A6985F datasheet(49/77 Pages) STMICROELECTRONICS

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A6985F

Application notes

Figure 36 shows the same regulators working synchronized. The MASTER regulator

(LX_5V trace) delivers the synchronization signal (SYNCH_3V3, SYNCH_5V pins are

connected together) to the SLAVE device (LX_3V3). The SLAVE regulator works in phase

with the synchronization signal which is out of phase with the MASTER switching operation.

Figure 36. Two regulators synchronized

Multiple A6985F devices can be synchronized to an external frequency signal fed to the

SYNCH pin. In this case the regulator set 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 - 2 MHz and the minimum

pulse width is 100 nsec (see Figure 37).

Figure 37. 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

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