MC44608_06 Datasheet, PDF(6/15 Page) ON Semiconductor – Few External Components Reliable and Flexible SMPS Controller

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MC44608_06 Datasheet, PDF (6/15 Pages) ON Semiconductor – Few External Components Reliable and Flexible SMPS Controller

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MC44608

In standby mode, this current can be lowered as due to the

activation of a 200 mA current source:

Ipkmax *stby

(Rcs(kW)

Rsense(W)

0, 2)

(A)

Switching Phase

200 mA

STANDBY

STARTUP

Oscillator Buffer Output

DMG

50/20 mV

Demag

> 24 mA

>120 m A

Idemag

Current Mirror

Rsense

Isense

Rcs

L.E.B.

Overcurrent

Comparator

+ OC

Figure 3. Current Sense

The current sense input consists of a filter (6.0 kW, 4.0 pF)

and of a leading edge blanking. Thanks to that, this pin is not

sensitive to the power switch turn on noise and spikes and

practically in most applications, no filtering network is

required to sense the current.

Finally, this pin is used:

â as a protection against over currents (Isense > I)

â as a reduction of the peak current during a Pulsed Mode

switching phase.

The overcurrent propagation delay is reduced by

producing a sharp output turn off (high slew rate). This

results in an abrupt output turn off in the event of an over

current and in the majority of the pulsed mode switching

sequence.

Demagnetization Section

The MC44608 demagnetization detection consists of a

comparator designed to compare the VCC winding voltage

to a reference that is typically equal to 50 mV.

This reference is chosen low to increase effectiveness of

the demagnetization detection even during startup.

A latch is incorporated to turn the demagnetization block

output into a low level as soon as a voltage less than 50 mV

is detected, and to keep it in this state until a new pulse is

generated on the output. This avoids any ringing on the input

signal which may alter the demagnetization detection.

For a higher safety, the demagnetization block output is

also directly connected to the output, which is disabled

during the demagnetization phase.

The demagnetization pin is also used for the quick,

programmable OVP. In fact, the demagnetization input

current is sensed so that the circuit output is latched off when

this current is detected as higher than 120 mA.

Figure 4. Demagnetization Block

This function can be inhibited by grounding it but in this

case, the quick and programmable OVP is also disabled.

Oscillator

The MC44608 contains a fixed frequency oscillator. It is

built around a fixed value capacitor CT successively

charged and discharged by two distinct current sources ICH

and IDCH. The window comparator senses the CT voltage

value and activates the sources when the voltage is reaching

the 2.4 V/4.0 V levels.

DMG

from Demag

logic block

ICH

SCH

OSC

Window

+ comp

Clock

2.4 V

SDCH

IDCH

Figure 5. Oscillator Block

The complete demagnetization status DMG is used to

inhibit the recharge of the CT capacitor. Thus in case of

incomplete transformer demagnetization the next switching

cycle is postpone until the DMG signal appears. The

oscillator remains at 2.4 V corresponding to the sawtooth

valley voltage. In this way the SMPS is working in the so

called SOPS mode (Self Oscillating Power Supply). In that

case the effective switching frequency is variable and no

longer depends on the oscillator timing but on the external

working conditions (Refer to DMG signal in the Figure 6).

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