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TPS7H4001-SP: SYNC1 VIH Threshold, Recommended Pull-Down Resistor?

Joshua Escobar
Prodigy 100 points
Part Number: TPS7H4001-SP
Other Parts Discussed in Thread: TPS50601A-SP,

I am designing a board with several TPS50601A-SP converters as well as one TPS7H4001-SP.

This post for the TPS50601A-SP suggests that a 3.3V external clock source can be used to successfully drive the SYNC pin of the converter when PVIN = VIN = 5V.

I was assuming the same would be true for the TPS7H4001-SP, but the datasheet specifies a VIH threshold of 3.5V for PVIN = VIN = 5V in Section 6.5 Electrical Characteristics.

Can you verify that VIH is in fact 3.5V when PIN = VIN = 5V? If so, would you recommend using the TPS7H4001-SP with PVIN = 5V and VIN = 3.3V to enable external synchronization with a 3.3V clock source?

Several posts for the TPS50601A-SP recommend a 1k pull-down resistor on the SYNC pin when using external synchronization mode (as opposed to the 10k resistor recommended in the datasheet). Should this same recommendation be applied to the SYNC1 pin for TPS7H4001-SP.

Thanks in advance!

  • 0
    TI__Genius 14540 points

    Hi Joshua,

    Yes, the designs are different between the TSP50601A-SP and the TPS7H4001-SP, therefore, it is possible to have different VIH, VIL specifications on the SYNC input.  With that said, I suspect there is a lot of margin to arrive at 3.5V.  I have asked if the design team has corner monte carlo sims to verify the simulated boundaries for this parameter.  I'll let you know once I hear back from this team.

    Regarding the SYNC termination, we found that this 1kohms helps mitigate SEE upsets on the SYNC pin when the device is using the internal oscillator.  When SYNC is being driven by an external clock the 1kohm resistor is not required.

    Thanks

    Christian

  • 0 in reply to Christian Yots
    Prodigy 100 points

    Hi Christian,

    Thanks for looking into this. Just to clarify -- when driving the SYNC pin with an external clock, there is no termination required on the line?

    I thought that the pull down was necessary to

    (1) prevent the SYNC pin from floating when not being driven by the external clock (a microcontroller is driving the SYNC pin in my application and it may take some time to start-up).

    (2) ensure that the converter will switch into internal oscillator mode if the SYNC signal is lost for any reason.

  • +1 in reply to Joshua Escobar
    TI__Genius 14540 points

    Hi Joshua,

    Yes, I agree with you. Since the internal oscillator can be used as a fail safe to a missing external clock, it is best to have the termination to mitigate single event effects when employed.  Also, as you state, the SYNC is pulled to a known bias and not floating when external clock is missing.

    Regarding the VIH of the external clock on the SYNC pin.  We found the following data to support operation below the 3.5V specification.

    Monte Carlo Simulation Design Verification 

    MIN VIH - 2.8968V-2.9028V

    Bench Characterization of 30 devices

    MIN VIH - 2.72V-2.88V

    Thanks

    Christian

  • 0 in reply to Christian Yots
    Prodigy 100 points

    Thanks Christian!

    I will keep PVIN = VIN = 5V given your confirmation that it should work with a 3.3V external clock source.

    Additionally, I will keep the 1k pull-down resistor on the SYNC pin.