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Original question:

TPS54538: Clarification on RT/SYNC Pin Voltage Limits with RC Circuit in TPS54538

TPS54338: RT/SYNC Pin Voltage Limits with RC Circuit

Koshi Ninomiya
Expert 5255 points
Part Number: TPS54338

Tool/software:

Dear Team,

My customer considers to use the TPS54338 with both RT mode and SYNC mode. Datasheet, page 16 to 17, "6.3.5 Switching Frequency Selection / Synchronization" describes the RC circuit "Figure 6-3. SYNC Mode Configuration" which can be used to interface the RT/SYNC pin.

This is AC-coupling circuit. 
Q1: When the SYNC signal amplitude is 0V to 3.3V, what is the amplitude of the RT/SYNC pin input voltage?

Q2: Does the ESD diode of GND-->RT/SYNC conduct every SYNC signal period and clamp the RT/SYNC pin negative voltage to the -Vf voltage?
       Is this behavior a problem for the TPS54338?

Best Regards,

Koshi Ninomiya

  • 0
    TI__Intellectual 1810 points

    Hi Koshi-san,

    For Q1, I would like to know what switching frequency are you using at SYNC

    For Q2, I did not understand the concern. The ESD diode would trigger only when the voltage at RT/SYNC pin goes less than 0. If you are applying 0-3.3V at SYNC, the voltage at RT/SYNC will not go negative ever.

    Regards,

    Himanshu 

  • 0 in reply to Himanshu Nigudkar
    TI__Expert 5255 points

    Hi Himanshu-san,

    For Q1, SYNC frequency is 1.9MHz.

    For Q2,
          What voltage is the RT/SYNC pin in RT mode?
          If SYNC not active before start-up, TPS54338 works at default clock (based on RT resistor). When SYNC clock active, TPS54338 works in SYNC mode.
          When SYNC clock (0V-3.3V) active, because of AC-coupling of capacitor, RT/SYNC pin is not 0V-3.3V at initial, right? At this time, does the ESD diode of GND-->RT/SYMC pin conduct?

    Best Regards,

    Koshi Ninomiya

  • 0 in reply to Koshi Ninomiya
    TI__Expert 5255 points

    Hi Himanshu-san,

    I look forward to your reply.

    Best Regards,

    Koshi Ninomiya

  • +1 in reply to Koshi Ninomiya
    TI__Intellectual 1810 points

    Hi Koshi-san,

    Apologies for the delay. Our team looked into the internal esd structure for this device. It uses back to back diodes on RT/SYNC pin, so it can tolerate up to -2V without any breakdown for triggering esd diodes.

    Regards,

    Himanshu