L7985 datasheet(9/42 Pages) STMICROELECTRONICS

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L7985

Functional description

5.1

Oscillator and synchronization

Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides

a constant frequency clock. Its frequency depends on the resistor externally connect to the

FSW pin. If the FSW pin is left floating, the frequency is 250 kHz; it can be increased as

shown in Figure 6 by an external resistor connected to ground.

To improve the line transient performance, keeping the PWM gain constant versus the input

voltage, the voltage feed-forward is implemented by changing the slope of the sawtooth

according to the input voltage change (see Figure 5.a).

The slope of the sawtooth also changes if the oscillator frequency is increased by the

external resistor. In this way a frequency feed-forward is implemented (Figure 5.b) in order

to keep the PWM gain constant versus the switching frequency (see Section 6.4 on page 18

for PWM gain expression).

On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of

180 ° with respect to the clock. This delay is useful when two devices are synchronized

connecting the SYNCH pin together. When the SYNCH pins are connected, the device with

a higher oscillator frequency typically works as the master, so the slave device switches at

the frequency of the master but with a delay of half a period. This minimizes the RMS

current flowing through the input capacitor (see the L5988D datasheet).

The SYNCH circuitry is also able to synchronize with a slightly lower external frequency, so

the frequency pre-adjustment with the same resistor on the FSW pin, as described below, is

suggested for a proper operation.

Figure 4. Oscillator circuit block diagram

The device can be synchronized to work at higher frequency feeding an external clock

signal. The synchronization changes the sawtooth amplitude, changing the PWM gain

(Figure 5.c). This change has to be taken into account when the loop stability is studied. To

minimize the change of PWM gain, the free-running frequency should be set (with a resistor

on the FSW pin) only slightly lower than the external clock frequency. This pre-adjusting of

the frequency changes the sawtooth slope in order to render the truncation of sawtooth

negligible, due to the external synchronization.

Clock

Generator

Ramp

Generator

FSW

Sawtooth

Clock

Synchronization

SYNCH

Clock

Generator

Ramp

Generator

FSW

Sawtooth

Clock

Synchronization

SYNCH

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