M51995 datasheet(13/27 Pages) MITSUBISHI

13 / 27 page

SWITCHING REGULATOR CONTROL

M51995AP/FP

MITSUBISHI (Dig./Ana. INTERFACE)

(

/ 27 )

Start-up circuit section

The start-up current is such low current level as typical 90µ

A,as shown in Fig.3,when the Vcc voltage is increased

from low level to start-up voltage Vcc(START).

In this voltage range,only a few parts in this IC,which has the

function to make the output voltage low level,is alive and

Icc current is used to keep output low level.The large voltage

difference between Vcc(START) and Vcc(STOP) makes start-up

easy,because it takes rather long duration from Vcc(START) to

Vcc(STOP).

Oscillator section

The oscillation waveform is the triangle one.The ON-duration

of output pulse depends on the rising duration of the triangle

waveform and dead-time is decided by the falling duration.

The rising duration is determined by the product of external

resistor RON and capacitor CF and the falling duration is mainly

determined by the product of resistor ROFF and capacitor CF.

(1)Oscillator operation when intermittent action

and OSC control circuit does not operate

Fig.4 shows the equivalent charging and discharging circuit

diagram of oscillator when the current limiting circuit does not

operate.It means that intermittent action and OSC control circuit

does not operate.

The current flows through RON from the constant voltage source

of 5.8V.CF is charged up by the same amplitude as RON

current,when internal switch SW1 is switched to "charging

side".The rise rate of CF terminal is given as

Vcc

(STOP)

Vcc

(START)

Icco

14mA

IccL

90µA

9.9V

16.2V

SUPPLY VOLTAGE Vcc(V)

Fig.3 Circuit current vs.supply voltage

RON X CF

VT - ON

(V/s) ................................................(1)

where VT - ON

4.5V

The maximum on duration is approximately given as

VT - ON

(VOSCH-VOSCL) X RON X CF

(s)........................(2)

where VOSCH

4.4V

VOSCL

2.0V

CF is discharged by the summed-up of ROFF current and one

sixteenth (1/16) of RON current by the function of Q2,Q3 and Q4

when SW1,SW2 are switched to "discharge side".

M51995

SW2

5.8V

1/16

SWITCHED BY

CHARGING AND

DISCHARGING

SIGNAL

DISCHARGING

CHARGING

SW1

T-ON

T-OFF

RON

ROFF

FROM

VF SIGNAL

Vz 4.2V

Fig.4 Schematic diagram of charging and discharging

control circuit for OSC.capacitor CF

Fig.5 OSC.waveform at normal condition (no-

operation of intermittent action and OSC.control

circuit)

VOH

VOL

VOSCH

VOSCL

4.4V

2.0V

So fall rate of CF terminal is given as

ROFF X CF

VT - OFF

(V/s)

16 X RON X CF

VT - ON

.....................(3)

The minimum off duration approximately is given as

(VOSCH-VOSCL) X CF

ROFF

VT-OFF +

16 X RON

VT-ON

(s) .....................................(4)

where VT - OFF

3.5V

The cycle time of oscillation is given by the summation of

Equations 2 and 4.

The frequency including the dead-time is not influenced by the

temperature because of the built-in temperature compensating

circuit.

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