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AN1061 APPLICATION NOTE

Figure 4. Oscillator Internal Circuit.

A resistor Rosc and a capacitor Cosc connected as shown in Figure 4, allow the setting of the

desired switching frequency in agreement with the below formula:

The oscillator capacitor, Cosc, is discharged by an internal mos transistor with 100W of Rdson

(Q1) and during this period the internal threshold is set at 1V by a second mos, Q2 . When the

oscillator voltage capacitor reaches the 1V threshold, the output comparator turns off the mos

Q1 and turns on the mos Q2, restarting the Cosc charge.

The oscillator block, shown in figure 5, generates a sawtooth wave signal that sets the switch-

ing frequency of the system.

Figure 5. Switching frequency vs. Rosc and Cosc.

This signal, compared with the output of the error amplifier, generates the PWM signal that will

modulate the conduction time of the power output stage.

The way the oscillator has been integrated,does not require additional external components to

benefit of the voltage feedforward function.

Osc

TO PWM

COMPARATOR

1V

R

5R

CLOCK

D97IN655A

-

+

F

SW

1

R

oasc

C

osc

⋅

()In 6

5

---





100 C

osc

⋅

+

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

=

0

20

40

60

80

R2(K

Ω)

5

10

20

50

100

200

500

fsw

(KHz)

D97IN630

0.82nF

1.2nF

2.2nF

3.3nF

4.7nF

5.6nF

Tamb=25˚C