MC44604 View Datasheet(PDF) - ON Semiconductor

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MC44604

High Safety Pulsed Mode Standby GreenLine PWM Controller

ON Semiconductor 

MC44604

The overvoltage section is enabled 5 ms after the regulator

has started to allow the reference Vref to stabilize.

By connecting external resistors to pin 6, the threshold

VCC level can be changed.

Rref

VCC

(Pin 1)

Vref enable

CSTARTUP

Pin 16

10

Vdisable

7.5 V or

12.5 V

10

STARTUP

14.5 V

Reference Block:

Voltage and Current

Sources Generator

(Vref, Iref, ...)

UVLO1

CUVLO1 (to SOFT−START)

Vdisable1

9.0 V

MC44604

Figure 38. VCC Management

Undervoltage Lockout Section

As depicted in Figure 39, an undervoltage lockout has

been incorporated to guarantee that the IC is fully functional

before allowing operation of the system.

Indeed, the VCC is connected to the non inverting input of

a comparator that has an upper threshold equal to 14,5 V

(Vstup−th) and a lower one equal to 7.5 V (Vdisable2) in

normal mode and 14.5 V and 12.5 V in Standby mode

(typical values) (Note 1).

This hysteresis comparator enables or disables the

reference block that generates the voltage and current

sources required by the system.

This block particularly, produces Vref (pin 16 voltage)

and Iref that is determined by the resistor Rref connected

between pin 16 and the ground:

Iref

+

Vref

Rref

where Vref + 2.5

V

(typically)

In addition to this, VCC is compared to a second threshold

level that is nearly equal to 9 V (Vdisable1) so that in normal

mode, a signal UVLO1 is generated to reset the maximum

duty cycle and soft−start block and so, to disable the output

stage (refer to Max. Duty Cycle and Soft−Start §) as soon as

VCC becomes lower than Vdisable1. In this way, the circuit is

reset and made ready for a next startup, before the reference

block is disabled (refer to Figure 26). In standby, UVLO1 is

not active (there is no need to discharge the soft−start

capacitor as the soft−start pin is maintained short circuited).

Note 1. In standby the difference between Vdisable2 and

Vstup−th is decreased not to have too low pulsed mode

frequencies.

Thus, finally in normal mode, the upper Vcc limit that

enables the output to be active, is 9.4 V (maximum value of

Vdisable1) and so the minimum hysteresis is 4.2 V.

[(Vstup−th)min = 13.6 V].

The large hysteresis and the low startup current of the

MC44604 make it ideally suited for off−line converter

applications where efficient bootstrap startup techniques are

required.

Standby Management

The MC44604 has been designed to detect the transitions

between the standby and normal mode and to manage each

mode in an optimal way.

In standby, the device monitors a pulsed mode that enables

to drastically reduce the power consumption.

Pulsed Mode

The MC44604 standby is preferably associated to a

flyback configuration as depicted in Figure 39.

Input

Voltage

VCC

Lp

MC44604

1 = Standby

0 = Normal

10

Mode

mP

Vstby

Regulator

Figure 39. Standby Flyback Configuration

In effect, by this means, all the output regulation levels are

divided by the ratio:

VHV

Vstby

where VHV is the normal mode high voltage regulation

level, Vstby is the standby mP supply voltage.

For instance, in the case of TV or monitors applications,

the output levels (except the mP supply voltage, Vstby) are

drastically reduced by a ratio in the range of 10.

Consequently, as the output voltages are reduced, the

losses due to the output leakage consumption, are practically

eliminated, without having to disconnect the loads.

Startup Operations

The choice of the right configuration (normal or standby)

is performed at each startup.

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