F72815
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,
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 turns on.
5V to 12V
3.3V/5V/12V
BOOT
VCC
C
BOOT
UGATE
PHASE
LGATE
Q1
Q2
VOUT
Fig 2. Power Supply Operation.
7.2 Power On Reset
The F72815 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.
7.3 Soft-Start
When the POR function initiates the digital soft-start sequence, PWM error amplifier reference inputs are forced to
track a voltage level proportional to the soft-start voltage. As the soft-start voltage slews up, the PWM comparator
regulates the output relative to the tracked soft-start voltage, and slowly charges the output capacitor(s). In the digital
soft-start sequence initiated by the POR function, the soft start function is used to prevent surge current from power
supply input during power on. The soft start duration is 3mS in typical case.
F72815
-6-
Aug, 2007
V0.14
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
