AN938 datasheet(7/33 Pages) STMICROELECTRONICS

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AN938

Power supply, UVLO and voltage reference

Doc ID 5410 Rev 10

7/33

2.1

Oscillator, sync and voltage feed-forward

One pin implements the oscillator function with inherent voltage feed-forward. A second pin

is dedicated to in/out synchronization.

A resistor Rosc and a capacitor Cosc, connected as shown in Figure 5, allow the setting of

the desired switching frequency given in Equation 1:

Equation 1

where Fsw is in kHz, Rosc in kΩ and Cosc in nF.

The oscillator capacitor Cosc is discharged by an internal mos transistor of 100 Ω of RDS(on)

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

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

mos Q1 and turns on mos Q2, charging the external capacitor Cosc. The oscillator block,

shown in Figure 5, generates a sawtooth wave signal that sets the switching frequency of

the system.

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

modulates the conduction time of the power output stage.

The design of the oscillator implements the voltage feed-forward function without any

additional external components.

Figure 6.

Device switching frequency vs. Rosc and Cosc

osc

⋅

()In 6

---

⎝⎠

⎛⎞

100 C

osc

⋅

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