FA7701V 데이터 시트보기 (PDF) - Fuji Electric

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FA7701V

FUJI Power Supply Control IC

Fuji Electric 

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(7) Timer latch short-circuit protection circuit

The short-circuit protection

circuit consists of two

comparators C1, C2(Fig. 6). In

steady operation, the output

6

Lower value of either 5.5V or Vcc terminal voltage

5

of S.C.DET comparator C2 is

4

set to High, and the CS terminal

is clamped by the 1.5V Zener

Start-up

3

diode,because the output of

error amplifier is about 1V. If the

converter output voltage drops

due to a short-circuit etc, when

the output voltage of error

amplifier rises. excesses 1.5V,

the output of S.C.DET

2.2V

2

1.5V

1

0

tp

momentary short circuit

soft start

Time

short circuit protection

short circuit

fig.8

comparator C2 is set to Low,

and then the clamp of Zener diode is turned off. As a result, the voltage of CS terminal rises up to the lower

value of either 5.5 V or the voltage of VCC terminal. If the voltage of CS terminal excesses 2.2V, the output of

S.C.P comparator C1 is set to High,and the circuit shuts down the output circuit of the IC. When it occurs, the

current consumption of the IC is 0.9mA(typ.) because the IC is set to OFF latch mode. The period (tp) between

the occurrence of a short-circuit in the converter output and the triggering of the short-circuit protection function

can be calculated by the following expression:

tp [ms]≒Cs × Rcs × ln(Vcc −1.5 ) 

Vcc − 2.2

Cs: capacitance of Cs[μF]

Rcs: Resistance of Rcs[kΩ]

Vcc: supply voltage [V]

Note) When the IC is used in a product with low VCC voltage, the period (tp) of the triggering of the short-

circuit protection described above fluctuates significantly. Therefore, sufficient care should be taken in such

cases.

Ex.) When Rcs=750kΩ, Cs=0.1μF:

Vcc=2.5V: tp≒90ms

Vcc=3.6V: tp≒30ms

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 Under voltage

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 2.2 V forcibly to the CS

terminal (1.5V,ZD clamped) Characteristics of the current and the voltage of CS terminal is shown in the

characteristics curve [CS terminal voltage vs. CS terminal sink current] in page 10. Be sure to use the IC up to

the recommended CS terminal current of 50μA.

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