AN557 datasheet(11/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

1) Oscillator charge current:

(For 15V < Vi < 45V)

2) Oscillator discharge current:

IDISCH = 20mA

3) Peak voltage ramp:

This formula is obtained in the following way: indicating with Ve the voltage of the emitter of the NPN tran-

sistor connected to Vcc, and V- the voltage at the inverted input of the comparator, one has:

(a)

(b)

by substituting (a) into (b), one obtains:

4) Valley voltage ramp:

Vth-L = 2Vbe

5) Switching frequency:

It should be noted that formula (5) does not take into account the discharge time of Cosc which cannot be

neglected when one is working at frequencies equal or higher than 200KHz. The discharge time is also

tied to the value of Cosc itself.

Analitycally one has:

from which is obtained the more closely approximate expression of the oscillator frequency:

During the discharge time of Cosc, a clock pulse is generated internally that is made subsequently avail-

able on the Sync. pin and that can be used to synchronize other regulators. (3 devices of the same family

maximum). The Sync. pulse generated has a typical range of 4.5V and the current availability is 4.5mA.

In general, it is better that the Sync pulse is at least 300-400ns in order to be able to synchronize a range

of existing regulators; to obtain this result, values of suggested capacitors, in different test circuits, have

CHARGE

–

os c

-------------------------

–

Ro sc

-------------------------

-----

–

-------------------------- R

⋅



 2V

–

-----

–



 2V

–

----------------------------------------------

–

-------------------------

os c Cosc

--------------------------

DISCH

–

20 m A

--------------------------------------- C

os c

⋅

osc

⋅

------------------------------

DISCH

---------------------------------------------------------

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