Справочник Пользователя для Fujitsu MB3788

14

■

■

■

■

If the load conditions of the switching regulator are stable, the outputs of comparators 1 and 2 do not change,
so the SP comparator outputs a High level.  At this time, the SCP pin (pin 15) is held at about 50 mV.

If the load conditions change suddenly due to a load short-circuit, for example, the output voltage of the com-
parator of the channel becomes a High-level signal (more than 2.1 V).  Then, the SVP comparator outputs a
Low level and transistor Q1 is turned off.  The short-circuit protection capacitor C

PE

 externally connected to the

SCP pin starts to charge.

V

PE

 = 50 mV + t

PE

 

×

 10

-6

/C

PE

0.65 = 50 mV + t

PE

 

×

 10

-6

/C

PE

C

PE

 = t

PE

 /0.6 (s)

Once the capacitor C

PE

 is charged to about 0.65 V, the SR latch is set and the output drive transistor is turned

off.  At this time, the duty cycle is made low and the output voltage of the SCP pin (pin 15) is held at Low level.
This closes the SR latch input to discharge C

PE

.

Fig. 5  Latch-Type Short-Circuit Protection Circuit

2.1 V

Low 

input

voltage 

protection 

circuit

SR latch-type 

circuit

1 

µ

A

C

PE

-
-
+

OUT

2.5 V

15

Q

2

Q

1

Comparator 1
Comparator 2

S

R

PWM

 comparator