MC44605 datasheet(10/20 Pages) ONSEMI

Part #	MC44605

Description	High Safety, Latched Mode, GreenLine PWM Controller for (Multi)Synchronized Applications

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MC44605

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10

is 9.4 V (maximum value of Vdisable 1) and so the minimum

hysteresis is 4.2 V. [(Vstup–th)min = 13.6 V].

The large hysteresis and the low start–up current of the

MC44605 make it ideally suited for off–line converter

applications where efficient bootstrap start–up techniques

are required.

Soft–Start Control Section

The Vcs value is clamped down to the pin 11 voltage.

So, if a capacitor is connected to this pin, its voltage

increases slowly at the start–up (the capacitor is charged by

an internal current source 0.4 Iref). So, Vcs is limited during

the start–up and then a soft–start is performed.

This pin can be used to inhibit the circuit by applying a

voltage that is lower than VSSinhi (refer to page 4).

Particularly, the MC44605 can be shutdown by connecting

the soft–start pin to ground.

As soon as Vdis1 is detected (that is Vcc lower than

Vdisable1), a signal UVLO1 is generated until the Vcc falls

down to Vdis2 (refer to the undervoltage lockout section §).

During the delay between the disable1 and the disable2,

using a transistor controlled by UVLO1, the pin 11 voltage

is made equal to zero in order to make the soft–start

arrangement ready to work for the next re–start.

Figure 4. Soft–Start

MC44605

Vref

UVLO1

Soft

Start

Capacitor

Vcs

0.4 Iref

VSSlnhi

Output

Inhibition

Pin 11

2.4 V

DZ

Oscillator Section (Figures 5 & 5b)

The

oscillator

and

synchronization

behavior

is

represented in Figure 5b.

The MC44605 oscillator achieves four functions:

— it fixes the free mode frequency

— it takes into account the synchronization signal

— it does not allow a new power switch conduction if the

flyback is not in a dead–time state when the circuit

works in demagnetization mode (pin 8 connected)

— it builds the Sf pulse required by the MPL block

During the operating mode, the oscillator sawtooth can

vary between a valley value (1.6 V typically) and a peak one

(3.6 V typically) and presents three distinct phases:

— the CT charge

— the CT discharge

— the phase during which the oscillator voltage is

maintained equal to its valley value. This happens at

the end of a discharge cycle when the synchronization

or demagnetization condition does not allow a new CT

charge phase. During this sequence, IREGUL

compensates the charge current Icharge.

The oscillator has two working modes:

— a free one when there is no synchronization

— a synchronized one.

In the free working, the oscillator grows up from its valley

value to its peak one for the charge phase and when once the

peak value is reached, a discharge sequence makes the CT

voltage decrease down to its valley value. When the

decrease phase is finished, a new charge cycle occurs if the

demagnetization condition is achieved (VDT high).

Otherwise there is a REGUL phase until VDT gets high.

In the synchronized mode, the charge cycle is only

allowed when the synchronization signal gets high while a

dead time has been detected (VDT high). This charge phase

is stopped when the synchronization signal has got low and

when the oscillator voltage is higher than Vint, the

intermediary voltage level used to generate the calibrated

pulse Sf by comparing the CT voltage to this threshold. So,

when these two conditions are performed, a discharge

sequence is set until the oscillator voltage is equal to its

valley value. Then, the CT voltage is maintained constant

thanks to the “REGUL” arrangement until the next

synchronization pulse.

In both cases, during the charge phase, a signal VS is

generated. When Sf becomes high. VS gets high and remains

in this state until the PWN latch is set of Sf is low. Then, VS

keeps low until the next Sf high level. This oscillator

behavior is obtained using the process described in

Figure 5b.

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