L5987 데이터 시트보기 (PDF) - STMicroelectronics

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L5987

3 A step-down switching regulator

STMicroelectronics 

L5987

5.1

Functional description

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 connected to

the FSW pin. In case 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 feedforward 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 feedforward is implemented (Figure 5.b) in order to

keep the PWM gain constant versus the switching frequency (see Section 6.4 on page 20

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 a

higher oscillator frequency works as a 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 4. Oscillator circuit block diagram

FSW

Clock

Generator

Clock

Synchronization

SYNCH

Ramp

Generator

Sawtooth

The device can be synchronized to work at a 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 the 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.

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