AN557 datasheet(12/52 Pages) STMICROELECTRONICS

12 / 52 page

AN557 APPLICATION NOTE

12/52

been selected. The typical duration of the synchronizing pulse with the suggested values of Cosc are as

follows:

Obviously, synchronize pulses of eccessive duration can greatly reduce the max duty-cycle and produce

distortions in the sawtooth of the synchronized regulator working as slave.

P.W.M.

Comparing the sawtooth signal generated by the oscillator and the output of the error amplifier, generates

the PWM signal which is sent to the driver of the output power stage. The PWM signal, in the path towards

the output stage, also encounters a latch block to prevent other pulses from being sent at same period to

the output, possibly damaging the power stage. In the PWM block, a duty-cycle limiter has also been in-

troduced. Such a limiter is obtained by taking advantage of the synchronizing pulse generated, the power

output stage is inhibited. Even if the error amplifier gives a large signal to the peak of the ramp, the power

stage will not be able to operate in DC, but will be switched off at each clock pulse. The max. obtainable

duty-cycle is higher than 90%; this, however depends on the working frequency and the value of Cosc.

Using the formulas 6) and 7) a precise calculation can be done.

SOFT START

The Soft Start function is essential for a correct startup of the device and for an output voltage that, at the

switch on, increases in a monotonous mode without dangerous output overvoltages and without over-

stress for the power stage.

Soft Start operates at the startup of the system and after an intervention of the thermal protection. Fig. 9

shows the simplified diagram of the startup functions. The function is carried out by means of an external

capacitor connected to the Soft Start pin, which is charged with a constant current of about 100

µA to a

value of around 7V. During the charging time, the output of the error transcon-ductance amplifier, because

of Q1, is forced to increase at the same rising edge time of the external softstart capacitor Css.

L497X Family (MULTIWATT PACKAGE)

Cosc (nf) - Rosc = 16KΩ

Sync (ns)

0.68

140

1230

1.2

270

1.5

330

2.2

450

3.3

680

4.7

1100

L497X Family (POWERDIP PACKAGE)

Cosc (nf) - Rosc = 30KΩ

Sync (ns)

1.2

230

1.5

280

2.2

420

3.3

600

4.7

900

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