M51995 datasheet(21/27 Pages) MITSUBISHI

21 / 27 page

SWITCHING REGULATOR CONTROL

M51995AP/FP

MITSUBISHI (Dig./Ana. INTERFACE)

(

/ 27 )

M51995A

CVcc1

MAIN

TRANSFORMER

THIRD

WINDING

CVcc2

GND

Vcc

Fig.27 DC source circuit for stable start-up

It is recommended to connect the capacitor between OVP

terminal and GND for avoiding the miss operation by the spike

noise.

The OVP terminal is connected with the sink current source

(~150µA) in IC when OVP does not operate,for absorbing the

leak current of the photo coupler in the application.

So the resistance between the OVP terminal and GND for leak-

cut is not necessary.

If the resistance is connected,the supply current at the OVP

reset supply voltage becomes large.

As the result,the OVP reset supply voltage may become higher

than the operation stop voltage.

In that case,the OVP action is reset when the OVP is triggered

at the supply voltage a little high than the operation stop

voltage.

So it should be avoided absolutely to connect the resistance

between the OVP terminal and GND.

(1)To avoid the miss operation of OVP

OVP circuit

M51995A

OVP

GND

Vcc

Fig.28 Peripheral circuit of OVP terminal

10k

PHOTO COUPLER

M51995A

MAIN

TRANSFORMER

THIRD

WINDING

CVcc

GND

Vcc

FIG.30 OVP setting method using the induced

third winding voltage on fly back system

The reset time may becomes problem when the discharge time

constant of CFIN • (R1+R2) is long. Under such the circuit

condition,it is recommended to discharge the CVCC forcedly and

to make the Vcc low value.This makes the OVP-reset time fast.

(2)Application circuit to make the OVP-reset time fast

For the over voltage protection (OVP),the induced fly back type

third winding voltage can be utilized,as the induced third

winding voltage depends on the output voltage.Fig.30 shows

one of the example circuit diagram.

(3)OVP setting method using the induced third winding

voltage on fly back system

M51995A

Fig.29 Example circuit diagram to make the

OVP-reset-time fast

GND

Vcc

TO MAIN

TRANSFORMER

CFIN

Cvcc

THE TIME CONSTANT OF

THIS PART SHOULD BE SHORT

470

Ω

OVP

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