Contents

Master / Slave configuration

Slave/Slave Configuration

Single Slave Configuration

UCC28950 SYNC and output inductor ripple cancellation

Colin Gillmor, 18 Jan 2016

This document attempts to clarify some aspects of the synchronisation function of the UCC28950. Additional information is in the datasheet and in the document at

Master / Slave configuration

The first case is where one UCC28950 operates as the Master and the other operates as the Slave. The details are explained in the datasheet.

The SYNC signal runs at twice the frequency of the OUTX signals. The Slave synchronises itself to the falling edge of the SYNC signal as shown in Figure 44 in the Datasheet. The Datasheet says “the Slave controller synchronizes its own clock to the falling edge of the synchronization signal thus operating 90° phase shifted versus the master converter’s frequency”. It happens that there are two falling edges per period on the SYNC signal for each OUTX period and the Slave can synchronise to either of them. This means that the Slave can synchronise either +90° or +270° (equivalent to -90°) with respect to the OUTX signals.

The currents in the Master and Slave output inductors will be 180° out of phase in either case and so it doesn’t matter whether the Slave is synchronised at 90° or 270°, both conditions are equivalent.Maximum cancellation of the ripple currents into the output filter of the PSU is achieved in both cases.

(Note: The output inductor switches at twice the frequency of the OUTX signals so a 90° or 270° phase shift at the OUTX signals is equivalent to 180° at the output inductor).

Typical waveforms for the Master / Slave configuration are shown below along with some screenshots.

/ Yel: Master, OUTA
Red: Master, OUTC
Blu: Slave, OUTA
Grn: Slave, OUTD
Slave has locked at 90° phase
/ Yel: Master, OUTA
Red: Master, OUTC
Blu: Slave, OUTA
Grn: Slave, OUTD
M4: SYNC
Slave has locked at 90° phase
/ Yel: Master, OUTA
Red: Master, OUTC
Blu: Slave, OUTA
Grn: Slave, OUTD
Slave has locked at 270° phase
/ Yel: Master, OUTA
Red: Master, OUTC
Blu: Slave, OUTA
Grn: Slave, OUTD
M4: SYNC
Slave has locked at 270° phase

Slave/Slave Configuration

The secondcase is where two UCC28950 controllers are both operated as Slaves and the SYNC signal is supplied externally. Again the details are explained in the datasheet.

The SYNC signal runs at twice the frequency of the OUTX signals. The Slaves synchroniseto the falling edge of the SYNC signal as shown in Figure 44 in the Datasheet. The Datasheet says “the Slave controller synchronizes its own clock to the falling edge of the synchronization signal thus operating 90° phase shifted versus the master converter’s frequency”. There are two falling edges per period on the SYNC signal for each OUTX period and a Slave can synchronise to either of them. This means that the Slaves can synchronise to the same falling edge or to different falling edges.

The currents in the Master and Slave output inductors will be in phase with each other in either case and so it doesn’t matter whether the Slaves synchronise to the same edge or different edges, both conditions are equivalent.There is no cancellation of the ripple currents into the output filter of the PSU is achieved in both cases.

/ Yel: Slave_1, OUTA
Red: Slave_1, OUTC
Blu: Slave_2, OUTA
Grn: Slave_2, OUTD
Slaves are locked in phase with each other
/ Yel: Slave_1, OUTA
Red: Slave_1, OUTC
Blu: Slave_2, OUTA
Grn: Slave_2, OUTD
M4: SYNC
Slaves are locked in phase with each other
/ Yel: Slave_1, OUTA
Red: Slave_1, OUTC
Blu: Slave_2, OUTA
Grn: Slave_2, OUTD
M4: SYNC
Slaves are locked 180° from each other
/ Yel: Slave_1, OUTA
Red: Slave_1, OUTC
Blu: Slave_2, OUTA
Grn: Slave_2, OUTD
Slaves are locked 180° from each other