SD42511

1.

PWM Dimming Function

SD42511 has internal PWM dimming function. When PWM voltage is open or connects to high level, the

chip is working normally; when PWM voltage 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 SD42511 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, we recommend to increase the output capacitor, then the LED level

will not be too low when shutdown to maintain the normal PWM dimming.

2.

Output Current Setting

Circuit).

S

LED

FB

OUT

R

V

V

I

−

=

3.

Bootstrap Circuit

SD42511 has Bootstrap circuit, which provides grid drive voltage to power MOSFET; the chip has diode to

charge the bootstrap capacitor by 5V standard voltage.

4.

Current Limiting Function

SD42511 has internal current limiting function, and the voltage on COMP is clamped at about 2.2V, the output

current of the power MOSFET is limited at about 1.6A by current comparator.

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.3

2008.11.19

Http://www.silan.com.cn

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