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

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M51996AP

SWITCHING REGULATOR CONTROL

MITSUBISHI ELECTRIC  

(2)Oscillator operation when the SOFT(soft

start) circuit is operating.

Output transistor is protected from rush current by CLM function

at the start time of power on.SOFT terminal is used to improve

the rising response of the output voltage of power

supply(prevention of overshooting).

The ON duration of output is kept constant,and the OFF

duration is extended as the SOFT terminal voltage becomes

lower by the soft start circuit of this IC.

The maximum value of extension is set internally at

approximately sixteen times of the maximum ON duartion.

The features of this method are as follows:

1 It is ideal for primary control as IC driving current is supplied

from the third widing of the main transformer at the start-up

because constant ON duration is obtained from start-up.

2 It is possible to get a wide dynamic range for ON/OFF ratio

by pulse-by-pulse current limit circuit.

3 The response characteristics at power-on is not affected by

input voltage as the pulse-by-pulse limit current value is not

affected by the input voltage.

Fig.6 shows the circuit diagram of the soft start.If SOFT terminal

voltage is low,T-OFF terminal voltage bocomes low and VT-OFF

in equations (3) and (4) become low.

TO REG

TERMINAL

TO REG

TERMINAL

RSOFT

SOFT

TERMINAL

CSOFT

Vz ~ 4.2V

DISCHARGING TRANSISTOR*

IC's INTERNAL CIRCUIT

*Active when operation stops.

T-OFF

TERMINAL

GND

TERMINAL

Fig.6 Circuit diagram of SOFT terminal section and T-

OFF terminal section

MITSUBISHI (Dig./Ana. INTERFACE)

M51996AP/FP

SWITCHING REGULATOR CONTROL

VOSCH

START FROM 0V

VOSCL

0

VOH

NO OUTPUT

PULSE

VOL

0

THE FIRST

OUTPUT PULSE

t

Fig.8 Relationship between oscillator waveform and

output waveform at start-up

Fig.7 shows the relationship between oscillator waveform and

output pulse.

If the SOFT terminal voltage is VSOFT,the rise rate of CF

terminal given as

~

VT - ON

RON • CF

(V/S) ..............................................................(5)

The fall rate of oscillation waveform is given as

~ VSOFT - VBE

RON • CF

+

VT - ON

16 • RON • CF (V/S) .............................(6)

where

VSOFT;SOFT terminal applied voltage

VBE ~ 0.65V

If VSOFT - VBE < 0, VSOFT - VBE = 0

If VSOFT - VBE > VT - OFF (~3.5V), VSOFT - VBE =VT - OFF

VOSCH

~ 4.4V

VOSCL

~ 2.0V

t

VOH

VOL

t

Fig.7 Oscillator waveform when the SOFT circuit is

operating

PWM comparator, PWM latch and current limit

latch section

Fig.9 shows the scematic diagram of PWM comparator and

PWM latch section. The on-duration of output waveform

coincides with the rising duration of CF terminal waveform,when

the no output current flows from F/B terminal.

When the F/B terminal has finite impedance and current flows

out from F/B terminal,"A" point potential shown in Fig.9 depends

on this current.So the "A" point potential is close to GND level

when the flow-out current becomes large.

"A" point potential is compared with the CF terminal oscillator

waveform and PWM comparator,and the latch circuit is set

when the potential of oscillator waveform is higher than "A"

point potential.

The latch circuit is reset during the dead-time of oscillation

(falling duration of oscillation current).So the "B" point potential

or output waveform of latch circuit is the one shown in Fig.10.

The final output waveform or "C" point potential is got by

combining the "B" point signal and dead-time signal

logically.(please refer to Fig.10)

( 11 / 22 )

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