DN61

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
1
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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
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2
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
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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.
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© Zetex Semiconductors plc 2006
DN61
Intentionally left blank
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DN61
Definitions
Product change
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service. Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights
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opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express written
approval of the Chief Executive Officer of Zetex Semiconductors plc. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
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The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
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Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
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“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
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Datasheet status key:
“Draft version”
This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue”
This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.
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© Zetex Semiconductors plc 2006