L7987L datasheet(12/41 Pages) STMICROELECTRONICS

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Functional description

L7987L

12/41

DocID026362 Rev 3

If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if the 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 L7987Ls can be connected to the same SYNCH pin;

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

180°.

The L7987L 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.

FSW

250kHz

12500

RFSW

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