F72814
7. Function Description
The F72814 is a single PWM DC-DC converter controller for application which requires high current such as motherboard,
graphic card etc. The F72814 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 PHASE negative voltage when heavy loading
without the need of a current sensing resistor and short circuit condition is detected through FB pin. If over current condition occurs,
the F72814 will initiate the soft start cycle. After two cycles and the fault condition persist, the controller will go into shut down
status. In shut down status, both gate drive signals will be low. To restart the controller, either recycling VCC supply or momentarily
pulling the OCSET pin below 0.15V. If under voltage condition occurs, it will keep hiccup mode in this event.
7.1 Bootstrap Operation
In the single power supply system, UGATE is powered by an external bootstrap circuit as Figure 3. The Boot capacitor, C
BOOT
is
charged and generated a floating reference voltage at PHASE pin. Typically, a 0.1uF C
BOOT
is enough for the most of applications.
The C
BOOT
capacitor is refreshed and charged to a voltage of “VCC – Diode Drop” while low side power MOS turns on.
5V to 12V
3.3V/5V/12V
VCC
BOOT
C
BOOT
UGATE
PHASE
LGATE
Q1
VOUT
Q2
Fig 2. Power Supply Operation.
7.2 Power On Reset
The F72814S Power-on-reset monitors VCC supply voltage and input voltage at OCSET pin. POR level is 4.1V with 0.5V
hysteresis at VCC voltage and 0.15V at OCSET voltage, The POR function initiates soft-start operation and resets control logic
after the supply voltages exceeds its POR threshold voltage.
F72814
-5-
V0.23P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
