SD42509 Datasheet, PDF (3/7 Pages) Silan Microelectronics Joint-stock – 1A WHITE LED DRIVER

SD42509
PIN DESCRIPTIONS
Pin No. Pin Name I/O
1
FB
I
2
LED
O
3
COMP
I/O
4
NC
-
5
BS
I
6
VIN
I
7
SW
O
8
GND
-
Description
Current sense pin ,connect inductance and sense resistor.
LED positive connecting end, the other end connects to the sense resistor.
Compensation pin, connects to external capacitor
No Connected.
Bootstrap pin, used to increase the power MOSFET voltage
Input supply voltage.
Power output.
Ground.
FUNCTION DESCRIPTION
The SD42509 is a current mode LED driver. The voltage on compensation pin COMP is proportional to the
current delivered to the load.
At the beginning of a cycle: the upper transistor M1 is off; the COMP pin voltage is higher than the current
sense amplifier output; and the current comparator’s output is low. The rising edge of the 280KHz CLK signal
sets the RS Flip-Flop. Its output turns on M1 thus connecting the SW pin and inductor to the input supply. 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. If the current sense amplifier plus slope compensation signal does
not exceed the comp voltage, then the falling edge of the CLK resets the Flip-Flop.
The Gm amplifier compares the output current to 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 acreases. The output current is stable at the set value by the adjusting of the loop.
1． Output Current Setting
The output current is determined by the sampling resistor and setting voltage. The sampling voltage of VFB-
VLED is 88mV, and adjust the output current by adjusting the sampling resistor Rs (refer to Typical Application
Circuit).
Iout = ^{VFB − VLED}
R_S
2． Bootstrap Circuit
SD42509 has Bootstrap circuit, which provides grid drive voltage to power MOSFET; the chip has diode to
charge the bootstrap capacitor by 5V standard voltage.
3． Current Limiting Function
SD42509 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
Http://www.silan.com.cn
REV:1.4 2009.02.26
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