M51995AFP datasheet(20/43 Pages) RENESAS

20 / 43 page

M51995AP/AFP

REJ03D0835-0300 Rev.3.00 Jun 06, 2008

Page 18 of 40

T-ON

From VF

signal

Vz = 4.2 V

T-OFF

RON

ROFF

M51995A

1/16

SW2

SW1

Charging

Discharging

5.8 V

Switched by

charging and

discharging signal

Figure 3 Schematic Diagram of Charging and Discharging Control Circuit for OSC Capacitor CF

2. Oscillator operation when intermittent action and OSC control circuit operates.

When over current signal is applied to CLM

+ or CLM− terminal, and the current limiting circuit, intermittent action

and OSC control circuit starts to operate. In this case T-OFF terminal voltage depends on VF terminal voltage, so

the oscillation frequency decreases and dead-time spreads.

The rise rate of oscillation waveform is given as

VT-ON

RON × CF

≈

(V/s)

………………………………………… (5)

The fall rate of oscillation waveform is given as

≈

(V/s)

………………… (6)

VVF − VVFO

ROFF × CF

VT-ON

× RON × CF

where VT-ON ≈ 4.5 V

VVF ; VF terminal voltage

VVFO ≈ 0.4 V

VVF − VFO = 0 if VVF − VVFO < 0

VVF − VVFO = VT-OFF if VVF – VVFO > VT−OFF ≈ 3.5 V

So when VVF > 3.5 V, the operation is just same as that in the no current limiting operation state.

The maximum on-duration is just same as that in the no-operation state of intermittent and oscillation control circuit

and is given as follows;

≈

(s)

………………… (7)

(VOSCH − VOSCL) × ROFF × CF

VT-ON

The minimum off-duration is approximately given as;

≈

(s)

………………… (8)

(VOSCH − VOSCL) × CF

VVF − VVFO

ROFF × CF

VT-ON

× RON × CF

The oscillation period is given by the summation of equation (7) and (8).

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