FA7700V, FA7701V

11

You can reset the off latch mode operation of the short-circuit

protection by either of the following ways: lowering the CS

voltage below 2.03V (typ.); lowering the Vcc voltage below the

Off threshold voltage of undervoltage lock out; 1.93V (typ.);

lowering the voltage of FB terminal below 1.5V (typ.)

The off latch mode action cannot be triggered by externally

applying voltage of over 2.2V forcibly to the CS terminal (1.5V,

ZD clamped). Characteristics of the current and the voltage of

CS terminal is shown in the characteristic curve (CS terminal

voltage vs. CS terminal sink current) on page 6. Be sure to use

the IC up to the recommended CS terminal current of 50

µA.

8. Output circuit

The IC contains a push-pull output stage and can directly drive

MOSFETs (FA7700: N ch, FA7701: P ch). The maximum peak

current of the output stage is a sink current of +150mA, and a

source current of –400mA. The IC can also drive NPN, and

PNP transistors. The maximum peak current in such cases is

±50mA. Be sure to design the output current considering the

rating of power dissipation.

9. Power good signal circuit/ Undervoltage lockout circuit

The IC contains a protection circuit against undervoltage

malfunctions to protect the circuit from the damage caused by

malfunctions when the supply voltage drops. When the supply

voltage rises from 0V, the circuit starts to operate at VCC of

2.07V (typ.) and outputs generate pulses. If a drop of the

supply voltage occurs, it stops output at VCC of 1.93V (typ.).

when it occurs, the CS terminal is turned to Low level and then

it is reset. The power good signal circuit monitors the voltage of

REF terminal, and stops output until the voltage of REF

terminal excesses approximately 2V to prevent malfunctions.

s Design advice

1. Setting the oscillation frequency

As described in item 2 “Oscillator” of “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

can be calculated by the following expression.

Timing resistor [k

Ω]

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 characteristic curves on page 5, “output duty

CS terminal voltage (Vcs)”, 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

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:

characteristic curve, Icc=1.2mA. When operating in Qg = 6nC,

fsw = 500kHz, Pd should be:

Pd

6

(1.2mA + 6nC

500kHz)

25.2mW

()