L6986 datasheet(46/68 Pages) STMICROELECTRONICS

46 / 68 page

Application notes

L6986

46/68

DocID025376 Rev 7

Multiple L6986 can be synchronized to an external frequency signal fed to the SYNCH pin.

In this case the regulator is phased to the reference and all the devices will work with 0°

phase shift.

The frequency range of the synchronization signal is 275 kHz - 1.4 MHz and the minimum

pulse width is 100 nsec (see Figure 34).

Figure 34. Synchronization pulse definition

Since the slope compensation contribution that is required to prevent subharmonic

oscillations in peak current mode architecture depends on the switching frequency, it is

important to select the same oscillator frequency for all regulators (all of them operate as

SLAVE) as close as possible to the frequency of the reference signal (please refer to

Table 6: fSW selection on page 11). As a consequence all the regulators have the same

resistor value connected to the FSW pin, so the slope compensation is optimized

accordingly with the frequency of the synchronization signal. The slope compensation

contribution is latched at power-up and so fixed during the device operation.

The L6986 normally operates in MASTER mode, driving the SYNCH line at the selected

oscillator frequency as shown in Figure 35 and Figure 36.

In SLAVE mode the L6986 sets the internal oscillator at 250 kHz typ. (see Table 6 on

page 11 - first row) and drives the line accordingly.

Figure 35. L6986 synchronization driving capability

In order to safely guarantee that each regulator recognizes itself in SLAVE mode during the

normal operation, the external master must drive the SYNCH pin with a clock signal

VCC INT

5 mA

0.7 mA

fOSC

150nsec typ.

HIGH LEVEL

LOW LEVEL

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