink area required to
extract heat from the LEDs. To maximize the light output at high ambient or operating
temperature conditions, the LEDs must have a sufficient thermal extraction path, otherwise the
thermal control circuit will effect current drive reduction in non-optimal conditions. The thermal
control threshold point is set by adjusting R2. For this design, three values (33k, 22k and 10k) were
evaluated. These values were chosen to give break points at approximately 25°C, 40°C and 60°C.
Note that the light output will not continually dim to zero - the thermal control is applying DC
control to the ADJ pin and therefore has a dimming ratio from maximum current of
approximately 5:1. Once the reduced DC level goes below the shutdown threshold of around
200mV, the LED drive current will fall to zero and the LEDs will be extinguished. The slope of the
current reduction is determined by the beta value of the thermistor. The larger the beta value, the
sharper will be the resultant current control response. The slope of the current reduction is also
affected by Q1's base emitter voltage (VBE) variation with temperature. Figure 3 shows the slope
starts to level off at higher temperatures due to the increasing influence of the approximately 2.2mV/°C change in the VBE of the transistor.
R6 2R
R7 1R
L1 100 uH
R8 1R
R9 1R
R 1 4 k7
D1
ZLLS1000
LED 1
Cree CLD7090
D2
ZLLS1000
W1
R10 0 R
Vin
R 2 typ 10 k
See table 2
W2
D4
ZLLS1000
D9
ZLLS1000
V Sense
C3
1uF
Not fitted
LED 3
Cree CLD 7090
Q1
ZXTP2039F
3
R4 0R
D3
ZLLS1000
4
5
C1
1uF
50 V
LED 2
Cree CLD7090
ADJ
D5
BZX284C6V2
ZXLD1350
LX
1
R5 10 k
Not fitted
GND
2
R3
10 k
Thermistor
B 3900
W3
Thermally connected
C
100nF
Not fitted
W4
Figure 2
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
Circuit diagram of ZXLD1350 MR16 lamp solution
59
www.zetex.com
DN83
Measured results for circuit of Figure 2 using 10k thermistor
with beta of 3900
400
LED current mA
350
300
250
R1 set for 25°C
200
R1 Set for 40°C
R1 Set for 60°C
150
100
50
0
0
20
40
60
80
100
120
140
Temperature °C
Figure 3
Measured response of thermal feedback control showing threshold point
Quantity Part reference Value
1
R1
4k7
1
R2
See Table 2 0603
resistor
1
R3
10K, 0603,
Beta = ~3900 thermistor
1
R4
0R, 0603 link
1
R5
10k, 0603 resistor
1
R6
2R, 0603 resistor
3
R7, R8, R9
1R, 0603 resistor
R10
0R 1206 link
5
D1, D2, D3,
ZLLS1000
D4, D9
1
D5
BZX284C6V2
1
C1
1uF
2
3
1
C2, C3
LED1, LED2,
LED3
Q1
1
L1
100␮H
1
IC1
ZXLD1350
XR-E
ZXTP2039F
Table 1
www.zetex.com
Description
Resistor, 1%, 0603
Resistor, 1% 0603
Source
Various
Various
Thermistor, 5% 0603
EPCOS
0R Link, 0603
Generic
Generic
Generic
0R Link, 1206
Schottky diode 40V, 1A
Various
Various
Various
Various
Various
Zetex
6.2V Zener diode
Capacitor 50v 1206 X7R
NMC1206X7R105K50F
C1206C105K5RAC7800
Not fitted
Cree XLamp power LED
Transistor, PNP
Alternative: FMMT717
MSS6132 100␮H
NPIS53D101MTRF
Zetex LED driver IC
NICcomponents
KEMET
Cree
Zetex
Coilcraft
NIC components
Zetex
Bill of materials
60
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
DN83
R2 = 33k
R2 = 22k
R2 = 10k
Temp.
R1 set for 25°C
R1 set for 40°C
R1 set for 60°C
0
350
350
350
20
350
350
350
25
352
350
350
40
280
345
350
60
177
235
342
80
131
148
248
100
104
120
171
120
81
90
Table 2: Thermal feedback control threshold point (resistor R2)
For three series-connected LEDs, the voltage can be from 12V minimum to 30V DC maximum. For
AC supplies, remember to include the 1.414 factor for RMS specified values - so for 20V AC (RMS),
this will provide a DC rail after the Schottky bridge of 28.3V. The nominal current is set at 350mA
with a 0.283⍀ sense resistor. The sense resistor is a combination component using 4 low cost,
commonly available values and allows current set point flexibility if the circuit is used as a
platform design for a series of products. For three series-connected LEDs, with a nominal supply
of 24V and a 100␮H inductor, the ZXLD1350 runs in continuous mode at approximately 500kHz.
The ZXLD1350 datasheet displays this information graphically, as shown in Figure 4 (for a sense
resistor of 330m⍀ in this case), which allows a fast assessment to be made of operating
conditions. The switching frequency will increase as the voltage on the ADJ pin decreases. As the
ZXLD1350 (and ZXLD1360) series of LED drivers use a hysteretic switching topology, the
switching frequency is dependent on several factors - input voltage, target current (including any
effect by voltage on the ADJ pin to reduce the current) and number of LEDs. An Excel based
calculator is available which allows "what-if" initial evaluation and is a useful tool for assessing
component and condition changes. Final designs should, of course, be verified by reality.
Operating Frequency vs Input Voltage
L=100uH, Rs=0.33 Ohms
600
Frequency (kHz)
500
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
400
300
200
100
Note: Please see the
ZXLD1350 datasheet
for complete
characterization
including efficiency,
duty cycle and current
variation charts for
various values of
inductor
0
5
10
15
20
25
30
VIN (V)
Figure 4
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
Example of operating frequency chart for the ZXLD1350
61
www.zetex.com
DN83
Higher current designs
The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which
with the hysteretic mode of operation (the current waveform will ramp ±15% about the nominal
current set point) provides sufficient margin. For higher current operation, the 1A rated ZXLD1360
offers similar design procedures and has the following features:
•
Up to 1A output current
•
Wide input voltage range: 7V to 30V
•
Internal 30V 400m NDMOS switch
•
Can drive up to 7 series connected 3W LEDs
(with due attention to thermal path design)
•
High efficiency (>90% for 7 LEDs)
•
Brightness control using DC voltage or PWM
•
Internal PWM filter
•
Optional soft-start
•
Up to 1MHz switching frequency
www.zetex.com
62
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
DN83
Board design
k
The Printed Circuit Board (PCB) design and circuit employed make it particularly suitable for use
in MR16 halogen lamp replacement units. The supply voltage range is nominally 12V AC or DC,
making it compatible and interchangeable with existing MR16 lamps. The printed circuit tracking
has been designed using only one side of the board, to facilitate the use of an aluminum or other
heat-conductive substrate where through-hole technology cannot be employed. A central hole is
provided to enable connection of the supply leads from the rear and for connection to a dimming
circuit, where this is required. Mounting holes are also provided. Gerber-format layout files for
this PCB are available from Zetex upon request. Please quote PCB number ZDB335.
2
LE D
3
R2
4
R
LED
R1
D5
W3
3
R
Q1
k
C2
R5
D3
IC1
W1 W4
D9
R9
R8
R7
R6
C1
R10
D4
C3
W2
D1
D2
L1
k
LED1
Top PCB overlay
Top PCB overlay and top copper
k
Figure 5
Top copper
2
LE
D
3
R2
4
R
LED
R1
D5
W3
R5
R3
k
C2
Q1
D3
IC1
D9
R9
R8
R7
R6
W1 W4
R10
C1
D4
C3
W2
D1
D2
L1
k
LED1
Figure 6
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
Composite view
63
www.zetex.com
DN83
Appendix A - ZXLD1350 operation
In normal operation, when voltage is applied at +VCC, the ZXLD1350 internal NDMOS switch is
turned on. Current starts to flow through the sense resistor, inductor L1, and the LEDs. The current
ramps up linearly, and the ramp rate is determined by the input voltage +VCC and the inductor L1.
This rising current produces a voltage ramp across the sense resistor. The internal circuit of the
ZXLD1350 senses the voltage across the sense resistor, and applies a proportional voltage to the
input of the internal comparator. When this voltage reaches an internally set upper threshold, the
NDMOS switch is turned off. The inductor current continues to flow through the sense resistor,
L1, the LEDs, the Schottky diode SD9, and back to the supply rail, but it decays, with the rate of
decay determined by the forward voltage drop of the LEDs and the Schottky diode. This decaying
current produces a falling voltage at the sense resistor, which, in turn, is sensed by the ZXLD1350.
A voltage proportional to the sense voltage across the sense resistor is applied at the input of the
internal comparator. When this voltage falls to the internally set lower threshold, the NDMOS
switch is turned on again. This switch-on-and-off cycle continues to provide the average LED
current set by the sense resistor.
Both DC and PWM dimming can be achieved by driving the ADJ pin through W3. For DC
dimming, the ADJ pin may be driven between 300mV and 1.25V. Driving the ADJ pin below
200mV will shutdown the output current. For PWM dimming, an external open-collector NPN
transistor or open-drain N-channel MOSFET can be used to drive the ADJ pin. The PWM
frequency can be low, around 100Hz to 300Hz, or high between 10kHz to 50kHz. For the latter case,
an on-chip filter derives the DC content and so for high frequency PWM input, the device will
operate essentially as for DC control input dimming. Generally, low frequency PWM control is
preferred as in this mode, the converter is shut down during PWM low signals and drives the
LEDs at the defined nominal current during PWM high signals - this ensures that the LEDs can are
always driven at the nominal current and therefore color temperature (CCT) shifts are minimized.
The capacitor C2 should be around 10nF to decouple high frequency noise at the ADJ pin for DC
dimming. Note - C2 should not be fitted when using the PWM dimming feature. The soft-start
time will be nominally 0.5ms without capacitor C2. Adding C2 will increase the soft start time by
approximately 0.5ms/nF
Please refer to the datasheets for the threshold limits, ZXLD1350 internal circuits, electrical
characteristics and parameters.
Issue 2 - August 2007
© Zetex Semiconductors plc 2007
64
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DN84
ZXSC400 Driving 3W high power LEDs
Ray Liu, Applications Engineer, Zetex Semiconductors
Description
This design note shows the ZXSC400 driving a single 3W LED. The input voltage ranges from 1.8V
to 3.6V with constant output current of 700mA down to 2.6V with an overall 80% of efficiency.
Figure 1 shows typical constant current solution with ZXSC400 driving one 3W LED. The input
voltage range allows the use of two alkaline batteries or one Lithium Ion cell (CR123A) for
portable flashlight applications.
Q1 and Q2 forms a pseudo Darlington pair which provide enough current gain for a switching
current up to 1.5A. In order to provide better switch off performance, a Schottky diode, D2, is used
to drain the base current from the base of Q1 directly. In order to achieve higher efficiency, current
monitor U2 is used to provide a low voltage drop LED current sensing through the low ohmic
resistor, R2. The LED current is converted to 300mV feedback voltage through R3.
D1
L1
VIN = 1.8V to 3V
R2
U2
VSENSE+
Q2
C1
R4
VSENSE-
IOUT
ZXCT1009
Q1
C2
U1
C1
VCC
VDRIVE
Stdn
ISENSE
GND
VFB
D2
R6
D1
R5
ZXSC400
C3
R3
R1
Figure 1
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
Schematic diagram
65
www.zetex.com
DN84
Ref.
Value
U1
Part number
Manufacturer
Comments
ZXSC400E6
Zetex
LED driver in SOT23-6
U2
ZXCT1009
Zetex
Current monitor in SOT23
Q1
ZXTN25012EFH
Zetex
Low sat NPN in SOT23
Q2
ZXTN25012EFL
Zetex
Low sat NPN in SOT23
D1
ZHCS2000
Zetex
2A Schottky in SOT23
D2
ZHCS400
Zetex
400mA Schottky
L1
15␮H
744 561 15
Wurth Electronik
ISAT = 3A DCR=60m⍀
R1
20m⍀ 1%
Generic
Generic
0805 size low ohmic
R2
50m⍀ 1%
Generic
Generic
0805 size low ohmic
R3
820⍀ 1%
Generic
Generic
0805 size
R4
82⍀ 5%
Generic
Generic
0805 size
R5
4.7⍀ 5%
Generic
Generic
0805 size
R6
10⍀ 5%
Generic
Generic
0805 size
C1
22␮F 10V 10%
Generic
Generic
1206 size X7R/X5R
C2
4.7␮F 10V 10%
Generic
Generic
1206 size X7R/X5R
C3
0.22␮F 16V
10%
Generic
Generic
C4
330pF/10V
Generic
Generic
0805 size
Table 1
www.zetex.com
Bill of materials
66
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
DN84
Intentionally left blank
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
67
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DN84
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
90
0.8
Output Current (A)
1.0
Efficiency (%)
100
80
70
60
50
0.6
0.4
0.2
0.0
3
2.8
2.6
2.4
2.2
2
1.8
3
2.8
2.6
Input Voltage (V)
Input Voltage vs Efficiency
2.2
2
1.8
Input Voltage vs Output Current
1.6
4
1.2
3
Output Voltage (V)
Input Current (A)
2.4
Input Voltage (V)
0.8
0.4
2
1
0
0.0
3
2.8
2.6
2.4
2.2
2
3
1.8
2.8
2.6
Input Voltage vs Input Current
Figure 2
www.zetex.com
2.4
2.2
2
1.8
Input Voltage (V)
Input Voltage (V)
Input Voltage vs Output Voltage
Performance graphs
68
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
DN85
ZXSC400 1W/3W buck LED drivers
Ray Liu, Applications Engineer, Zetex Semiconductors
Description
In Figure 1, ZXSC400 is configured as a high efficiency buck LED driver. The target applications
are either 1W (350mA) or 3W (700mA) drivers for white LED driven from a 4 cell battery, or a 2
alkaline cell input for flashlights. The supply voltage for ZXSC400 reference design is:
VIN = 3.8V to 6V.
Parts lists for 1W and 3W design are shown in Table 1 and Table 2 respectively. Performance data
is measured based on two different LED's VF binning with 0.3V VF difference.
C3
L1
VIN = 4V to 6V
D1
C2
U1
VCC
C1
Q1
VDRIVE
Stdn
VFB
GND
ISENSE
R3
R2
ZXSC400
R1
Figure 1
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
Schematic diagram
69
www.zetex.com
DN85
Ref. Value
Package Part number
Manufacturer Contact details
Notes
U1
N/A
SOT23-6 ZXSC400E6
Zetex
www.zetex.com
LED Driver
Q1
N/A
SOT23
ZXTN25012EFL Zetex
www.zetex.com
Low sat NPN
transistor
D1
40V/0.75A SOT23
BAT750
Zetex
www.zetex.com
40V/0.75A
Schottky diode
L1
47␮H
N/A
744052470
Wurth
Elektronik
www.we-online.com ISAT = 520mA
R1
62m⍀
0805
Generic
N/A
0805 1%
R2
10⍀
0805
Generic
N/A
0805 5%
R3
47⍀
0805
Generic
N/A
0805 5%
C1
4.7␮F/10V 1206
Generic
N/A
X7R/X5R
C2
100pF/10V 0805
Generic
N/A
COG/NPO
C3
1uF/10V
Generic
N/A
X7R/X5R optional
0805
Table 1
www.zetex.com
Bill of materials for 1W LED
70
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
DN85
Performance
90
0.4
Output Current (A)
0.5
Efficiency (%)
100
80
70
60
0.3
0.2
0.1
VF~3.3
VF~3.6
VF~3.3
50
6.2
5.8
5.4
5
4.6
4.2
3.8
6.2
5.8
Input Voltage (V)
5.4
5
4.6
4.2
3.8
Input Voltage (V)
Input Voltage vs Efficiency
Input Voltage vs Output Current
0.4
4
0.3
Output Voltage (V)
Input Current (A)
VF~3.6
0.0
3
0.2
2
0.1
1
VF~3.3
VF~3.6
VF~3.3
0.0
VF~3.6
0
6.2
5.8
5.4
5
4.6
4.2
3.8
6.2
Input Voltage (V)
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
5.4
5
4.6
Input Voltage (V)
4.2
3.8
Input Voltage vs Output Voltage
Input Voltage vs Input Current
Figure 2
5.8
Performance graphs for 1W design
71
www.zetex.com
DN85
Ref. Value
Package Part number
Manufacturer Contact details
Notes
U1
N/A
SOT23-6 ZXSC400E6
Zetex
www.zetex.com
LED Driver
Q1
N/A
SOT23
ZXTN25012EFL Zetex
www.zetex.com
Low sat NPN
transistor
D1
40V/1A
SOT23
ZHCS1000
Zetex
www.zetex.com
40V/1A Schottky
diode
L1
33␮H
N/A
722065330
Wurth
Elektronik
www.we-online.com Isat=1.6A
R1
30m⍀
0805
Generic
N/A
0805 1%
R2
10⍀
0805
Generic
N/A
0805 5%
R3
47⍀
0805
Generic
N/A
0805 5%
C1
10␮F/10V 1210
Generic
N/A
X7R/X5R
C2
100pF/10V 0805
Generic
N/A
COG/NPO
C3
2.2uF/10V 1206
Generic
N/A
X7R/X5R optional
Table 2
www.zetex.com
Bill of materials for 3W LED
72
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DN85
Intentionally left blank
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
73
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DN85
Performance
0.8
100
0.7
Output Current (A)
Efficiency (%)
90
80
70
60
VF~3.2
0.6
0.5
0.4
0.3
0.2
0.1
VF~3.5
50
VF~3.2
VF~3.5
0.0
6.4
6
5.6
5.2
4.8
4.4
4
6.4
6
Input Voltage (V)
5.6
5.2
4.8
4.4
4
Input Voltage (V)
Input Voltage vs Efficiency
Input Voltage vs Output Current
0.8
4
0.6
Output Voltage (V)
Input Current (A)
0.7
3
0.5
0.4
2
0.3
0.2
1
VF~3.2
0.1
VF~3.5
VF~3.2
0.0
VF~3.5
0
6.4
6
5.6
5.2
4.8
4.4
4
6.4
Input Voltage (V)
www.zetex.com
5.6
5.2
4.8
Input Voltage (V)
4.4
4
Input Voltage vs Output Voltage
Input Voltage vs Input Current
Figure 3
6
Performance graphs for 3W design
74
Issue 1 - August 2007
© Zetex Semiconductors plc 2007
DN86
Reduced component count and compact reference
design for MR16 replacement lamps using multiple
1W LEDs
Silvestro Russo, October 2007
Introduction
MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that usually employ a halogen
filament capsule as the light source. They are used in many retail and consumer lighting
applications where their size, configurability, spot-lighting capability and aesthetics provide utility
and creativity. Low efficiency, heat generation and halogen capsule handling issues are among
the disadvantages of the technology. They typically operate from 12V DC or 12V AC, using
conventional electromagnetic transformers.
LEDs offer a more energy efficient and no radiated heat solution to replace some halogen lamp
applications.
This reference design is intended to fit into the base connector space of an MR16 style LED lamp.
The design has been optimized for part count and thermal performance. The design can be used
with up to 3 1W LEDs in the Lens section. This can be arranged to suit the luminary designer's
requirements.
Figure 1
MR16 application with ZXLD1350
Data sheet
It is recommended that this design note is used with the data sheet for the ZXLD1350 see
http://www.zetex.com/3.0/pdf/ZXLD1350.pdf
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
75
www.zetex.com
DN86
Description
The system diagram of the MR16 lamp solution with ZXLD1350 and ZXSBMR16PT8 is shown in
Figure 2, and Table 1 provides the bill of materials.
Figure 2
System diagram of ZXLD1350 MR16 Lamp Solution
The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic
NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which
with the hysteretic mode of operation (the inductor current waveform will ramp +/-15% about the
nominal current set point) provides sufficient margin. The main features of the ZXLD1350 are:
•
Up to 380mA output current
•
Wide input voltage range: 7V to 30V
•
Internal 30V 400mA NDMOS switch
•
High efficiency (>90% possible)
•
Up to 1MHz switching frequency
The ZXSBMR16PT8 is a new space saving and thermally efficient device specifically designed for
the critical requirements of MR16 applications. The device encompasses a full bridge and a
freewheeling diode realized using extremely low leakage 1A, 40V Schottky diodes to allow a
nominal 12V AC input operations. The Schottky bridge together with the embedded freewheeling
diode enhance the system efficiency compared to the standard silicon diodes in a compact
format. The reference design has solder tag pins to bypass the bridge rectifier should the final
lamp design be used for purely DC operation.
As the ZXLD1350 has a hysteretic switching topology, the switching frequency is dependent on
several factors - input voltage, target current and number of LEDs. An Excel based calculator is
available for system initial evaluation and component choice.
See http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls
System efficiency and LED current have been measured keeping the ADJ pin floating and the
current in the device at its rated value. The input impedance of the ADJ pin is high (200K) and is
susceptible to leakage currents from other sources. Anything that sinks current from this pin will
reduce the output current. In order to avoid any kind of electromagnetic coupling a guard track
around this pin is used.
www.zetex.com
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Issue 1 - October 2007
© Zetex Semiconductors plc 2007
DN86
Quantity
Part reference
Value
Description
Source
1
R1
0.33⍀
Resistor, 1%, 0805
Various
2
C1, C2
150µF/20V
Type D SMD Tantalum Cap
Kemet
1
C3
0.1µF/25V
SMD 0805 X7R
NIC
Componenents
1
C4
1µF/25V
SMD 1210 X7R
NIC
Componenents
1
L1
100µH
MSS6132-104
Coilcraft
1
U1
ZXLD1350
LED driver IC
ZETEX
1
U2
ZXSBMR16PT8
Schottky bridge rectifier
and freewheeling diode
ZETEX
Table 1
Bill of Material
Referring to circuit schematic in Figure 2; the jumper connection could be used utilizing a zero
ohm resistor, in order to enable the pure DC operations.
Care has to be taken in this case, since the system is not reverse polarity protected.
In Figure 3 the circuit layout is shown, highlighting its space saving features and compactness.
Both bottom layer and top layer are shown to display effective devices arrangement.
TOP COPPER AND SILKSCREEN
BOTTOM COPPER AND SILK SCREEN
Figure 3 Circuit layout
The main layout design suggestions are:
•
All thin devices on one side
•
Employ a star connection for ground tracks
•
Use a ground ring protecting ADJ pin
•
Check that:
•
Tracks connecting R1 to ZXLD1350 are as short as possible (being sense tracks)
•
The filter capacitor C3 is connected as close as possible to the Vin pin
•
The freewheeling current path is as short as possible to ensure system precision and efficiency
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
77
www.zetex.com
DN86
Circuit board views
Top Layer
Bottom Layer
MR16 Pins
- Anode
LED connections
+ Cathode
Figure 4 Circuit board views
Choice of Inductor and switching circuit layout
A 100 µH screened inductor was chosen to set the nominal frequency around 250kHz. A screened
inductor is chosen to minimize radiated EMI. The layout with any switching regulator is crucial to
minimize radiated EMI. This reference design keeps the critical track lengths to a minimum.
Ground areas have been maximized around critical areas.
Circuit performances
Circuit performances have been evaluated taking into account two main parameters, the system
efficiency and the current precision.
The reference current is set to a nominal 300mA but can be adjusted to any value up to 350mA
by changing the sense resistor Rsense according to the formula:
Iref = 0.1/R1
For
R1 = 0.33⍀
[A]
Iref = 300mA
Æ
In Table 2 the data related to the system supplied with a DC voltage ranges from 12V to 15V. For
these tests the Schottky bridge was included. The most important parameters are the system
efficiency and the error between the rated LED current (300mA) and the actual LED current. In the
DC case the frequency ranges between 150kHz and 300kHz, depending on the input voltage.
Whatever the input voltage, the efficiency is higher than 87% and the error lower than 2%.
Vin [V]
Iin[A]
Vout[V]
Iout[A]
Efficiency
Current
Accuracy
12.000
0.275
9.80
0.296
87,9%
1.3%
13.000
0.252
9.78
0.294
87.7%
2.0%
14.000
0.232
9.76
0.294
87.6%
2.0%
15.000
0.220
9.75
0.294
87.4%
2.0%
Table 2 DC input voltage
Table 3 shows the data related to the system supplied with an AC electromagnetic transformer.
Using a SMD tantalum capacitor will save space and avoid using a larger aluminum electrolytic
capacitor. This will improve the reliability of the system and stabilize performance during its
lifetime. There is a trade off between physical size, reliability, cost and average LED current.
Typical output voltages from a nominal 12V AC transformer can be ±10%. With 3 LEDs the voltage
across these will be around 10V. If the input capacitor value is lower then 200µF, the AC input
waveform is distorted (as can be seen in figure 8). When the rectified AC is not sufficiently
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© Zetex Semiconductors plc 2007
DN86
smoothed the ripple may drop below the combined LED forward voltage which stops the
switching regulator and so reduces the average current in the LEDs. This will also reduce the
average lumens output.
C1 [µF]
Vin [V]
Iin[A]
Vout[V]
Iout[A]
Efficiency
Current
Accuracy
100
12.70
0.303
9.28
0.225
54%
25%
150
12.60
0.394
9.50
0.271
52%
10%
200
12.53
0.432
9.55
0.293
52%
2%
300
12.50
0.386
9.70
0.295
60%
2%
Table 3 AC input voltage
Figures 5 to 7 show the input voltage ripple and LX voltage varying the input capacitance value
Cin = C1 + C2 + C3. The higher the input capacitance the higher to output current precision and
the average lumens outputs. The case with Cin=300µF has the best performance both as efficiency
and current precision. Reducing the input capacitance the output current precision will decrease
up to 25% with system efficiency always above 50%.
Figure 5 Cin=300µF
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
Figure 6 Cin=200µF
79
www.zetex.com
DN86
Figure 7 Cin=150µF
Figure 8 Cin=100µF
Figure 5 to 8: input ripple and LX voltage (Ch3 is the LX pin voltage and Ch4 is the input voltage)
Gerber plots and further assistance are available from your local Zetex contact or Distributor.
You can contact your local sales office by email.
[email protected]
[email protected]
[email protected]
Conclusion
A compact, reliable, efficient and minimum part count solution can be realized using the
ZXLD1350, ZXSBMR16PT8, and associated passive components. The compact design in the
connector housing keeps the temperature sensitive semiconductors as far from the heat
generating LEDs as possible. A compromise between LED current and size of capacitance is
necessary for the final solution which accounts for efficiency, accuracy, size, and component
count.
This is the first design note in a series of reference designs MR16 variants solutions and options.
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80
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
AN44
A high power LED driver for low voltage halogen replacement
Introduction
LED lighting is becoming more popular as a replacement technology for Halogen low voltage
lighting, primarily because of the low efficiency, reliability and lifetime issues associated with
Halogen bulbs.
Discussed below is a novel approach for driving high power LED's as a replacement for low
voltage halogen lighting systems.
A typical schematic diagram is shown in Figure 1.
Figure 1
Schematic diagram
Operation
Please refer to the typical schematic diagram in Figure 1.
On period, TON
The ZXSC300 turns on Q1 until it senses 19mV (nominal) on the ISENSE pin.
The current in Q1 to reach this threshold is therefore 19mV/R1, called IPEAK.
With Q1 on, the current is drawn from the battery and passes through C1 and LED in parallel.
Assume the LED drops a forward voltage VF. The rest of the battery voltage will be dropped across
L1 and this voltage, called V(L1) will ramp up the current in L1 at a rate di/dt = V(L1)/L1, di/dt in
Amps/sec, V(L1) in volts and L1 in Henries.
The voltage drop in Q1 and R1 should be negligible, since Q1 should have a low RDS(on) and R1
always drops less than 19mV, as this is the turn-off threshold for Q1.
VIN = VF + V(L1)
TON = IPEAK x L1/ V(L1)
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So TON can be calculated, as the voltage across L1 is obtained by subtracting the forward LED
voltage drop from VIN. Therefore, if L1 is smaller, TON will be smaller for the same peak current
IPEAK and the same battery voltage VIN. Note that, while the inductor current is ramping up to
IPEAK, the current is flowing through the LED and so the average current in the LED is the sum of
the ramps during the TON ramping up period and the TOFF ramping down period.
Off period, TOFF
The TOFF of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7µs. Note that, if relying on
this for current ramp calculations, the limits are 1.2µs min., 3.2µs max.
In order to minimize the conductive loss and switching loss, TON should not be much smaller than
TOFF. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300
and 310 are operated only up to 200 kHz. Given the fixed TOFF of 1.7µs, this gives a TON of (5µs 1.7µs) = 3.3µs minimum. However, this is not an absolute limitation and these devices have been
operated at 2 or 3 times this frequency, but conversion efficiency can suffer under these
conditions.
During TOFF, the energy stored in the inductor will be transferred to the LED, with some loss in the
Schottky diode. The energy stored in the inductor is:
½ x L x IPEAK2 [Joules]
Continuous and discontinuous modes (and average LED current)
If TOFF is exactly the time required for the current to reach zero, the average current in the LED
will be IPEAK/2. In practice, the current might reach zero before TOFF is complete and the average
current will be less because part of the cycle is spent with zero LED current. This is called the
‘discontinuous’ operation mode and is shown in Figure 2.
Figure 2
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For continuous mode
If the current does not reach zero after 1.7µs, but instead falls to a value of IMIN, then the device
is said to be in ‘continuous’ mode. The LED current will ramp up and down between IMIN and IPEAK
(probably at different di/dt rates) and the average LED current will therefore be the average of
IPEAK and IMIN, as shown in Figure 3.
Figure 3
Design example
(Refer to Figure 1 and Table 1)
Input = VIN = 12V
LED forward drop = VLED = 9.6V
VIN = VLED+VL
Therefore VL = (12 - 9.6) = 2.4
The peak current = VSENSE / R1
(R1 is RSENSE) = 24mV/50mR = 480mA
TON = IPEAK x L1/V(L1)
680mAx22μH
T ON --------------------------------------- = 6.2μs
2.4
These equations make the approximation that the LED forward drop is constant throughout the
current ramp. In fact it will increase with current, but they still enable design calculations to be
made within the tolerances of the components used in a practical circuit. Also, the difference
between VIN and VLED is small compared to either of them, so the 6.2µs ramp time will be fairly
dependent on these voltages.
Note that, for an LED drop of 9.6V and a Schottky drop of 300mV, the time to ramp down from
680mA to zero would be:
680mAx22μH
TDIS --------------------------------------- = 1.5μs
( 9.6 + 0.3 )
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As the TOFF period is nominally 1.7µs, the current should have time to reach zero. However, 1.5µs
is rather close to 1.7µs and it is possible that, over component tolerances, the coil current will not
reach zero, but this is not a big issue as the remaining current will be small. Note that, because of
the peak current measurement and switch-off, it is not possible to get the dangerous ‘inductor
staircasing’ which occurs in converters with fixed TON times. The current can never exceed IPEAK,
so even if it starts from a finite value (i.e. continuous mode) it will not exceed the IPEAK. The LED
current will therefore be approximately the average of 680mA and zero = 340mA (it will not be
exactly the average, because there is a 200ns period at zero current, but this is small compared
with the IPEAK and component tolerances).
Ref
Value
Part number
Manufacturer Contact details
Comments
U1
ZXSC310E5
Zetex
www.zetex.com
LED Driver in SOT23-5
Q1
ZXMN6A07F
Zetex
www.zetex.com
N-channel MOSFET in
SOT23
www.zetex.com
1A Schottky diode in
SOT23
D1
1A / 40V
ZHCS1000
Zetex
D2
6V8
Generic
Generic
L1
22␮H
DO3316P-223 Coilcraft
R1
50m⍀
Generic
Generic
0805 size
R2
1k2⍀
Generic
Generic
0805 size
C1
100␮F/25V
Generic
Generic
C2
1␮F/10V
Generic
Generic
C3
2.2␮F/25V
Generic
Generic
Table 1
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6V8 Zener diode
www.coilcraft.com
Bill of materials
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Typical performance graphs for 12V system
Figure 4
Performance graphs for 12V system
By changing the value of R2 from 1k2⍀ to 2k2⍀ the operating input voltage range can be adjusted
from 30V to 20V, therefore the solution is able to operate from the typical operating voltage
supplies of 12V and 24V for low voltage lighting.
Typical performance graphs for 24V system
Figure 5
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Performance graphs for 24V system
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Useful formulae for calculations
The input power from the battery during TON (assuming discontinuous operation mode) is VIN *
IPEAK/2. The average input current from the battery is therefore this current multiplied by the ratio
of TON to the total cycle time:
T ON
I PEAK
---------------- × -------------------------------2
T ON × T OFF
It can be seen from this how the average battery current will increase at lower VIN as TON becomes
larger compared to the fixed 1.7µs TOFF. This is logical, as the fixed (approximately) LED power
will require more battery current at lower battery voltage to draw the same power.
The energy which is stored in the inductor equals the energy which is transferred from the
inductor to the LED (assuming discontinuous operation) is:
½ * L1 * IPEAK2 [Joules]
I PEAK × L1
T ON = -------------------------------------------( V BATT – V LED )
Therefore, when the input and the output voltage difference are greater, the LED will have more
energy which will be transferred from the inductor to the LED rather than be directly obtained
from the battery. If the inductor size L1 and peak current IPEAK can be calculated such that the
current just reaches zero in 1.7µs, then the power in the LED will not be too dependent on battery
volts, since the average current in the LED will always be approximately IPEAK/2.
As the battery voltage increases, the TON necessary to reach IPEAK will decrease, but the LED
power will be substantially constant and it will just draw a battery current ramping from zero to
IPEAK during TON. At higher battery voltages, TON will have a lower proportional of the total cycle
time, so that the average battery current at higher battery voltage will be less, such that power
(and efficiency) is conserved.
The forward voltage which is across the Schottky diode detracts from the efficiency. For example,
assuming VF of the LED is 6V and VF of the Schottky is 0.3V, the efficiency loss of energy which is
transferred from the inductor is 5%, i.e. the ratio of the Schottky forward drop to the LED forward
drop. The Schottky is not in circuit during the TON period and therefore does not cause a loss, so
the overall percentage loss will depend on the ratio of the TON and TOFF periods. For low battery
voltages where TON is a large proportion of the cycle, the Schottky loss will not be significant. The
Schottky loss will also be less significant at higher LED voltages (more LED's in series) as Schottky
drop becomes a lower percentage of the total voltage.
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Getting more out of the ZXLD1350 - dimming techniques
Ray Liu, Systems Engineer, Zetex Semiconductors
Introduction
The ZXLD1350 has a versatile adjust pin that can be used in many ways to adjust the brightness
of the LED by controlling the current in the LED. This application note deals with some the ways
in which dimming the LED can be achieved and discusses the merits of the techniques. These
dimming methods discussed include PWM dimming both with a low and high frequency signals,
DC voltage control and resistive dimming.
Low frequency dimming
Low frequency dimming is preferred for LED dimming since the LED instantaneous driving
current is constant. The color temperature of the LED is preserved at all dimming levels. Another
advantage of low frequency dimming is that the dimming level can down to 1%. Hence result in
dimming range of 100:1.
Choice of frequency
To avoid visible flicker the PWM signal must be greater than 100Hz. If you choose too high a
frequency the internal low pass filter will start to integrate the PWM signal and produce a non
linear response. Also the soft start function of the ADJ pin will cause a delay on the rising a falling
edge of the PWM signal. This can give a non-linearity in the LED current which will have a greater
affect as frequency increases.
An upper limit of 1kHz is suggested. The effect of audible noise in the inductor may need to be
considered. This may happen in some inductors with loose windings and will be more noticeable
at PWM frequencies of 1kHz than 100Hz.
If the PWM frequency is less than approximately 500Hz, the device will be gated 'on' and 'off' and
the output will be discontinuous, with an average value of output current given by:
0.1 D PWM
I OUT ≈ -------------------------RS
[for 0<DWPM<1]
ADJ
PWM
ZXLD1350
GND
GND
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High frequency dimming
High frequency dimming is preferred if system required low radiated emission and in/output
ripple. But dimming range is reduced to 5:1. The ZXLD1350 has an internal low pass filter which
integrates the high frequency PWM signal to produce a DC dimming control.
If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified
minimum value, the device will remain active and the output will be continuous, with a nominal
output current given by:
0.1 D PWM
I OUT ≈ -------------------------RS
[for 0.16< DPWM <1]
ADJ
PWM
ZXLD1350
GND
GND
Input buffer transistor
For PWM dimming an input bipolar transistor with open collector output is recommended. This
will ensure the 200mV input shutdown threshold is achieved.
It is possible to PWM directly without a buffer transistor. This must be done with caution. Doing
this will overdrive the internal 1.25V reference. If a 2.5V input level is used at 100% PWM (DC) the
output current into the LED will be 2X the normal current which may destroy the ZXLD1350.
Overdriving with a 5V logic signal is very likely to damage the device as it exceeds the ADJ pin
voltage rating.
Soft start and decoupling capacitors
Any extra capacitor on the ADJ pin will affect the leading and falling edge of the PWM signal. Take
this into account as the rise time will be increased by approximately 0.5ms/nF.
Compare this with a 100Hz PWM. 50% duty cycle Ton and Toff are 5ms at 1% duty cycle Ton is
0.1ms. 1nF on the ADJ pin will cause 0.5ms rise time which result in an error and limitation in
dimming at low duty cycles.
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DC voltage dimming
The ADJ pin can be overdriven by an external DC voltage (VADJ), as shown, in order to override
the internal voltage reference and adjust the output current to a value above or below the nominal
value.
ZXLD1350
ADJ
+
GND
DC
GND
The nominal output current is then given by:
0.08 × V ADJ
I OUT ≈ -------------------------------RS
[for 0.3< VADJ <2.5V]
Figure 1 shown the relationship of LED current against VADJ with VADJMAX =1.25V (VIN = 12V).
Note that 100% brightness setting corresponds to VADJ = VREF = 1.25V with RS = 300m⍀. The
minimum dimming ratio is governed by the VADJON which is 250mV typically. In this case, the
minimum dimmable current is 20% of full LED current. This gives dimming ratio of 1:5.
IOUT vs VADJ (RS = 300m⍀)
350
300
250
mA
200
150
100
50
0
0
0.25
0.5
0.75
1
1.25
V
Figure 1
Typical output current versus ADJ voltage with RS = 300m⍀
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Switching frequency is another factor to consider for DC voltage dimming. Figure2 shows the
relationship of switching frequency current against VADJ with L=100␮H. As VADJ decreases,
switching frequency increases. Care had to be taken for choosing the right inductor to achieve the
desirable operating frequency range with the aid of the ZXLD1350 calculator.
Figure 2
Typical switching frequency versus ADJ voltage with RS = 300m⍀, L=100␮H
In order to maximize the dimming ratio, we could increase the maximum value of VADJ to 2.5V. In
this case, the minimum dimmable current is 10% of full LED current. This gives dimming ratio of
1:10. RSENSE should then be increased by 2X RS. This will slightly decrease the efficiency by 1 to
2%.
Figure 3 shows the relationship of LED current against VADJ with VADJMAX = 2.5V (VIN = 12V).
IOUT vs VADJ (RS = 600m⍀)
350
300
mA
250
200
150
100
50
0
0
0.5
1
1.5
2
2.5
V
Figure 3
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Typical output current versus ADJ voltage with RS = 600m⍀
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Figure 4
Typical switching frequency versus ADJ voltage with RS = 600m⍀, L=100␮H
The input impedance of the ADJ pin is 200k⍀ ±20%. This may be the factor to consider if the DC
dimming voltage is from a relatively high output resistance. Figure 5 shows a typical circuit that
would provide 1.25V dimming voltage.
VIN
R1
4.7k
VR1
10k
ADJ
GND
ZTLV431
Figure 5
ZXLD1350
Typical circuit of DC voltage dimming
The ZTLV431 acts as a shunt regulator to generate an external 1.25V reference voltage. The
reference voltage is applied to pot VR1 to provide dimming voltage of 0-1.25V.
Using an external regulator affects the accuracy of the current setting. If a 1% reference is used
the LED current will be more accurate than using the internal reference.
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Resistor dimming
By connecting a variable resistor between ADJ and GND, simple dimming can be achieved.
Capacitor CADJ is optional for better AC mains interference and HF noise rejection. Recommend
value of CADJ is 0.22␮F.
RS
ZXLD1350
ADJ
*CADJ
RADJ
GND
GND
The current output can be determined using the equation:
( 0.08 ⁄ R S ) × R ADJ
I OUT = -----------------------------------------------( R ADJ + 200k )
Note that continuous dimming is not possible with a resistor. At some point the shutdown
threshold will be reached and the output current reduced to zero. This can occur below 300mV.
Note that a 1M⍀ resistor will load the VREF on the ADJ pin. The VREF will now be divided down by
the nominal 200k VREF resistance and the 1M RADJ. The nominal voltage will now be
approximately 1V. RS will need to be adjusted to set the maximum current.
The +/20% tolerance of the input resistance should also be understood. See table below:
RADJ k⍀
Rint nom.
k⍀
Rint min.
k⍀
Rint max.
k⍀
VADJ
nominal
% error
from
nominal
due to Rint
min.
% error
from
nominal
due to Rint
max.
1000
200
160
240
1.041
3.4%
-3.3%
500
200
160
240
0.892
6.1%
-5.7%
200
200
160
240
0.625
11.1%
-10.0%
100
200
160
240
0.416
15.4%
-13.3%
Table 1
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IOUT
0.35
0.3
0.25
0.2
IOUT
0.15
0.1
0.05
0
-0.05
0
200
Figure 6
400
600
800
1000
Typical output current against pot resistance
If linear pot is used, the output current change is not linear against shaft rotation.
In order to make the output current more linear, a log type pot is used.
IOUT vs shaft rotation
350
300
IOUT (mA)
250
200
150
100
50
0
0
10
20
30
40
50
60
70
80
90
100
Shaft rotation (%)
Figure 7
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Output current against shaft rotation of log type pot
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Getting more out of the ZXLD1350 - high output current
Ray Liu, Systems Engineer, Zetex Semiconductors
Introduction
The ZXLD1350 is a continuous mode inductive step-down converter, designed for driving single
or multiple series connected LEDs efficiently from a voltage source higher than the LED voltage.
The device operates from an input supply between 7V and 30V and provides an externally
adjustable output current of up to 350mA. In order to obtain higher output current to drive LEDs
with higher power, a high current externally connected output stage is required.
700mA driver for multiple 3W LEDs in series
This driver is designed to drive up to six 3W LEDs in series which could deliver total output power
of 15W with an overall efficiency of around 90%.
RS2
LED1
LEDN
L1
RS1
D1
R3
Q3
V6
6
VIN
+
4
ISENSE
U1
C1
R1
LX GND ADJ
1
2
3
R2
D2
C2
Q1
Q2
0
Figure 1
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Schematic of 700mA driver
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Part list
Table 1
Part ref.
Part no.
Remark
U1
ZXLD1350
Q1
FCX619
Q2
FMMT619
Q3
FMMT619
D1
ZLLS1000
D
25.6V Zener diode
L1
68␮H 1A
RS1
150m⍀
RS2
2.2⍀
R1
2.2K⍀
R2
470⍀
R3
15K⍀
C1
3.3␮F 50V
X5/7R or other low ESR cap
C2
0.1␮F
Optional
Circuit description
The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal
inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base
of transistor Q1 which acts as the power output switch.
Transistor Q3 and Zener diode D2 form a simple regulator to supply a constant voltage to the
driver stage. The voltage at emitter of Q3 is around 5V. This helps to provide a stable driving
current to both Q1 and Q2. The driving currents are around 2mA and 9mA respectively.
Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time
of the output switch are less than 70ns when input supply voltage is 30V.
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Typical performance graphs
5 LEDs in series with total VF = 17.9V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
700
600
500
60
400
50
21
22
23
24
25
26
27
28
29
21
30
22
23
24
25
26
27
28
29
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (5 LEDs)
Output Current vs Input Voltage (5 LEDs)
4 LEDs in series with total VF = 14.9V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
700
600
500
60
400
50
18
20
22
24
26
28
18
30
20
22
24
26
28
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (4 LEDs)
Output Current vs Input Voltage (4 LEDs)
3 LEDs in series with total VF = 11.1V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
60
700
600
500
400
50
15
20
25
15
30
Efficiency vs Input Voltage (3 LEDs)
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25
30
Input Voltage (V)
Input Voltage (V)
Output Current vs Input Voltage (3 LEDs)
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Typical performance graphs (cont.)
2 LEDs in series with total VF = 7.7V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
700
600
500
60
400
50
12
15
18
21
24
27
12
30
15
18
21
24
27
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (2 LEDs)
Output Current vs Input Voltage (2 LEDs)
1 LED in series with VF = 3.8V
100
800
Efficiency (%)
Output Current (mA)
90
80
70
60
700
600
500
400
50
10
15
20
25
10
30
Efficiency vs Input Voltage (1 LED)
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20
25
30
Input Voltage (V)
Input Voltage (V)
Output Current vs Input Voltage (1 LED)
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A driver for supply voltage up to 16V
This driver is a simplified version to the 700mA driver described above. The driver is designed to
drive up to 3 Luxeon® K2 LEDs in series which could deliver a total output power of 10W with a
maximum input supply voltage of 16V.
LED1
LEDN
RS1
L1
D1
V6
+
6
4
VIN
ISENSE
R1
R2
U1
C1
Q1
LX GND ADJ
1
2
3
C2
Q2
0
Figure 2
Schematic of 1A driver
Part List
Table 2
Part ref.
Part no.
U1
ZXLD1350
Q1
ZXTN25020DFH
Q2
ZXTN25020DFH
D1
ZLLS2000
L1
47␮H 1.5A
RS
100m⍀
R1
4.7K⍀
R2
1.5K⍀
C1
4.7␮F 25V
X5/7R or other low ESR cap
C2
0.1␮F
Optional
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Remark
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Circuit description
This circuit is similar to the 700mA driver described above. The output driver consists of two NPN
transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX
switch signal. The collector of Q2 is connected to the base of transistor Q1 which act as the power
output switch.
Unlike the 700mA driver, the driving current to both Q1 and Q2 varies with the input supply
voltage. Hence, the maximum input supply voltage is limited to 16V. The driving current to Q1 is
between 5mA and 10mA with input supply voltage between 8V and 16V. Lowering the maximum
supply voltage to 16V enables us to use a lower voltage BJT with better switching performance.
Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time
of the output switch are less than 60ns when input supply voltage is 16V.
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Typical performance graphs
2 LEDs in series with total VF = 7.1V
1100
Output Current (mA)
100
Efficiency (%)
90
80
70
60
50
900
700
500
10
11
12
13
14
15
16
10
11
12
13
14
15
16
Input Voltage (V)
Output Current vs Input Voltage (2 LEDs)
Input Voltage (V)
Efficiency vs Input Voltage (2 LEDs)
1 LED in series with total VF = 3.5V
1100
Output Current (mA)
100
Efficiency (%)
90
80
70
60
50
900
700
500
8
10
12
14
16
8
Input Voltage (V)
Efficiency vs Input Voltage (1 LED)
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12
14
16
Input Voltage (V)
Output Current vs Input Voltage (1 LED)
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Feed forward compensation for ZXSC300 LED driver
Yong Ang, Application Engineer, Zetex Semiconductors
Input voltage feed forward compensation for ZXSC300 to improve control of the
LED current
Introduction
The ZXSC300 LED drivers do not directly control the LED current. As a consequence the LED
current is dependent of the input voltage. This application note describes a way of reducing the
supply voltage dependency by a method of supply voltage feed forward compensation. The
method can also be used to provide temperature compensation of the LED.
The ZXSC300 works on the PFM control scheme where the LED current is simply regulated by
controlling the peak current through transistor Q1. The internal voltage threshold of current sense
pin is around 19mV and transistor Q1 is switched off when its current reaches the preset
threshold, thereby necessitating fewer external components required. However, this threshold
value is invariant to the supply voltage level. In the event where input voltage increases, peak Q1
current will stay the same and current delivered to the LED creeps up which could potentially
damage the LED if it exceeds the maximum rated current of the device.
The circuit diagram in Figure 1 shows how to apply input voltage and thermal correction to a
typical LED. A simple design guide for a single LED driver has also been put forward. The
equations can generate a design capable of sourcing up to 200mA LED current, when used with
the Zetex high current gain NPN transistor-ZXTN25012EFH.
Input voltage feed forward compensation
Normally, IPK is set by the output current threshold voltage VISENSE divided by RSENSE. As the
input voltage increases, the inductor ripple current level ⌬I decreases because the transistor off
time, TOFF is fixed by the ZXSC300
⌬I = (VOUT - VIN) • TOFF ÷ L
L discharges at a flatter slope to a higher minimum choke current IMIN = IPK - ⌬I, before transistor
Q1 is turned on again.
Figure 1
Circuit diagram of ZXSC300 with feed forward and thermal compensation
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Consequently, the average current IAV flowing through L increases and a shorter transistor on
time, TON is required to charge boost inductor to the preset threshold current level IPK
TON = ⌬I • L ÷ VIN
By making the aforementioned assumptions for turn-on period and average coil current, the
output power delivered to the LED is now determined from
POUT = VLED • IAV • TOFF ÷ (TON + TOFF)
Therefore, a higher power and LED current is delivered to the LED at high VIN for a fixed RSENSE
and this elevated current could potentially damage the LED if it exceeds the maximum rated
current of the device.
Ignoring the effect of thermistor RT for the moment, a 100⍀ resistor ROFF can be inserted in series
with RSENSE and feed forward resistor Rfb (see Figure 1) to inject a slight voltage offset across
resistor RSENSE. This enables a lower Q1's current to build up the required VISENSE to turn the
driver off, which regulates the LED current. The Rfb value has to be sufficiently big to lower
dissipation and to prevent circuit from stalling. The circuit could stall at high input voltage if Rfb
drops 19mV or more across 100⍀ resistor forcing the driver off all the time.
It must be noted that ISENSE pin threshold on ZXSC300 has a positive temperature coefficient of
0.4%/°C. If a circuit nominal operating temperature is higher than 65°C, it could give
approximately 20% increase in average LED current from that in 25°C ambient. When a feed
forward network is used, this injects an offset voltage to the threshold pin. For instance, if an
offset voltage of 9.5mV is used, the effective VISENSE temperature coefficient becomes double.
Therefore, it is essential that thermal compensation is used with a feed forward approach.
Feed forward components calculation
For initial estimation, the associated IAV(VMAX) that delivers the required LED current can be
determined from
IAV(VMAX) = POUT ÷ (F • TOFF • VOUT)
Where the transistor switching frequency F is given by
F = VIN(MAX) ÷ VOUT ÷ TOFF
Figure 2
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Example of current and voltage waveforms for circuit using ZXSC300
with feed forward network
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IAV(VMAX) is used to establish the required DC current rating, IDC for boost inductor L.
The minimum inductor current is given by,
IMIN(VMAX) = IAVE – 0.5 • (VOUT – VIN(MAX)) • TOFF /L
A high L value is recommended to minimize errors due to propagation delays at high input
voltage, which results in increased ripple and lower efficiency.
And the maximum inductor current which relates to the Q1 peak current is
IPK(VMAX) = 2 • IAVE - IMIN(VMAX)
In practice, a higher IPK(VMAX) value can be used to account for the VCE saturation and switching
edge loss in the transistor.
The value of feed forward resistor Rfb is selected to give IPK(VMIN) at worse case input voltage and
IPK(VMAX) at maximum input voltage. The internal VISENSE threshold on the ZXSC300 is typically
19mV with ±25% tolerance at 25°C. RSENSE has to drop less voltage than that demanded by
VISENSE as Rfb will make a contribution to satisfy the threshold, which lowers IPK value with
increasing input. Allowing for the positive temperature coefficient on ISENSE pin, effective
threshold voltage level at operating temperature TAMB is;
[email protected] = 19mV ± 25% • 0.4%/°C • (TAMB - 25°C).
At low supply voltage VIN(MIN)
[email protected] = IPK(VMIN) • RSENSE + VIN(MIN) • 100⍀ ÷ (Rfb + 100⍀)
Whilst at VIN(MAX),
[email protected] = IPK(VMAX) • RSENSE + VIN(MAX) • 100⍀ ÷ (Rfb + 100⍀)
Solving the above simultaneous equations gives the required RSENSE and Rfb resistor values.
These design equations are also available as a spreadsheet calculator from Zetex website at
www.zetex.com/zxsc300feedforward
Figure 3 shows the measured LED current against variation in the input voltage with feed forward
compensation. For comparison purpose, the same measurement is repeated with feed forward
network removed, in which case the LED current at low supply is 3 times lower than that at
nominal input voltage level.
70
LED Current (A)
60
50
Feed forward
compensation
No feed forward
network
40
30
20
10
0
1.6 1.8
2
2.2 2.4 2.6 2.8
3
Input Voltage (V)
Figure 3
LED current discrepancy for ZXSC300 with feed forward compensation
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The improvement in LED current regulation through feed forward compensation is self-evident.
Although some discrepancy in LED current persists at low supply, this is predominantly due to
the dependency of internal VISENSE threshold level on the input voltage level.
To incorporate thermal compensation into the design, Rfb can be made up from a series
combination of normal resistor R1 and NTC RT. During start-up condition, the printed circuit
board’s and LED’s temperatures are low, hence RT has high resistance. As circuit temperature
rises to its design operating value, the effective feed forward resistance drops, increasing the
offset voltage on ISENSE pin, which in turn matches the elevated VISENSE value and hence
regulates the actual output current fed to the LED.
For instance, the required effective feed forward resistor value (R1+ RT) for 25°C ambient start-up
can be determined from
Rfb = VIN(MAX) • 100⍀ • (19mV ± 25% - IPK(VMAX) • RSENSE)
And the required normal resistor R1 is equivalent to Rfb - RT.
For this design, three NTC values (3.3K⍀, 4.7K⍀ and 6.8K⍀) are recommended. These resistors
with MURATA 0603 or 0805 size NTC thermistors with beta-constant value of 3950K are chosen to
give good current control response at both normal operating temperature and start-up
conditions. The NTC works to reduce the peak transistor current, facilitating thermal feedback
control to ensure that LED current and lumen maintenance expectation are achieved. Note that it
is sometimes difficult to achieve perfect LED current matching between start-up and normal
operating temperature. In extreme cases of large temperature gradients, the average LED current
should be lower at start-up giving less lumen output, and then ramps up to the rated current once
it reaches the normal operating temperature. Furthermore, the thermistor can be thermally
coupled to the LED to provide response tracking and prevent overheating.
Conclusion
Two or three additional external components can be used to provide input voltage feed forward
for ZXSC300. This serves to ensure that the LED current is closely regulated. The LED current
regulation improves significantly when feed forward compensation is employed. The LED current
at the worse case input voltage increases from 33% to 64% of the nominal LED current with a feed
forward network. The remaining discrepancy is predominantly due to the dependency of the
VISENSE threshold level on the input voltage level.
In applications where the circuit is designed to operate in elevated ambient temperature, a NTC
thermistor can be incorporated to facilitate thermal feedback control and prevent over heating.
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