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FA7701V

FUJI Power Supply Control IC

Fuji Electric 

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CS terminal is charged through the resistor connected to the power supply , and then the output pulses begin to

widen gradually as the operation of soft start. In steady operation, the pulse width is determined based on the

voltage of the error amplifier③, and then the output voltage is stabilized. The Dead Time control voltage (④DT

voltage) of FA7700 and FA7701 has different characteristics to adjust the ICs to various types of power supply

circuits being controlled and also to reduce external discrete components as many as possible. FA7700 is

developed for fly-back circuits, and boost circuits, and the DT voltage is set in the IC so that the maximum output

duty cycle is fixed to 80%(min.). (Maximum output duty cycle changes according to operation frequencies.—

See P10 “Maximum output duty vs. temperature”.) It prevents magnetic saturation of the transformer or the like

when a short-circuit in the output circuit occurs. FA7701 is developed for buck circuits, and it is designed for the

maximum output duty cycle of 100%. The timing chart of PWM comparator is described in Fig. 5.

(5) Soft start function

As described in Fig. 6, RCS is connected

between CS terminal and VCC terminal, and

Cs is connected between CS terminal and

GND. The voltage of CS terminal rises

when starting the power supply, because Cs

is charged by Vcc through Rcs. The soft

start function starts by charging a capacitor

Cs connected to PWM comparator. To

estimate the soft start period, the time(ts)

between the start and the moment when the

width of output pulse reaches 50% is

calculated by:

VCC

Rcs

CS

8

Cs

FB

0.3 V

5.5V

1.5V

C2

S.C.DET

1.5V

REF OFF

C3

ON/OFF

Output

off

C1

S.C.P

2.2V

ts [ms]≒Cs × Rcs × ln( Vcc ) 

fig.6

Vcc − 0.88

Cs: Capacity of Cs [μF]

Rcs: resistance of Rcs [kΩ]

Vcc: supply voltage [V]

The maximum current flowing in Rcs should be within the recommended value(50μA max.).

VCC − 1.5 < RCS[MΩ] < VCC − 1.5 (IL: leak current of capacitor Cs)

50uA + IL

1uA + IL

Note) This IC operates ON/OFF function by the CS terminal(CS<0.3V typ. :OFF), then it turns off the internal

bias voltage VREF (off mode). Therefore, you can not connect the resistor “Rcs” between CS terminal and REF

terminal, and can connect the resistor only to VCC terminal.

    

(6) ON/OFF circuit

Vcc

The ON/OFF function can be controlled by external signal to the CS

terminal,the IC becomes off mode .When the CS terminal voltage is

below 0.30V(typ.), the output of ON/OFF comparator C3 is set to LOW,

and the internal power source VREF is shut off, then the IC is switched

to the off mode. The power consumption in the off mode is 40μA(typ.).

A sample circuit is given in Fig. 7.

ON/OFF

CS

Cs

fig.7

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