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TLV743P: reverse voltage prevention

Part Number: TLV743P

Hi

My customer has question.

Customers want to use TLV 743. Refer to the attached file for the configuration.
I understand that it is usual to prevent diode backflow from LDO into output from output.
However, since the way of using this customer is a circuit for Backup, it is necessary to provide a Diode at the attached part.
In this case, is Diode can be placed on input side or output side OK?

TLV743P.pdf

Best regard

T Kishi

  • Hi T Kishi,

    Since your application has enough headroom (Vin - Vout), you can place a diode in series before the input capacitor of the LDO to protect upstream devices from reverse current. The diode must be placed before the input capacitor of the LDO so that any reverse current through the LDO will charge the input capacitor and raise the voltage local to the IN pin of the LDO. Once Vout no longer exceeds Vin reverse current will not flow.

    Very Respectfully,
    Ryan
  • Hi Ryan

    Please tell me one more.

    If a diode is inserted before the input capacitor, I think that the front stage of the LDO will be protected.
    However, LDO itself is not protected, so voltage difference should occur. (It should lower the Vin by the forward voltage of the internal parasitic diode.)
    In this case, is there no problem?

    And, since the above-mentioned voltage difference occurs in the device whose maximum rating of the Out terminal is described as
    "Vin + 0.3", I think that it is impossible to place a diode on the input side.
    → Is this recognition correct?

    Regard
    T Kishi
  • Hi T Kishi,

    Ryan is out on Vacation until after Thanksgiving so I wanted to follow up with you so you aren't left hanging for a week.

    You've recognized a potential flaw with this standard methodology of implementing reverse current protection externally. If the input capacitor is very large there it is theoretically possible to damage the device but it would probably need to be on the order of milli-farads as we see 100uF-450uF commonly used in this type of use-case in the automotive sector.

    The only risky scenario I can think of is an application using a large input cap and a high current LDO connected to a load that is very inductive (either itself or the connection to it), since a sudden change in in the load could cause an inductive spike. However these inductive events are usually limited in the current they actually drive back into the part.

    For the more likely scenario where the input voltage drops, the diode will keep the input from discharging into the input supply and the LDO will go into dropout and won't be exposed to any reverse current.

    Kyle Van Renterghem
    Applications & Validation Manager
    Linear & Low Dropout Regulators
  • Hi Kyle

    Thank you for reply.

    When adding diodes for reverse flow protection to the input side,that if the Absolute maximum rating of Vout is written as "Vin + 0.3",
    there is no problem unless the input capacitor is small and it is not an inductive load.
    → Is this recognition correct?

    Regard

    T Kishi

  • Hi T Kishi,

    Your understanding is correct.

    Kyle Van Renterghem
    Applications & Validation Manager
    Linear & Low Dropout Regulators