M51995AP/AFP

REJ03D0835-0201 Rev.2.01 Nov 14, 2007

Page 17 of 40

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.

Waveform of

CF terminal

Waveform of VOUT

terminal in Max

ON dutycondition

VOSCH

≈ 4.4 V

VOSCL

≈ 2.0 V

VOH

VOL

Figure 2 OSC Waveform at Normal Condition

(no-operation of intermittent action and OSC control circuit)

1. Oscillator operation when intermittent action and OSC control circuit does not operate

Figure 3 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 5.8 V. 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

VT-ON

RON × CF

≈

(V/s)

………………………………………… (1)

where VT-ON

≈ 4.5 V

The maximum on duration is approximately given as

≈

(s)

………………… (2)

(VOSCH − VOSCL) × RON × CF

VT-ON

where VOSCH

≈ 4.4 V

VOSCL

≈ 2.0 V

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”.

So fall rate of CF terminal is given as

≈

+

(V/s)

………………… (3)

VT-OFF

ROFF × CF

VT-ON

16

× RON × CF

The minimum off duration approximately is given as

≈

(s)

…………………… (4)

(VOSCH − VOSCL) × CF

VT-OFF

ROFF

+

VT-ON

16

× RON

where VT−OFF ≈ 3.5 V

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.