FAC2786FE

2015.01. 26

10/13

Revision No : 0

Function Description

Main Control Loop

The FAC2786 is a constant frequency current mode switching

regulator. During normal operation, the internal N-channel

power MOSFET is turned on each cycle when the oscillator

sets an internal RS latch and would be turned off when an

internal current comparator (ICMP) resets the latch. The

peak inductor current at which ICMP resets the RS latch is

controlled by the voltage on the COMP pin, which is the

output of the error amplifier (EAMP). An external resistive

divider connected between VOUT and ground allows the

When the load current increases, it causes a slight decrease

the COMP volt- age to increase until the average inductor

current matches the new load current.

VIN Power-On-Reset (POR) and EN

Under-voltage Lockout

The FAC2786 keep monitoring the voltage on VIN pin to

prevent wrong logic operations which may occur when VIN

voltage is not high enough for the internal control circuitry

to operate. The VIN POR has a rising threshold of 4.1V

(typical) with 0.5V of hysteresis.

An external under-voltage lockout (UVLO) is sensed at the

EN pin. The EN UVLO has a rising threshold of 2.5V with 0.2V

of hysteresis. The EN pin should be connected a resistor

divider from VIN to EN.

After the VIN and EN voltages exceed their respective

voltage thresholds, the IC starts a start-up process and then

ramps up the output voltage to the setting of output voltage

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the FAC2786. When the junction temperature

MOSFET, allowing the devices to cool. The thermal sensor

allows the converter to start a start-up process and regulate

the output voltage again after the junction temperature

cools by 50ºC.

The OTP is designed with a 50ºC hysteresis to lower the

increasing lifetime of the lC.

Enable / Shutdown

Driving EN to ground places the FAC2786 in shutdown. When

in shutdown, the internal power MOSFET turns off, all

internal circuitry shuts down

Current-Limit Protection

The FAC2786 monitors the output current, flowing through

the N-Channel power MOSFET, and limits the IC from

damages during overload, short-circuit and over voltage

conditions.

Frequency Foldback

The foldback frequency is controlled by the FB voltage.

When the FB pin voltage is under 0.6V, the frequency of the

oscillator will be reduced to 110kHz. This lower frequency

allows the inductor current to safely discharge, thereby

preventing current runaway. The oscillator’s frequency will

switch to its designed rate when the feedback voltage on FB

rises above the rising frequency foldback threshold (0.6V,

typical) again

Over-Voltage Protection

The over-voltage function monitors the output voltage by FB

pin. When the FB voltage increases over 120% of the

reference voltage, the over-voltage protection comparator

will force the low-side MOSFET gate driver high. This action

actively pulls down the output voltage. As soon as the

output voltage is within regulation, the OVP comparator is

disengaged. The chip will restore its normal operation.