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TPS543C20A: Behavior during external SYNC signal loss

Part Number: TPS543C20A

Hello, 

In our design, we use one TPS543C20A and two TPS543B20 to generate three power rails from 12V.

In order to avoid too much current demand on the 12V, we apply on each converter a external signal on the sync pin (each signal is phase shifted by 120° from the others to make the converters drawing current alternaltively).

These 3  signals are generated by a microcontroller. The issue we have is the following : while reprogramming the microcontroller, the sync signals aren't generated anymore during a few seconds. Then, the SYNC pins of the converters do not receive the signal anymore, which make the power rails going down. We expected the converters to toggle on its internal pulse generator (frequency set by RT pin) when loss of signal on SYNC pin occurs, but it seems this is not the case.

Is that a normal behaviour ? Is there way to keep this synchronization feature and having ability to keep power-on state even if synchronization signal is lost ?

The converter is set is standalone mode, with API/BB off (MODE = open). We have verified that the frequency set by RT pin is below the frequency of external sync clock.

See schematic extract for one of the power rails (+12V to +5V) : https://ibb.co/1dfBnjd

Thanks.

Alexandre

  • Alexandre, 

    The device is not designed to adjust its MODE functionality when the SYNC signal is lost. When the device is set in standalone mode, the RT resistor will determine the frequency, not the phase shift. When the device is configured in sync-in mode, RT pin will determine the phase shift and not the frequency.

    I am surprised to hear that you are setting the converter in standalone mode, as standalone mode does not provide phase shift functionality. 

    For clarity, what resistors are applied to the MODE and RT pins? I am unable to access the link 

    Best regards,
    Britton

  • Hi,

    Phase shifting is managed in the microcontroller, which delivers 3 sync signals for the 3 converters which delivers 3 different voltages.

    Each converter is set in standalone mode. MODE = open, RT = 30.8kohm (freq = 648KHz). Synchronization signals are at 672kHz.

  • Alexandre,

    Thanks for the clarification. My apologies for misunderstanding initially. 

    I can confirm that what you are seeing is the expected behavior. In all cases, both standalone operation and multi-phase operation, the loss of the sync signal will terminate switching until the sync signal is returned. Unfortunately, there is no way around this.

    Best regards,

    Britton