M51996AP データシートの表示（PDF） - MITSUBISHI ELECTRIC

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M51996AP

SWITCHING REGULATOR CONTROL

MITSUBISHI ELECTRIC  

MITSUBISHI (Dig./Ana. INTERFACE)

M51996AP/FP

SWITCHING REGULATOR CONTROL

F/B

~ 7.1V

5.8V

6S

1S

POINT A

-

+

*1

PWM

COMP

LATCH

CURRENT

POINT B

OSC WAVEFORM

OF CF TERMINAL

TO

OUTPUT

WAVEFORM OF

CLM+ TERMINAL

POINT C

VTHCLM~ 200mV

M51996

POINT D

CF

CLM+

*2 FROM

OSC

*1 Resistor to determine current limit sensitivety

*2 High level during dead time

Fig.9 PWM comparator PWM latch and

current limit latch section

WAVEFORM

OF O.S.C. &

POINT A

OSC WAVEFORM

WAVEFORM AT POINT A

CURRENT LIMIT

SIGNAL TO SET

LATCH

WAVEFORM OF

VOUT TERMINAL

Fig.11 Operating waveform of current limiting circuit

To eliminate the abnormal operation by the noise voltage,the

low pass filter,which consists of RNF and CNF is used as shown

in Fig.12.

It is recommended to use 10 to 100Ω for RNF because such

range of RNF is not influenced by the flow-out current of some

200µA from CLM+ terminal and CNF is designed to have the

enough value to absorb the noise voltage.

M51996

POINT B

POINT C

Fig.10 Waveforms of PWM comparator input point A,

latch circuit points B and C

VOUT

CLM+

GND

POINT

D

RNF

CNF

RCLM

Current limiting section

When the current-limit signal is applied before the crossing

instant of "A" pint potential and CF terminal voltage shown in

Fig.9,this signal makes the output "off" and the off state will

continue until next cycle.Fig.11 shows the timing relation among

them.

If the current limiting circuit is set,no waveform is generated at

output terminal, however this state is reset during the

succeeding dead-time.

So this current limiting circuit is able to have the function in

every cycle,and is named "pulse-by-pulse current limit".

There happen some noise voltage on RCLM during the switching

of power transistor due to the snubber circuit and stray

capacitor of the transformer windings.

Fig.12 Connection diagram of current limit circuit

Voltage detector circuit(DET) section

The DET terminal can be used to control the output voltage

which is determined by the winding ratio of fly back transformer

in fly-back system or in case of common ground circuit of

primary and secondary in feed forward system.

The circuit diagram is quite similar to that of shunt regulator

type 431 as shown in Fig.13.As well known from Fig.13 and

Fig.14,the output of OP AMP has the current-sink ability,when

the DET terminal voltage is higher than 2.5V

( 12 / 22 )

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