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 20

μS, 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).

S

SENSE

IN

OUT

R

V

V

I

−

=

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

REV:1.1

2009.06.26

Http://www.silan.com.cn

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