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LP5907-Q1: reverse protection

K.Hamamoto
Guru 12155 points
Part Number: LP5907-Q1

Hi

This device does not have a reverse protection such as VIN < VOUT.  However as is often the case, Vin will be go down under Vout during it is in Vin shutdown.

When it is at the above case, Vout > Vin + 0.3V (e.g. it is for 50ms) Is there any concerns? Or it is not allowed because Vout is over Abs Max ratings?

(This is just example)

In the case of customer board, after power down, Vout= Vin +0.38V for 50ms.

If the differences is about 0.3V, I think the body diode of internal FET is not turned on.

If the body diode is not turned on, the reverse current does not flow the body diode. 

Regards,
Koji Hamamoto

  • 0
    TI__Guru 55940 points

    Hi Koji Hamamoto,

    You are correct that LP5907-Q1 does not have a dedicated reverse current protection circuit.  When Vout exceeds Vin, current may flow from OUT to IN through the body diode.  We did not design, test or characterize the body diode for reverse current.  If reverse current is expected in your application, we recommend either placing a diode in parallel to the LDO in order to provide a robust path for the reverse current or placing a diode in series before the input capacitor of the LDO.  Placing the diode in parallel with the LDO will protect the LDO without requiring additional headroom (Vin - Vout) but can degrade the PSRR of the system due to leakages through the diode.  Placing the diode in series requires additional headroom to account for the diode drop; however, it will protect upstream devices without impacting PSRR.  When reverse current flows for a diode placed in series before the input capacitor of the LDO, the input capacitor will charge preventing further reverse current.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Guru 12155 points
    Hi Ryan,

    Thank you for your support.

    I understood.
    And I would like to make sure about the reverse current. Is the trigger voltage (means voltage threshold of starting to flow the reverse current) 0.3V? (that is why the Abs Max rating of VOUT defined as VIN+0.3V)

    At the customer situation, the difference between VIN and VOUT is 0.38V(VOUT-VIN) when it shutdown. However it looks the revers current does not flow from VOUT to VIN.

    This is just a opinion, if the reverse current does not flow internally from VOUT to VIN. IC might not be damaged even if VOUT is VIN+0.3V. Is my understanding correct?

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Guru 55940 points
    Hi Koji Hamamoto,

    Anytime that the Abs Max ratings are exceeded, there is the potential of damage to the LDO. There could be several reasons why you are not measuring reverse current. For example, it is possible that the particular body diode of the device you are measuring has a forward voltage of greater than 0.3 V (we do not characterize or test the body diode); however, there is also the possibility that the device has been slightly damaged already. Since your application has the potential for reverse current, it is highly recommended that you add external circuitry to protect the LDO. If you do not implement the external protection circuitry it is possible that the LDO may be damaged.

    Very Respectfully,
    Ryan