F72820

Jul 2007

V0.24P

-4-

F72820

6. Function Description

The F72820 is a single PWM DC-DC converter controller for application requiring high current such as motherboard,

graphic card etc. The F72820 uses voltage mode PWM controller with fast transient response (driving two external

N-channel MOSFETs in a synchronous rectified buck converter topology) and integrates some functions such as

soft-start, over current and short circuit protections etc. Over current protection is achieved by monitoring the voltage

drop across the high side MOSFET, eliminating the need for a current sensing resistor and short circuit condition is

detected through the FB pin. If fault conditions occur, the F72820 would initiate the soft start cycle. After three cycles

and the fault condition persist, the controller will go into shut down status. In shut down, both gate drive signals will

be low. To restart the controller, either recycle the Vcc supply or momentarily pull the OCSET pin below 1.25V.

6.1 Bootstrap Operation

In single power supply system, the UGATE is powered by an external bootstrap circuit as Fig 3. The Boot capacitor,

side power MOS turning on.

5V

VOUT

Q1

Q2

UGATE

LGATE

PHASE

BOOT

VCC

Fig 3. Single 5V Power Supply Operation.

6.2 Dual Power Operation

The F72820 Power-on-reset monitors VCC supply voltage and input voltage at the OCSET pin. POR level is 3.8V

with 0.5V hysteresis at VCC voltage and 1.25V at OCSET voltage, The POR function initiates soft-start operation

and reset control logic after these supply voltages exceed their POR threshold voltages.