L7987 datasheet(12/37 Pages) STMICROELECTRONICS

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Functional description

L7987

12/37

DocID025589 Rev 3

If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if FSW pin is

connected to an external resistor the programmed switching frequency can be increased up

to 1.5 MHz, as shown in Figure 5. The required RFSW value (expressed in k) is estimated

by Equation 1:

Equation 1

Figure 5. Switching frequency programmability

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 a).

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

external resistor. In this way a frequency feed-forward is implemented (Figure 6 b) 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 a period. This helps reducing the RMS current flowing

through the input capacitor. Up to five L7987s can be connected to the same SYNCH pin;

however, the clock phase shift from master switching frequency to slaves input clock is

180°.

The L7987 device can be synchronized to work at a higher frequency, in the range 250 kHz

- 1500 kHz, providing an external clock signal on the SYNCH pin. The synchronization

changes the sawtooth amplitude, also affecting the PWM gain (Figure 6 c). 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.

FSW

250kHz

12500

RFSW

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