SD46520 Datasheet, PDF (4/8 Pages) Silan Microelectronics Joint-stock – 400KHZ 2A STEP-DOWN DC-DC CONVERTER

SD46520
PIN DESCRIPTIONS
Pin No. Pin Name I/O
1
BS
I/O
2
IN
I
3
SW
I/O
4
GND
G
5
FB
I
6
COMP
I/O
7
EN
I
8
NC
I
Description
Bootstrap pin.
Connect 10nF capacitor to SW pin.
Input supply voltage.
Switch pin.
Connect with inductor.
Ground.
Feedback voltage input pin.
Compensation pin.
Enable input pin.
No connection.
FUNCTION DESCRIPTIONS
SD46520 is a current mode DC-DC convertor with PWM control. The working process is as follows.
At the beginning of a cycle, the switching MOSFET is off, the freewheeling MOSFET is on, SW is connected to
the ground; the external bootstrap capacitor between BS and SW is charged by 5V voltage via internal Schottky
diode. The bootstrap capacitor voltage is used as the power supply of the driver of switch MOSFET to make it
work normally.
When the bootstrap capacitor voltage is higher than the switching MOSFET’s threshold voltage, the control loop
circuit starts to work. The rising edge of the 400kHz clock signal sets the RS Flip-Flop. Its output turns on the
switching MOSFET, the input supply is connecting to the output capacitor via the inductor, and the inductor
current is increasing linearly and charges the output capacitor. The inductor current is sensed and amplified by
the current sense amplifier. Ramp compensation is summed to current sense amplifier’s output and compared to
the error amplifier output by the current comparator.
When the sum of current sense amplifier’s output plus slope compensation signal exceeds the comp pin voltage,
the RS Flip-Flop is reset. If the sum of current sense amplifier’s output plus slope compensation signal does not
exceed the comp voltage during one cycle, the falling edge of the CLK resets the Flip-Flop. After RS Flip-flop is
reset, the switching MOSFET is off, the freewheeling MOSFET is on, and the external Schottky diode is on at the
same time which carries most of the inductor current, the inductor current decreases linearly. When the next
rising edge of the clock arrives, sets the RS Flip-flop again, the switching MOSFET is on, which is cycled and the
output capacitor is charged to the output voltage setting.
The voltage of COMP pin is the integral of the voltage difference between FB feedback voltage and 1.22V
reference voltage. It is proportional to the peak inductor current. If COMP voltage increases, the inductor current
and the output current also increase. When COMP voltage is rising to the high clamp voltage 2.3V, the output
current is up to the limited value.
Enable control
Enable pin EN has the enable and under voltage lock two functions. When EN voltage is lower than 1V, the chip
is off; when EN voltage is higher than 1V but lower than 2.495V, the chip is enable, while COMP is pulled down
to the ground and the switching MOSFET is not working, so this is under voltage lock state; When EN voltage is
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0 2009.04.07
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