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,
CBOOT is charged and generated a floating reference voltage at phase pin. Typically a 0.1uF CBOOT is enough for the
most of applications. The CBOOT capacitor is refreshed and charged to a voltage of “VCC – Diode Drop” while low
side power MOS turning on.
5V
BOOT
VCC
UGATE
Q1
PHASE
LGATE
VOUT
Q2
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.
F72820
-4-
Jul 2007
V0.24P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
