Functional description

L5983

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4.1

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 FSW

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

shown in Figure 6 by external resistor connected to ground.

To improve the line transient performance and to keep 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 5.4 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 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 L5988D Data sheet].

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 changing has to 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 FSW pin) only slightly lower than the external clock frequency. This pre-adjusting

of the frequency will change the sawtooth slope in order to get negligible the truncation of

sawtooth, due to the external synchronization.

Clock

Generator

Ramp

Generator

FSW

Sawtooth

Clock

Synchronization

SYNCH

Clock

Generator

Ramp

Generator

FSW

Sawtooth

Clock

Clock

Synchronization

SYNCH