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TPS54338: External sync

David Anderle
Prodigy 30 points
Part Number: TPS54338


Tool/software:

Hi,

I'm designing a PCB using the above TPS54338. This SMPS takes 14.5V as input, and outputs 3.4V, which is then powering the MCU and a ADP5076 dual boost converter. Since this PCB has sensitive analog signals on it, I wanted to sync up the ADC sampling clock (2MHz) with the TPS54338's and the ADP5076's switching frequency by driving the SYNC pins from the MCU once it has booted (~1s after 3.4V comes online). However, I am getting ambiguous signs whether this is possible at all as the SYNC clock would be provided AFTER the TPS54338/ADP5076 is powered on.

On the ADP5076 I can play around with the EN pins from the MCU, but I cannot disable the TPS54338 from the micro as obviously that would power the MCU down. 

Could you please check if I can apply the SYNC clock at any point in a certain configuration? If not, would it be OK to simply toggle the EN pin or VIN must be off before the clock is applied?

Thanks!

  • 0
    TI__Expert 3120 points

    Hi David,

    • If SYNC is active before start-up, TPS54x38 works in SYNC clock.

    • If SYNC not active before start-up, TPS54x38 works at default clock (based on RT resistor). When SYNC clock active, TPS54x38 works in SYNC mode.

    – If SYNC clock is out of range (200kHz to approximately 2.2MHz), TPS54x38 works at the end frequency (200kHz, 2.2MHz).

    – SYNC clock is not locked during operation.

    Best regards,

    Usha.

  • 0 in reply to Alla Ushasri
    Prodigy 30 points

    Hi Usha, 

    Thank you for looking this up. My confusion arises from the "during operation" part, as it's not clear if the clock needs to be applied before VIN, or it's enough to disable the chip (while VIN is present and EN is pulled low), apply the clock, and re-enable the TPS54338?

    Thanks,

    David

  • 0 in reply to David Anderle
    TI__Expert 3120 points

    Hi David,

    After output voltage getting regulated then you can apply SYNC clock to set it to 2MHz.

    Best regards,

    Usha.

  • 0 in reply to Alla Ushasri
    Prodigy 30 points

    HI Usha,

    I'm sorry, I'm not sure I understand this and it's not answering the question regarding the EN/VIN. 

    David

  • 0 in reply to David Anderle
    TI__Expert 3120 points

    Hi David,

    No need to pull EN low, you can SYNC clock with Vin being there.

    Best regards,

    Usha.

  • 0 in reply to Alla Ushasri
    Prodigy 30 points

    Hi Usha,

    But then this goes against "SYNC clock is not locked during operation."

    David

  • 0 in reply to David Anderle
    TI__Expert 3120 points

    Hi David,

    Let me clarify — the phrase "SYNC clock is not locked during operation” means the TPS54338 does not permanently latch the external SYNC at start-up. Instead, the device continuously follows the external clock as long as it’s within the valid 200 kHz to ~2.2 MHz range.

    So:

    * You do not need to apply SYNC before VIN or toggle EN.
    * The converter can start up normally at its RT frequency, and once your MCU provides the 2 MHz SYNC clock, the TPS54338 will transition into SYNC mode.
    * If the SYNC clock is later removed, the device simply reverts to the RT/default frequency.

    In short: you can safely power the device with VIN present and apply SYNC later without disabling EN.

    Best regards,
    Usha

  • 0 in reply to Alla Ushasri
    Prodigy 30 points

    Hi Usha,

    Understood, thank you for the explanation. I thougth that "locking" refers to how the internal phase-locked loop would "lock on" to the external clock.

    David