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BUF602: Power supply sequencing of the BUF602. The VCC- pin pulls down on VCC+

Andy Khan
Prodigy 65 points
Part Number: BUF602
Other Parts Discussed in Thread: LM27762

I am using the BUF602 in a design with +/-5v supply rails. My +5v is coming from an LT3045 and my -5v is coming from an LM27762. Both of these regulators are supplied by the same +5.5v supply voltage to the board.

I'm finding an issue with powering sequencing of the BUF602. When the -5v supply rail is enabled before the +5v rail, pin 5 of the BUF602 (VCC+) is pulled down to -0.5v and prevents the positive regulator (LT3045) from starting up. The positive regulator is stuck low by being forced low through the BUF602 VCC+.

Have you seen this with other regulators? Is there a recommendation on how to sequence the supplies or any method I can use to prevent the VCC+ pin of the BUF602 from being pulled down?

  • 0
    TI__Expert 6050 points

    Hi Andy,

    Typically, the recommendation when powering an op amp with dual supplies is to use a power supply that comes up to both supply levels very close in time to each other rather than sequencing each on. We do not characterize this behavior as all tests we run at dual supply configuration, have supplies that come up at levels very close in time to each other.

    My understanding is that the LM27762 is capable of outputting both +/-5V supply rails at the same time. Is there a reason you have for using this to power up only one supply rail instead of both?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 65 points

    Hello.  We needed a cleaner +5v voltage for other LNA's in the system, so it was simpler from a PCB routing aspect to use the +5v that was already available near all of the BUF602 devices in the design.

    Is there any test data that describes how much pulldown typically occurs when the VCC- is powered but the VCC+ is not powered?

    Would the amount of "pulldown", as I'm calling it, be dependent on the temperature of the circuit?  I'm curious about the mechanism that is actually causing this.

    Thanks.

  • 0 in reply to Andy Khan
    TI__Expert 6050 points

    Hi Andy, 

    Let me check with the team on your questions and I will provide an update early next week. 

    Thanks,

    Nick