FAN5307 Datasheet, PDF (8/13 Pages) Fairchild Semiconductor – High-Efficiency Step-Down DC-DC Converter

N-channel rectifier switch. The device stays in sleep
mode until the output voltage falls below the low thresh-
old. The FAN5307 enters the PWM mode as soon as the
output voltage can no longer be regulated in PFM with
constant peak current.
100% Duty Cycle Operation
As the input voltage approaches the output voltage and
the duty cycle exceeds the typical 95%, the converter
turns the P-channel transistor continuously on. In this
mode the output voltage is equal to the input voltage
minus the voltage drop across the P-channel transistor:
V_OUT = V_IN – I_LOAD × (R_dsON + R_L), where
R_dsON = P-channel switch ON resistance
I_LOAD = Output current
R_L = Inductor DC resistance
Soft Start
The FAN5307 has an internal soft-start circuit that limits
the inrush current during start-up. This prevents possible
voltage drops of the input voltage and eliminates the out-
put voltage overshoot. The soft-start is implemented as a
digital circuit increasing the switch current in four steps to
the P-channel current limit (520mA). Typical start-up time
for a 10µF output capacitor and a load current of 200mA
is 500µs.
Short-Circuit Protection
The switch peak current is limited cycle by cycle to a typ-
ical value of 520mA. In the event of a output voltage
short circuit the device operates at minimum duty cycle,
therefore the average input current is typically 100mA.
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FAN5307 Rev. 1.0.4