ZETEX ZXLD1352ET5TA

A Product Line of
Diodes Incorporated
ZXLD1352
350mA LED driver with internal switch and enhanced PWM dimming
range
Description
The ZXLD1352 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. Depending upon
supply voltage and external components, this
can provide up to 8 watts of output power.
The ZXLD1352 includes the output switch and
a high-side output current sensing circuit,
which uses an external resistor to set the
nominal average output current.
Output current can be adjusted above, or
below the set value, by applying an external
control signal to the 'ADJ' pin.
The ADJ pin will accept either a DC voltage or a
PWM waveform. DC voltages between 0.3V and
2.5V allow adjustment of output current from
25% to 200% of nominal. 1000:1 adjustment of
output current is possible using PWM control.
Applying a voltage of 0.2V or lower to the ADJ
pin turns the output off and switches the device
into a low current standby state.
The device is assembled in a TSOT23-5 pin
package.
Features
Applications
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Simple low parts count
Internal 30V NDMOS switch
350mA output current
Single pin on/off and brightness control
using DC voltage or PWM
1000:1 PWM dimming range
Soft-start
High efficiency (up to 95%(*))
Wide input voltage range: 7V to 30V
40V transient capability
Output shutdown
Up to 1MHz switching frequency
Inherent open-circuit LED protection
Typical 4% output current accuracy
Low voltage halogen replacement LEDs
Automotive lighting
Low voltage industrial lighting
LED back-up lighting
Illuminated signs
(*) Using standard external components as specified under electrical characteristics. Efficiency is dependent upon the
number of LEDs driven and on external component types and values.
Pin connections
Typical application circuit
D1
ZLLS1000
LX 1
5
VIN
Rs
VIN
(12V - 30V)
0.33
GND 2
L1 47␮H
4 ISENSE
ADJ 3
C1
TSOT23-5
Top view
1␮F
PWM
VIN
ISENSE
ADJ
ZXLD1352
LX
GND
GND
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ZXLD1352
Absolute maximum ratings (voltages to GND unless otherwise stated)
Input voltage (VIN)
-0.3V to +30V (40V for 0.5 sec)
ISENSE voltage (VSENSE)
+0.3V to -5V (measured with respect to VIN)
LX output voltage (VLX)
-0.3V to +30V (40V for 0.5 sec)
Adjust pin input voltage (VADJ)
-0.3V to +6V
Switch output current (ILX)
500mA
Power dissipation (Ptot)
(Refer to package thermal de-rating curve on page 18)
450mW
Operating temperature (TOP)
-40 to 105°C
Storage temperature (TST)
-55 to 150°C
Junction temperature (Tj MAX)
150°C
These are stress ratings only. Operation above the absolute maximum rating may cause device failure. Operation at
the absolute maximum ratings, for extended periods, may reduce device reliability.
Thermal resistance
Junction to ambient (R⍜JA)
200°C/W
Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated) (*)
Symbol
VIN
Parameter
Conditions
Min.
Typ.
7
Input voltage
30
VSU
Internal regulator start-up threshold VIN rising
4.8
IINQoff
Quiescent supply current
with output off
ADJ pin grounded
20
IINQon
Quiescent supply current
with output switching
ADJ pin floating
f=250kHz
VSENSE
Mean current sense threshold
voltage
(defines LED current setting
accuracy)
Measured on ISENSE pin with
respect to VIN
VADJ =1.25V
95
ISENSE
ISENSE pin input current
VSENSE =VIN -0.1
VREF
Internal reference voltage
Measured on ADJ pin with
pin floating
⌬VREF /⌬T
Temperature coefficient of VREF
VADJ
External control voltage range on
ADJ pin for dc brightness control (†)
VADJoff
DC voltage on ADJ pin to switch
device from active (on) state to
quiescent (off) state
VADJ falling
0.15
VADJon
DC voltage on ADJ pin to switch
device from quiescent (off) state to
active (on) state
VADJ rising
0.2
RADJ
Resistance between ADJ pin and
VREF
ILXmean
Continuous LX switch current
ILX(leak)
µA
250
500
µA
105
mV
%
1.25
10
1.25
1.29
µA
V
ppm/°C
2.5
V
0.2
0.25
V
0.25
0.3
V
65
k⍀
35
1.5
2
V
100
0.3
LX switch leakage current
© Diodes Incorporated 2008
30
50
LX Switch ‘On’ resistance
Issue 1 - August 2008
1.21
Unit
V
±15
VSENSEHYS Sense threshold hysteresis
RLX
Max.
0.37
A
2
⍀
1
µA
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ZXLD1352
Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated) (*) (continued)
Symbol
DPWM(LF)
Parameter
Conditions
Duty cycle range of PWM signal
applied to ADJ pin during PWM
dimming mode
Min.
PWM frequency 100Hz - 1KHz
PWM amplitude= VREF
Measured on ADJ pin
0.01
Max.
Unit
1
1000:1
Brightness control range
fLX
Typ.
Operating frequency
(See graphs for more detail)
ADJ pin floating
L=100µH (0.82⍀)
IOUT=350mA @ VLED=3.4V
Driving 1 LED
250
KHz
TONmin
Minimum switch ‘ON’ time
LX switch ‘ON’
200
ns
TOFFmin
Minimum switch ‘OFF’ time
LX switch ‘OFF’
200
ns
fLXmax
Recommended maximum operating
frequency
DLX
Recommended duty cycle range of
output switch at fLXmax
TPD
Internal comparator propagation
delay
1
0.3
MHz
0.7
50
ns
NOTES:
(*) Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified over
a -40°C to +105°C temperature range, are guaranteed by design, characterization and process control.
(†) 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE
threshold and output current proportionally.
Pin description
Name
Pin No.
Description
LX
1
Drain of NDMOS switch
GND
2
Ground (0V)
ADJ
3
Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(VADJ= VREF =1.25V giving nominal average output current
IOUTnom=0.1/RS)
• Drive to voltage below 0.2V to turn off output current
• Drive with DC voltage (0.3V<VADJ<2.5V) to adjust output current from 25% to 200%(†) of IOUTnom
• Drive with PWM signal from open-collector or open-drain transistor, to adjust output current.
Adjustment range 1% to 100% of IOUTnom for 100Hz < f < 1KHZ
• Connect a capacitor from this pin to ground to define soft-start time. Soft-start time is
approx.0.5ms/nF)
ISENSE
4
Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom=0.1/RS
(Note: RSMIN=0.27⍀ with ADJ pin open-circuit)
VIN
5
Input voltage (7V to 30V). Decouple to ground with 1µF or higher X7R ceramic capacitor close to
device
Ordering information
Device
ZXLD1352ET5TA
Issue 1 - August 2008
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Reel size
(mm)
Reel width
(mm)
Quantity per reel
Device mark
180
8
3,000
1352
3
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ZXLD1352
Block diagram
RS
VIN
L1
D1
VIN
VIN
ISENSE
LX
R1
Current sense circuit
Voltage
regulator
5V
+
Shutdown
circuit
C1
Comparator
+
ADJ
50k
Vref
MN
R2
600KHz
1.25V
R3
GND
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ZXLD1352
Device description
The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a self-oscillating
continuous-mode buck converter.
Device operation (Refer to block diagram and Figure 1 - Operating waveforms)
Operation can be best understood by assuming that the ADJ pin of the device is unconnected and
the voltage on this pin (VADJ) appears directly at the (+) input of the comparator.
When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN
and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is
forced across internal resistor R1 by the current sense circuit and produces a proportional current
in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of
the comparator. When this reaches the threshold voltage (VADJ), the comparator output switches
low and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of VADJ.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The current
decays at a rate determined by the LED and diode forward voltages to produce a falling voltage
at the input of the comparator. When this voltage returns to VADJ, the comparator output switches
high again. This cycle of events repeats, with the comparator input ramping between limits of
VADJ ± 15%.
Switching thresholds
With VADJ =VREF, the ratios of R1, R2 and R3, define an average VSENSE switching threshold of
100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is
then defined by this voltage and Rs according to:
IOUTnom=100mV/RS
Nominal ripple current is ±15mV/RS
Adjusting output current
The device contains a low pass filter for noise suppression between the ADJ pin and the threshold
comparator and an internal current limiting resistor (50k nom) between ADJ and the internal
reference voltage. This allows the ADJ pin to be overdriven with either DC or PWM signals to
adjust the output current. The filter is first order, comprising one section with a cut-off frequency
of nominally 600kHz.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The ADJ pin drives the shutdown circuit. When the input voltage to this circuit falls below the
threshold (0.2V nom), the internal regulator and the output switch are turned off. The voltage
reference remains powered during shutdown to provide the bias current for the shutdown circuit.
Quiescent supply current during shutdown is nominally 20␮A and switch leakage is below 1␮A.
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ZXLD1352
VIN
LX voltage
0V
Toff
Ton
VIN
85mV
115mV
100mV
VSENSE-
SENSE voltage
VSENSE+
IOUTnom +15%
IOUTnom
Coil current
IOUTnom -15%
0V
Comparator
input voltage
0.15VADJ
VADJ
0.15VADJ
Comparator
output
5V
0V
Figure 1
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Operating waveforms
6
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ZXLD1352
Typical operating waveforms [VIN=12V, RS=0.3⍀, L=100µH]
Normal operation. Output current (Ch3) and LX voltage (Ch1)
2
3
Ch3 100mA
M 400μs 5.0 S/s
Ch 2 20.0V
A Ch2 \ 12.0 V
200 ns/pt
Start-up waveforms. Output current (Ch3), LX voltage (Ch2)
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ZXLD1352
Typical operating conditions
For typical application circuit driving 1W Luxeon® white LED(s) at VIN =12V and Tamb=25°C unless
otherwise stated.
Duty Cycle vs Input Voltage
L=100uH, Rs=0.33 Ohms
Efficiency vs No. of LEDs
L=100uH, Rs=0.33 Ohms
100
1.2
95
1
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
85
80
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
0.8
Duty Cycle
Efficiency (%)
90
0.6
0.4
0.2
75
0
70
5
10
15
20
25
5
30
10
15
Operating Frequency vs Input Voltage
L=100uH, Rs=0.33 Ohms
25
30
Output current variation with Supply Voltage
L=100uH, Rs=0.33 Ohms
600
8
6
Deviation from nominal set current (%)
500
Frequency (kHz)
20
VIN (V)
VIN (V)
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
400
300
200
100
4
2
0
5
10
15
20
-2
25
30
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
-4
-6
0
5
10
15
20
25
30
-8
VIN (V)
VIN (V)
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ZXLD1352
Typical operating conditions (continued)
Efficiency vs No. of LEDs
L=47uH, Rs=0.33 Ohms
Duty Cycle vs Input Voltage
L=47uH, Rs=0.33 Ohms
1
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
95
90
85
80
75
70
Duty Cycle
Efficiency (%)
100
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
0.8
0.6
0.4
0.2
0
5
10
15
20
25
30
5
10
VIN (V)
20
25
30
VIN (V)
Operating Frequency vs Input Voltage
L=47uH, Rs=0.33 Ohms
Output Current Variation vs Supply Voltage
L=47uH, Rs=0.33 Ohms
20
800
700
600
500
400
300
200
100
0
Deviation from nominal
set current (%)
Frequency (kHz)
15
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
5
10
15
20
25
© Diodes Incorporated 2008
10
5
0
-5 5
10
15
20
-10
30
25
30
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
-15
VIN (V)
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VIN (V)
9
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ZXLD1352
Typical operating conditions (continued)
Efficiency vs No. of LEDs
L=220uH, Rs=0.33 Ohms
Duty Cycle vs Input Voltage
L=220uH, Rs=0.33 Ohms
1
100
0.8
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
90
85
Duty Cycle
Efficiency (%)
95
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
0.6
0.4
0.2
80
75
0
5
10
15
20
25
30
5
10
15
VIN (V)
25
30
25
30
Output Current Variation vs Input Voltage
L=220uH, Rs=0.33 Ohms
Operating Frequency vs Input Voltage
L=220uH, Rs=0.33 Ohms
2
350
1
250
Deviation from nominal set current (%)
300
Frequency (kHz)
20
VIN (V)
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
200
150
100
50
0
5
10
15
20
-1
-2
-3
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
-4
-5
0
5
10
15
20
25
-6
30
VIN (V)
VIN (V)
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ZXLD1352
Typical operating conditions (continued)
Vref vs Vin over nominal supply voltage range
Vref vs Vin at low supply voltage
1.2425
1.4
1.2
Vref (V)
Vref (V)
1
1.242
0.8
0.6
0.4
0.2
0
1.2415
5
10
15
20
25
0
30
1
2
3
4
6
7
8
9
10
Supply Current vs Vin (Quiescent)
Supply Current vs Vin (Operating)
500
20
4 00
15
300
I in (u A)
Iin (u A )
5
Vin (V)
Vin (V)
200
10
5
100
0
0
10
5
15
20
25
0
30
0
Vin (V)
5
10
15
20
25
30
Vin (V)
Output Current vs VADJ
35 0
30 0
Iout mean (mA)
2 50
20 0
Rs=0.3 Ohm
Rs=0.56 Ohm
Rs=1 Ohm
150
100
50
0
0
0.5
1
1.5
2
2.5
3
VADJ (V)
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ZXLD1352
Typical operating conditions (continued)
Output Current Change vs Temperature
VIN=7V, L=100uH, Rs=0.33 Ohms
VADJ vs Temperature
L=100uH, Rs=0.33 Ohms
2
Deviation from nominal set value
(%)
1.255
Vadj (V)
1.25
Vin = 7V
Vin = 9V
Vin = 12V
Vin = 30V
1.245
1.24
1.235
-50
0
50
100
1
0
-60
-40
-20
0
20
80
100
120
140
100
120
140
100
120
140
-2
150
Temperature (Deg C)
LX Switch 'On' Resistance vs Temperature
Output Current Change vs Temperature
VIN=12V, L=100uH, Rs=0.33 Ohms
2.6
0. 5
Deviation from nominal set value
(%)
2.4
2.2
2
Ohms
60
-1
Temperature (Deg C)
1.8
1.6
1.4
1.2
0.25
0
-60
-40
-40
-20
0
40
20
60
80
100
120
140
-20
0
20
40
60
80
-0.25
1
-60
40
-0.5
160
Temperature (Deg C)
Temperature (Deg C)
Output Current Change vs Temperature
VIN=30V, L=100uH, Rs=0.33 Ohms
Deviation from nominal set value
(%)
4
3
2
1
0
-60
-40
-20
0
20
40
60
80
Temperature (Deg C)
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ZXLD1352
Application notes
Setting nominal average output current with external resistor RS
The nominal average output current in the LED(s) is determined by the value of the external
current sense resistor (RS) connected between VIN and ISENSE and is given by:
IOUTnom = 0.1/RS [for RS>0.27⍀]
The table below gives values of nominal average output current for several preferred values of
current setting resistor (RS) in the typical application circuit shown on page 1:
RS (⍀)
Nominal average
output current (mA)
0.27
370
0.3
333
0.33
300
0.39
256
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V).
Note that RS=0.27⍀ is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value.
It is possible to use different values of RS if the ADJ pin is driven from an external voltage. (See
next section).
Output current adjustment by external DC control voltage
The ADJ pin can be driven by an external dc voltage (VADJ), as shown, to adjust the output current
to a value above or below the nominal average value defined by RS.
+
ADJ
ZXLD1352
GND
DC
GND
The nominal average output current in this case is given by:
IOUTdc = 0.08*VADJ/RS [for 0.3< VADJ <2.5V]
Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above
1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 370mA maximum.
The input impedance of the ADJ pin is 50k⍀ ±25%.
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ZXLD1352
Output current adjustment by PWM control
Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as
shown below, to adjust the output current to a value above or below the nominal average value
set by resistor RS:
PWM
VADJ
ADJ
0V
ZXLD1352
GND
GND
Driving the ADJ input via open collector transistor
The recommended method of driving the ADJ pin and controlling the amplitude of the PWM
waveform is to use a small NPN switching transistor as shown below:
ADJ
PWM
ZXLD1352
GND
GND
This scheme uses the 50k resistor between the ADJ pin and the internal voltage reference as a
pull-up resistor for the external transistor eg MMBT3904.
Driving the ADJ input from a microcontroller
Another possibility is to drive the device from the open drain output of a microcontroller. The
diagram below shows one method of doing this:
MCU
10k
ADJ
ZXLD1352
GND
If the NMOS transistor within the microcontroller has high Gate / Drain capacitance, this
arrangement can inject a negative spike into ADJ input of the ZXLD1352 and cause erratic
operation but the addition of a Schottky clamp diode (eg Diodes Inc. SD103CWS) to ground and
inclusion of a series resistor (3.3k) will prevent this. See the section on PWM dimming for more
details of the various modes of control using high frequency and low frequency PWM signals
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ZXLD1352
Shutdown mode
Taking the ADJ pin to a voltage below 0.2V will turn off the output and supply current will fall to
a low standby level of 20µA nominal.
Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increase
output current above the 100% nominal average value. (See graphs for details).
Soft-start
An external capacitor from the ADJ pin to ground will provide soft-start delay, by increasing the
time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the
rate of rise of the control voltage at the input of the comparator. The graph below shows the
variation of soft-start time for different values of capacitor.
Soft Start Time vs Capacitance from ADJ pin to Ground
10
Soft Start time (ms)
8
6
4
2
0
0
5
15
10
20
25
Capacitance (nF)
Inherent open-circuit LED protection
If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so
the device will not be damaged, unlike in many boost converters, where the back EMF may
damage the internal switch by forcing the drain above its breakdown voltage.
Capacitor selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in
series with the supply 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 1␮F is acceptable if the input source is close to the device, but higher
values will improve performance at lower input voltages, especially when the source impedance
is high. The input capacitor should be placed as close as possible to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better
dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this
application and should NOT be used.
A table of recommended manufacturers is provided below:
Manufacturer
Murata
Website
www.murata.com
Taiyo Yuden
www.t-yuden.com
Kemet
www.kemet.com
AVX
www.avxcorp.com
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ZXLD1352
Inductor selection
Recommended inductor values for the ZXLD1352 are in the range 47␮H to 220␮H.
Higher values of inductance are recommended at higher supply voltages in order to minimize
errors due to switching delays, which result in increased ripple and lower efficiency. Higher
values of inductance also result in a smaller change in output current over the supply voltage
range. (See graphs). The inductor should be mounted as close to the device as possible with low
resistance connections to the LX and VIN pins.
The chosen coil should have a saturation current higher than the peak output current and a
continuous current rating above the required mean output current.
Suitable coils for use with the ZXLD1352 are listed in the table below:
Part No.
L
(␮H)
DCR
(⍀)
DO1608C
47
47
68
100
220
47
0.64
0.38
0.58
0.82
0.55
0.27
MSS6132ML
CD104-MC
NP04SB470M
ISAT
(A)
0.5
0.56
0.47
0.39
0.53
0.38
Manufacturer
CoilCraft
Sumida
Taiyo Yuden
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times
within the specified limits over the supply voltage and load current range.
The following equations can be used as a guide, with reference to Figure 1 - Operating
waveforms.
LX Switch 'On' time
LΔI
T ON = --------------------------------------------------------------------------------------V IN – V LED – I avg ( R S + rL + R LX )
Note: TONmin>200ns
LX Switch 'Off' time
LΔI
T OFF = ---------------------------------------------------------------------V LED + VD + I avg ( R S + rL )
Note: TOFFmin>200ns
Where:
L is the coil inductance (H)
rL is the coil resistance (⍀)
Iavg is the required LED current (A)
⌬I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (⍀)
VD is the diode forward voltage at the required load current (V)
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ZXLD1352
Example:
For VIN =12V, L=47␮H, rL=0.64⍀, VLED=3.4V, Iavg =350mA and VD =0.36V
TON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622␮s
TOFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21␮s
This gives an operating frequency of 546kHz and a duty cycle of 0.34.
These and other equations are available as a spreadsheet calculator from the Zetex website.
Go to www.zetex.com/ZXLD1352
Note that in practice, the duty cycle and operating frequency will deviate from the calculated
values due to dynamic switching delays, switch rise/fall times and losses in the external
components.
Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal
supply voltage. This helps to equalize the undershoot and overshoot and improves temperature
stability of the output current.
Diode selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance
Schottky diode with low reverse leakage at the maximum operating voltage and temperature. The
recommended diode for use with this part is the ZLLS1000. This has approximately ten times
lower leakage than standard Schottky diodes, which are unsuitable for use above 85°C. It also
provides better efficiency than silicon diodes, due to a combination of lower forward voltage and
reduced recovery time.
The table below gives the typical characteristics for the ZLLS1000:
Diode
Forward voltage
at 100mA
(mV)
Continuous
current
(mA)
Reverse Leakage
At 30V 85°C
(␮A)
Package
Manufacturer
ZLLS1000
310
1000
300
TSOT23
Zetex
If alternative diodes are used, it is important to select parts with a peak current rating above the
peak coil current and a continuous current rating higher than the maximum output load current.
It is very important to consider the reverse leakage of the diode when operating above 85°C.
Excess leakage will increase the power dissipation in the device.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will
increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to
ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the
specified maximum value.
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ZXLD1352
Reducing output ripple
Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled
across the LED(s) as shown below:
Rs
VIN
LED
Cled
L1
D1
VIN
ISENSE
LX
ZXLD1352
A value of 1␮F will reduce nominal ripple current by a factor three (approx.). Proportionally lower
ripple can be achieved with higher capacitor values. Note that the capacitor will not affect
operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of
LED voltage.
Operation at low supply voltage
The internal regulator disables the drive to the switch until the supply has risen above the startup threshold (VSU). Above this threshold, the device will start to operate. However, with the
supply voltage below the specified minimum value, the switch duty cycle will be high and the
device power dissipation will be at a maximum. Care should be taken to avoid operating the
device under such conditions in the application, in order to minimize the risk of exceeding the
maximum allowed die temperature. (See next section on thermal considerations).
Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient
to prevent the device from switching below approximately 6V. This will minimize the risk of
damage to the device.
Thermal considerations
When operating the device at high ambient temperatures, or when driving maximum load
current, care must be taken to avoid exceeding the package power dissipation limits. The graph
below gives details for power derating. This assumes the device to be mounted on a 25mm2 PCB
with 1oz copper standing in still air.
Maximum Power Dissipation
500
Power (mW)
400
300
200
100
0
-50
-30
-10
10
30
50
70
90
110
130
Ambient Temperature (Deg C)
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ZXLD1352
Note that the device power dissipation will most often be a maximum at minimum supply
voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of
unsuitable coils, or excessive parasitic output capacitance on the switch output.
Thermal compensation of output current
High luminance LEDs often need to be supplied with a temperature compensated current in order
to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted
remotely from the device, so for this reason, the temperature coefficients of the internal circuits
for the ZXLD1352 have been optimized to minimize the change in output current when no
compensation is employed. If output current compensation is required, it is possible to use an
external temperature sensing network - normally using Negative Temperature Coefficient (NTC)
thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network
can be used to drive the ADJ pin in order to reduce output current with increasing temperature.
Layout considerations
LX pin
The LX pin of the device is a fast switching node, so PCB tracks should be kept as short as
possible. To minimize ground 'bounce', the ground pin of the device should be soldered directly
to the ground plane.
Coil and decoupling capacitors
It is particularly important to mount the coil and the input decoupling capacitor close to the device
to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important
to take account of any track resistance in series with current sense resistor RS.
ADJ pin
The ADJ pin is a high impedance input, so when left floating, PCB tracks to this pin should be as
short as possible to reduce noise pickup. A 100nF capacitor from the ADJ pin to ground will
reduce frequency modulation of the output under these conditions. An additional series 10k⍀
resistor can also be used when driving the ADJ pin from an external circuit (see below). This
resistor will provide filtering for low frequency noise and provide protection against high voltage
transients.
10k
ADJ
100nF
ZXLD1352
GND
GND
High voltage tracks
Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage due to
board contamination. Any such leakage may raise the ADJ pin voltage and cause excessive
output current. A ground ring placed around the ADJ pin will minimize changes in output current
under these conditions.
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ZXLD1352
Dimming output current using PWM
When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage
VADJ and a low level of zero, the output current will be switched on and off at the PWM frequency,
resulting in an average output current IOUTavg proportional to the PWM duty cycle. (See Figure 2
VADJ
Toff
Ton
PWM Voltage
0V
IOUTnom
0.1/Rs
Output Current
IOUTavg
0
Figure 2
Low frequency PWM operating waveforms
The average value of output current is given by:
IOUTavg = 0.1DPWM/RS [for DPWM >0 01]
PWM dimming is preferable to DC dimming if optimum LED 'whiteness' is required. It will also
provide the widest possible dimming range (approx. 1000:1) and higher efficiency at the expense
of greater output ripple.
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ZXLD1352
Package outline - TSOT23-5
DIM
A
A1
A2
b
c
D
E
E1
e
e1
L
L2
a°
Millimeters
Min.
0.01
0.84
0.30
0.12
Inches
Max.
1.00
0.10
0.90
0.45
0.20
Min.
0.0003
0.0330
0.0118
0.0047
2.90 BSC
2.80 BSC
1.60 BSC
0.95 BSC
1.90 BSC
0.30
0.114 BSC
0.110 BSC
0.062 BSC
0.0374 BSC
0.0748 BSC
0.50
0.0118
12°
4°
0.25 BSC
4°
Max.
0.0393
0.0039
0.0354
0.0177
0.0078
0.0196
0.010 BSC
12°
Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches
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ZXLD1352
Definitions
Product change
Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or
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 Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
rights arising from such use or otherwise. Diodes Inc. does not assume any legal responsibility or will not be held legally liable (whether
in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business,
contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Diodes 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 Diodes Incorporated. 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.
Reproduction
The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a
representation relating to the products or services concerned.
Terms and Conditions
All products are sold subjects to Diodes Inc and conditions of sale, and this disclaimer (save in the event of a conflict between the two
when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Diodes Zetex sales office.
Quality of product
Diodes Zetex Semiconductors Limited is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Diodes Inc. or one of our regionally
authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com or www.diodes.com
Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales
channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
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.
Green compliance
Diodes Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or
exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions.
All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance
with WEEE and ELV directives.
Product status key:
“Preview”
Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
“Last time buy (LTB)”
Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
Production has been discontinued
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.
Diodes Zetex sales offices
Europe
Americas
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Corporate Headquarters
Zetex GmbH
Kustermann-Park
Balanstraße 59
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Telefon: (49) 89 45 49 49 0
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[email protected]
Zetex Inc
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Hauppauge, NY 11788
USA
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3701-04 Metroplaza Tower 1
Hing Fong Road, Kwai Fong
Hong Kong
Diodes Incorporated
15660 N. Dallas Parkway
Suite 850, Dallas,
X57248, USA
Telephone: (1) 631 360 2222
Fax: (1) 631 360 8222
[email protected]
Telephone: (852) 26100 611
Fax: (852) 24250 494
[email protected]
Telephone (1) 972 385 2810
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