FA7700V データシートの表示（PDF） - Fuji Electric

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FA7700V

FUJI Power Supply Control IC

Fuji Electric 

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8． Description of each circuit

(1) Reference Voltage Circuit

This circuit consists of the reference voltage circuit using band gap reference, and also serves as the power

supply of the internal circuit. The precision of output is 2.23V±3%. It is stabilized under the supply voltage of

2.5V or over. The precision of reference voltage of error amplifier circuit is 0.88V±2%, and the reference

voltage circuit is connected to the non-inverting input of the error amplifier circuit.

(2) Oscillator

The oscillator generates a triangular waveform by charging and discharging the

built-in capacitor. A desired oscillation frequency can be determined by the value

of the resistor “RT”connected to the RT terminal (Fig. 1). The built-in capacitor

voltage oscillates between approximately 0.66V and 1.1V with almost the same

charging and discharging gradients. You can set the desired oscillation frequency

by changing the gradients using the resistor connected to the RT terminal. (Large

RT: low frequency, small RT: high frequency) The oscillator waveform cannot be

observed from the outside because a

terminal for this purpose is not provided.

1.1V

RT value : small

The oscillator output is connected to the

PWM comparator.

0.66V

(3) Error Amplifier Circuit

The IN(-) terminal (Pin3) is an inverting input terminal. The non-

Inverting input is internally connected to the reference voltage

(0.88V±2%; 25℃). The FB terminal (Pin4) is the output of the

error amplifier. Gain setting and phase compensation setting is

done by connecting a capacitance and a resistor between the FB

terminal and the IN(-) terminal. Vout which is the output voltage of

DC to DC converter can be calculated by:

Vout

R1

3

IN(‑)

R2

　　　　Vout = VB × R1 + R2

R2

Gain AV between the Vout and the FB terminal can be calculated by:

OSC

1 RT

RT Fig.1

RT value : large

fig.2

RNF

Er.AMP

+

VB

(0.88V)

4

FB

PWM

fig.3

　　　　 AV = − RNF

R1

(4) PWM comparator

The PWM comparator has 4 input

terminals. (Fig. 4) The oscillator output

① is compared with the CS terminal

voltage ② , and the error amplifier

voltage ③ ,then, the lower voltage

between ② and ③ is preferred.

While the preferred voltage is lower

than the oscillator output, the PWM

comparator output is LOW. While the

preferred voltage is higher than the

oscillator output, the PWM comparator

output is HIGH(Fig. 5). When the IC

starts, the capacitor connected to the

①Oscillation output

‑

②CS terminal voltage

+

③Error Amplifier output

+

④DT voltage

+

③Error Amplifier output

①Oscillation output

PWM

output pulse

PWM output

fig.4

②CS terminal voltage

④DT voltage

fig.5

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