R6986 datasheet(44/62 Pages) STMICROELECTRONICS

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Application notes

R6986

44/62

DocID026996 Rev 2

7.5

Synchronization (LNM)

Beating frequency noise is an issue when multiple switching regulators populate the same

application board. The R6986 synchronization circuitry features the same switching

frequency for a set of regulators simply connecting their SYNCH pin together, so preventing

beating noise. The master device provides the synchronization signal to the others since the

SYNCH pin is I/O able to deliver or recognize a frequency signal.

For proper synchronization of multiple regulators, all of them have to be configured with the

same switching frequency (see Table 6 on page 11), so the same resistor connected at the

FSW pin.

In order to minimize the RMS current flowing through the input filter, the R6986 device

provides a phase shift of 180° between the master and the SLAVES. If more than two

devices are synchronized, all slaves will have a common 180° phase shift with respect to

the master.

Considering two synchronized R6986 which regulates the same output voltage (i.e.:

operating with the same duty cycle), the input filter RMS current is optimized and is

calculated as:

Equation 43

The graphical representation of the input RMS current of the input filter in the case of two

devices with 0° phase shift (synchronized to an external signal) or 180° phase shift

(synchronized connecting their SYNCH pins) regulating the same output voltage is provided

in Figure 31. To dimension the proper input capacitor please refer to Chapter 7.6.1: Input

capacitor selection.

Figure 31. Input RMS current

IRMS

IOUT

------------

1 2D

–





if D < 0.5



IOUT

------------

2D 1

–

 22D

–





if D > 0.5













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