AN487 APPLICATION NOTE

4/13

A resistor, between Rosc and GND , defines a current that is mirrored internally to charge the oscillator

capacitor on the Cosc pin. The voltage at pin.Rosc is a function of Vcc value for the implementation of the

feed-forward function (oscillator slope proportional to Vcc). A comparator is sensing the voltage across

Cosc capacitor and dis-carge it when the ramp exceedes an upper threshold proportional to Vcc for the

implementation of the feed-forward function. The Cosc discharge current is internally controlled at a value

of about 20 mA. The lower threshold of the comparator is about 1.3V (2VBE). Here are reported basic

equations for the oscillator:

for 15V

(1)

(2)

for 15V

(3)

(4)

(5)

(6)

By which :

(7)

During the discharge time of Cosc a clock pulse is generated that is available on pin.SYNC and that can

be used to synchronize max 3 devices of the same family. See also fig. 6 and fig. 7 for the switching fre-

PWM

The comparison between oscillator sawtooth and error amplifier output generates the PWM signal that

feeds the driving stages. A PWM latch structure is implemented to avoid multiple pulses that could be dan-

gerous for the power stage. A maximum duty cycle limitation is implemented in the PWM stage. Such

limitation is obtained by the synchronization pulse generated in the oscillator section during the Cosc dis-

charge time. When the pulse is present the driver is inhibited. In this way even if the error amplifier output

completely overcomes the oscillator sawtooth, the power stage can not work in DC conditions, but is

switched off during the clock pulse allowing a maximum duty cycle tipically in the range 90 - 95 % .

SOFT START

Soft start (see fig.8) is an essential function for correct start-up and to obtain a monotonically increasing

output voltage, without overstressing the output power stage. Soft start operates at the start-up of the sys-

tem and after the intervention of thermal protection. The function is realized through a capacitor connected

to soft start pin, which is charged at constant current(about 100uA) up to a value of about 7V.

During the charging time , through PNP transistor Q1 the voltage at the output of the transconduc-tance

amplifier is forced to increase with the same rising speed of Css capacitor. As the capacitor is charged,

I

CHARGE

V

CC

9V

BE

–

R

OSC

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

=

V

TH .HIGH

V

CC

9V

BE

–

9

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

2V

BE

+

=

F

SW ITC H

9

R

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

≅

T

DISCH

V

TH .HIGH

V

TH .LOW

–

() C

os c

⋅

20m A

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

=

1

R

9

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

T

DISCH

+

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