MC44608

http://onsemi.com

6

In stand–by mode, this current can be lowered as due to the

activation of a 200

µA current source:

Ipk

max

*stby +

1

* (Rcs(kW) 0, 2)

Rsense(W)

(A)

Figure 2. Current Sense

+

–

Switching Phase

1 V

STAND–BY

10

&

2

Isense

L.E.B.

Overcurrent

OC

200 A

m

START–UP

Comparator

Rcs

Rsense

The current sense input consists of a filter (6k

W, 4pF) 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:

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

– 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

sequense.

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 50mV.

This reference is chosen low to increase effectiveness of

the demagnetization detection even during start–up.

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

µA.

Figure 3. Demagnetization Block

&

1

DMG

> 24 A

Output

Current Mirror

Oscillator Buffer

+

–

Idemag

50/20 mV

>120 A

m

m

Demag

RQ

S

DMG

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 succesively 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

2.4V/4V levels.

Figure 4. Oscillator Block

+

–

from Demag

4 V

2.4 V

&

Window

OSC

ICH

comp

DMG

SDCH

IDCH

logic block

SCH

Clock

CT

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 2.4V 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 5).