SD42524
1A High Power LED Driver with 6½36V Input
DESCRIPTION
The SD42524 is a step-down PWM control LED driver with a built-in
power MOSFET. It achieves 1A continuous output current in 6~36V
input voltage range. It provides thermal shutdown circuit, current limit
circuit and PWM dimming circuit.
The SD42524 adopts current mode control which provides fast
transient response and simple loop stabilization design. It has high
efficiency up to more than 96%, and has excellent constant current
characteristic when the input/output voltage changed, with the
excellent current accuracy within ±1%
FEATURES
* 6-36V input voltage range
* Maximum 1A output current
* 0.40Ω built-in power MOSFET
* Integrate Jitter function
* Thermal compensation
* PWM dimming function
* 280kHz fixed frequency
* Excellent constant current accuracy ±1%
* High efficiency up to 96%
* Thermal shutdown
* Cycle-by-cycle over current protection
APPLICATIONS
* MR16 LED spotlight
* LED illuminance
* LED street lamp
ORDERING INFORMATION
Part No.
SD42524
Package
SOP-8-225-1.27
Marking
SD42524
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.1
2009.06.26
Page 1 of 11
SD42524
amplifier output; and the current comparator’s output is low. The rising edge of the 280KHz CLK signal sets the
RS Flip-Flop whose output turns on M1 thus VIN, Rs, LED, inductor and M1 comprises a circuit loop and the
inductor current increases. The inductor current is sensed and amplified by the current sense amplifier. Ramp
compensation is summed to current sense amplifier output and compared to the Gm amplifier output by the
current comparator. When the current sense amplifier plus ramp compensation signal exceeds the comp pin
voltage, the RS Flip-Flop is reset and the chip reverts to its initial M1 off state. The inductor energy storage
released via the circuit loop composed by Rs, LED, inductor and M1 and the current decreases. When next clock
cycle arrives, the upper transistor M1 is on to enter next switch cycle. If the current sense amplifier plus slope
compensation signal does not exceed the comp voltage in one cycle, then the falling edge of the CLK resets the
Flip-Flop.
The Gm amplifier compares the output current with the threshold current (threshold current is set by internal).
When the output current is higher than threshold current, the COMP pin’s voltage is lower down. Since the
COMP pin’s voltage is proportional to the peak inductor current a decrease in its voltage decreases current
delivered to the output. When the output current is lower than threshold current, the COMP pin’s voltage is up
while the output current increases. The output current is stable at the set value by adjusting the circuit loop.
1.
PWM dimming function
SD42524 has internal PWM dimming function. When PWM voltage is connects to high level, the chip is working
normally; when PWM voltage is open or connects to low level, COMP pin disconnects with the chip, the charge of
the capacitor is in hold state, voltage level holds, and the output of current comparator is high, transistor is off
without output current. When PWM signal is high, COMP pin is connected with the chip to improve the startup
speed of the chip. Adjust the output current by control the duty of external PWM signal.
The minimum setup time of SD42524 PWM dimming is less than 20μS, and the maximum PWM dimming ratio is
up to 500:1. When high dimming ratio is needed, the dimming frequency is recommended 500Hz below; or else
the dimming frequency can be up to 2 KHz. When PWM pin is low, output is shutdown. In normal station, LED pin
holds above 2V, and if the LED or other components have electric leakage which makes the LED pin is low and
caused change on PWM dimming, the output capacitor is recommend to be increased, then the LED level will not
be too low when shutdown to maintain the normal PWM dimming.
2.
Output current setting
The output current is determined by the sampling resistor and setting voltage. The sampling voltage of VIN-
VSENSE(Drop voltage on Rs) is 88mV, and adjust the output current by adjusting the sampling resistor Rs (refer
to Typical Application Circuit).
I
OUT
=
V
IN
−
V
SENSE
R
S
3.
Current limiting
SD42524 has internal current limiting function, and the voltage on COMP is clamped at about 1.9V, the output
current of the power MOSFET is limited at about 1.9A by current comparator.
4.
Frequency jitter
SD42524 has internal frequency jitter function to improve the EMI performance of the system. The internal
frequency is hopping in a very small range to reduce the single frequency radiation which simplifies the EMI
design.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.1
2009.06.26
Page 4 of 11
SD42524
5.
Thermal compensation and linear dimming function
SD42524 has linear dimming and thermal compensation functions.
Linear dimming function: Set the output current by adjust the ratio of R1 and R2. The relation between voltage
on pin ADJ and output current Iout is shown below:
V
A DJ
≥
1.22V
,
Iout
=
0.088
Rs
0.1V
<
V
A DJ
<
1.22V
,
Iout
=
0.088
×
Rs
V
A DJ
1.22
Thermal compensation function: the application circuit is shown above, use NTC resistor to take place of R2 for
thermal compensation. In normal temperature, the voltage on R2 is higher than 1.22V, and 100% current is
delivered to output; when LED temperature detected by R2 is higher than the setting value Tth, the voltage on
R2 is lower than 1.22V, and output current will decrease following the rising temperature. Connect NTC R2 with
other resistor in series to get different LED falling curves under different current.
If thermal compensation and linear dimming are not used, connect ADJ to V
DD
.
The relation between LED output current and temperature
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.1
2009.06.26
Page 5 of 11
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