MC44604_05 datasheet(15/22 Pages) ONSEMI

Part #	MC44604

Description	High Safety Pulsed Mode Standby GreenLine TM PWM Controller

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MC44604

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15

The Sawtooth Generation

In the steady state, the oscillator voltage varies between

about 1.6 V and 3.6 V.

Indeed, the sawtooth is obtained by charging and

discharging an external capacitor CT (Pin 10), using two

distinct current sources = Icharge and Idischarge. In fact, CT

is permanently connected to the charging current source

(0.4 Iref) and so, the discharge current source has to be

higher than the charge one to be able to decrease the CT

voltage. This condition is performed, its value being

(2 Iref).

Two comparators are used to generate the sawtooth. They

compare the CT voltage to the oscillator valley and peak

values. The comparison to the low value enables to detect the

end of the discharge phase while the comparison to the high

value determines when the charge cycle must be stopped. A

latch (LDISCH) memorizes the oscillator state.

Figure 30. Oscillator

10

CT

1 V

Vref

0.4 IREF

CVOS PROT

COSC HIGH

COSC LOW

0

1

IREGUL

1.6 V

Q

MC44604

COSC REGUL

Vosc prot

Vosc

R

S

LOSC

3.6 V

Q

S

R

DISCH

01

Vdemag out

CT< 1.6 V

DISCHARGE

IDISCHARGE

Now, in addition to the charge and discharge cycles, a

third state can exist. This phase can be produced when at the

end of the discharge phase, the oscillator has to wait for a

demagnetization pulse before re−starting. During this

delay, the CT voltage must remain equal to the oscillator

valley value (

X1.6 V). So, a third regulated current source

IREGUL controlled by COSC REGUL, is connected to CT in

order to perfectly compensate the (0.4 Iref) current source

that permanently supplies CT.

On−time is only allowed during the oscillator capacitor

charge. So, the maximum duty cycle is 80%. (Note 1)

The demagnetization condition is taken into account by a

second latch (Losc). (Refer to demagnetization § for further

details.)

Oscillator Frequency

The oscillator frequency can be deducted using the

following equations:

T

charge +

C

T • D

V

.

I

charge

T

discharge +

C

T • D

V I

discharge

where:

Tcharge is the oscillator charge time

DV is the oscillator peak to peak value

Icharge is the oscillator charge current

and

Tdischarge is the oscillator discharge time

Idischarge is the oscillator discharge current

So, as:

fosc = 1 /(Tcharge + Tdischarge) if the REGUL

arrangement is not activated, the following equation can

be obtained:

fosc X

0·395

R

ref •

C

T

Demagnetization Block (Note 2)

To enable the output, the Losc latch complementary output

must be low. Now, this latch reset is activated by the LDISCH

output during the discharge phase. So, to restart, the Losc has

to be set (refer to Figure 30). To perform this, the

demagnetization signal must be low.

In a fly−back, a good means to detect the demagnetization

consists in using the VCC winding voltage. Indeed this

voltage is:

−

negative during the on−time,

−

positive during the off−time,

−

equal to zero for the dead−time with generally a

ringing (refer to Figure 31).

That is why, the MC44604 demagnetization detection

consists of a comparator that can compare the VCC winding

voltage to a reference that is typically equal to 65 mV.

Note 1. The output is disabled by the signal Vosc prot when VCT

is lower than 1 V. (Refer to Figure 29 and Figure 30.)

Note 2. The demagnetization detection can be inhibited by

connecting pin 8 to the ground.

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MC44604 Datasheet(PDF) 15 Page - ON Semiconductor

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