FA7701V 查看數據表（PDF） - Fuji Electric

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FA7701V

FUJI Power Supply Control IC

Fuji Electric 

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9． Design Advice

(1) Setting the oscillation frequency

As described in Section 8(1), “Description of Each Circuit,” a desired oscillation frequency can be determined by

the value of the resistor connected to the RT terminal. When designing an oscillation frequency, you can set any

frequency between 50kHz and 1MHz. You can roughly obtain the oscillation frequency from the characteristic

curve “Oscillation frequency(fosc) vs. timing resistor resistance(RT)” or the value can be calculated by the

following expression.

fosc = 3000 × RT −0.9

RT

=



3000

fosc

1.11

fosc: oscillation frequency [kHz]

RT: timing resistor [kΩ]

This expression, however, can be used for rough calculation, the value obtained is not guaranteed. The

operation frequency varies due to the conditions such as tolerance of the characteristics of the ICs, influence of

noises, or external discrete components. When determining the values, be sure to verify the effectiveness of the

values of the components in an actual circuit.

(2) Operation around the maximum or the minimum output duties

As described in P9 of characteristic curves of “FB terminal voltage (VFB) vs.output duty cycle” and “CS terminal

voltage (Vcs) vs. output duty cycle”, the linearity of the output duty of this IC drops around the minimum output

duty and the maximum output duty(FA7701 only). This phenomena are conspicuous when operating in a high

frequency(when the pulse width is narrow). Therefore be careful when using high frequency.

(3) Restriction of external discrete components

To achieve a stable operation of the ICs, the value of external discrete components connected to Vcc, REF, CS,

FB terminals should be within the recommended operational conditions.

(4) Loss Calculation

Since it is difficult to measure IC loss directly, the calculation to obtain the approximate loss of the IC connected

directly to a MOSFET is described below.

When the supply voltage is Vcc, the current consumption of the IC is Icc, the total input gate charge of the driven

MOSFET is Qg, the switching frequency is fsw, the total loss Pd of the IC can be calculated by:

Pd ≒ Vcc×(Icc+Qg×fsw).

The values in this expression is influenced by the effects of the dependency of supply voltage, the

characteristics of temperature, or tolerance. Therefore,be sure to verify appropriateness of the value considering

the factors above under all applicable conditions.

Example)

When VCC=6V, in the case of a typical IC, from the characteristic curve, Icc=1.2mA. When operating in

Qg=6nC, fsw=500kHz, Pd should be:

Pd≒6×(1.2mA+6nC×500kHz)≒25.2mW

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