The UCC2895x allows flexible configuration of converters operating in synchronized mode by connecting all SYNC pins together and by configuration of the controllers as leader and/or followers. The controller configured as leader (resistor between RT and VREF) provides synchronization pulses at the SYNC pin with the frequency equal to 2X the converter frequency F_SW(nom) and 0.5 duty cycle. The controller configured as a follower (resistor between RT and GND and 825-kΩ resistor between SS_EN pin to GND) does not generate the synchronization pulses. The follower controller synchronizes its own clock to the falling edge of the synchronization signal thus operating 90° phase shifted versus the leader converter’s frequency F_SW(nom).

The output inductor in a full bridge converter sees a switching frequency which is twice that seen by the transformer. In the case of the UCC2895x this means that the output inductor operates at 2 × F_SW(nom). This means that the 90° phase shift between leader and follower controllers gives a 180° phase shift between the currents in the output inductors and hence maximum ripple cancellation. For more information about synchronizing more than two UCC2895x devices, see Synchronizing Three or More UCC28950 Phase-Shifted, Full-Bridge Controllers (SLUA609).

If the synchronization feature is not used then the SYNC pin may be left floating, but connecting the SYNC pin to GND through a 10kΩ resistor will reduce noise pickup and switching frequency jitter.

• If any converter is configured as a follower, the SYNC frequency must be greater than or equal to 1.8 times the converter frequency.
• follower converter does not start until at least one synchronization pulse has been received.
• If any or all converters are configured as followers, then each converter operates at its own frequency without synchronization after receiving at least one synchronization pulse. Thus, If there is an interruption of synchronization pulses at the follower converter, then the controller uses its own internal clock pulses to maintain operation based on the R_T value that is connected to GND in the follower converter.
• In leader mode, SYNC pulses start after SS pin passes its enable threshold which is 0.55 V.
• follower starts generating SS/EN voltage even though synchronization pulses have not been received.
• TI recommends that the SS on the leader controller starts before the SS on the follower controller; therefore SS/EN pin on leader converter must reach its enable threshold voltage before SS/EN on the follower converter starts for proper operation. On the same note, TI also recommends that the T_MIN resistors on both leader and follower are set at the same value.

Figure 6-16 SYNC_OUT (leader Mode) Timing Diagram

Figure 6-17 SYNC_IN (follower Mode) Timing Diagram