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R7986A

Functional description

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5.1

Oscillator and synchronization

Figure 3 shows the block diagram of the oscillator circuit. The internal oscillator provides

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

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

shown in Figure 5 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 4.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 4.b) in order

to keep the PWM gain constant versus the switching frequency (see Section 6.4 on page 19

for PWM gain expression).

The synchronization signal is generated on the SYNCH pin. 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 pins together. When the SYNCH pins are connected, the device

with higher oscillator frequency works as 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).

Figure 3. 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 4.c). This change must be taken into account when the loop stability is studied. To

minimize the change of the 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

Clock

Synchronization

SYNCH