L6986I datasheet(31/60 Pages) STMICROELECTRONICS

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6.4

Synchronization

The synchronization feature helps the hardware designer to prevent beating frequency noise that is an issue

when multiple switching regulators populate the same application board.

6.4.1

Embedded master - slave synchronization

The L6986I 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), so the same resistor connected at the FSW pin.

In order to minimize the RMS current flowing through the input filter, the L6986I 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 L6986I which regulate the same output voltage (i.e.: operating with the same duty

cycle), the input filter RMS current is optimized and is calculated as:

IRMS IOUT2∙ 2D∙ 1−2D if D<0.5

IOUT2∙ 2D−1∙2−2D if D>0.5

(29)

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 the figure below. To dimension the proper input capacitor please

refer to the dedicate section.

Figure 36. Input RMS current

The figure below shows two not synchronized regulators with unconnected SYNCH pin.

L6986I

Synchronization

DS13647 - Rev 1

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