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TLV704: Noise immunity

Caglar Cihan
Expert 6235 points
Part Number: TLV704
Other Parts Discussed in Thread: TPS709

Dear Team,

My customer has been using TLV70433 and they have a problem with noise immunity test.

At frequencies higher than 150MHz in radiated immunity test, the output regulation get worse if the input of the device is supplied from an SMPS stage. However, if they keep the SMPS running, but the input of the device is supplied from a battery, then there is no issue.

What could be the problem here?

Best Regards,

Caglar

  • 0
    TI__Guru 55940 points

    Hi Caglar,

    150 MHz is well outside of the LDO Bandwidth.  In the PSRR curve, the LDO contribution is rolling off and it is the output capacitor that provides the extra PSRR after the trough at ~2 kHz for the 1 mA line (~5 kHz for the 50 mA line).  Any signal at 150 MHz will pass straight through the LDO without any reduction.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Expert 6235 points
    Hi Ryan,

    thanks! the problem here is not the noise passing through the LDO. the problem is that this causes output voltage to loose tight regulation. the voltage deviates from the set value. The output supplies and MSP430 and display. they can significantly see the affect on the LCD display.
    what could be the solution to fix this?
    They tried to add a very small resistance between ground pin and ground which helped to fix, but this now requires a layout change. any other idea? or an alternative pintopin product to offer to be tested?

    Best Regards,

    Caglar
  • 0 in reply to Caglar Cihan
    TI__Guru 55940 points
    Hi Caglar,

    Thanks for the clarification. Unfortunately it sounds like the radiated signal is penetrating the package of TLV704 and is impacting internal nodes.

    The pinout for TLV704 is rather unique so we do not have much to offer in ways of pin-to-pin alternatives to try. TPS709 does have an alternative pinout that may work for your application. The TPS709B will work as a pin-to-pin alternative as long as long as pin 5 is left floating.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Expert 6235 points

    Hi Ryan,

    As you pointed out, there is no pin-to-pin alternative to TLV704, so there is a need for relayout. They need to be sure about the radiated immunity performance of TPS709 family. Are we sure about the noise immunity performance of TPS709 family?

    By the way, do you think standard regulators are more immune to noise compared to LDOs? If so, what is the mechanism behind this?

    Best Regards,

    Caglar