Figure 5. Switching frequency programmability

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RFSW [kOhm]

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

voltage feed-forward is implemented by changing the slope of the sawtooth ramp, according to the input voltage

change (Figure 6. Feed-forward6a).

The slope of the sawtooth also changes if the oscillator frequency is programmed by the external resistor. In this

manner, a frequency feed-forward is implemented (Figure 6. Feed-forward6b) in order to keep the PWM

modulator gain constant versus the switching frequency.

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 pins together. When

SYNCH pins are connected, the device with a higher oscillator frequency works as master, so the slave device

switches at the frequency of the master but with a delay of half period. This helps reducing the RMS current

flowing through the input capacitor. Up to five A7987s can be connected to the same SYNCH pin; however, the

clock phase shift from master switching frequency to slaves input clock is 180 °.

The A7987 can be synchronized to work at a higher frequency, in the range 250 kHz-1500 kHz, providing the

SYNCH pin with an external clock signal. The synchronization changes the sawtooth amplitude, also affecting the

PWM gain (Figure 6. Feed-forward6c). This change must be taken into account when the loop stability is studied.

In order 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 slave IC switching frequency keeps the truncation of the ramp sawtooth negligible.

In case two or more (up to five) A7987 SYNCH pins are tied together, the A7987 IC with higher programmed

switching frequency is typically the master device; however, the SYNCH circuit is also able to synchronize with a

slightly lower external frequency, so the frequency pre-adjustment with the same resistor on FSW pin, as

suggested above, is required for a proper operation.

A7987

Oscillator and synchronization

DS12928 - Rev 3

page 9/36