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TPS7A49,TPS7A30,TPS7A47,TPS7A33 Reverse current diode

Other Parts Discussed in Thread: TPS7A49

Dear all.

Does the reverse current diode need these IC?

Regards,
PAN-M

  • Hi Pan-M,

    As per the abs. max table Vout should not exceed Vin + 0.3 V.

    Regards,
    David
  • We have not incorporated a reverse current diode in our design.
    We get sometimes a board from a customer that appears to have a defective TPS7A49. Its output voltage shifts several hundred mV with changing DC loads from 40mA to 140mA. Some boards have a shifted output voltage of +1V with respect to the nominal output voltage.

    Our LDO regulator circuit with TPS7A49:
    Preferred circuit from figure 29 of the datasheet with Vin=23V, Cin=10uF, Vout=19.5Vout, Cout=50uF, CFF=10nF, CNR/SS=10nF. It is loaded with and average of 40mA. Our circuit is fed from a main supply from which also other dc/dc converters are fed.

    Possible cause:
    When the main supply is shut down the other dc/dc converters draw lots of current from the input capacitors so the input voltage of the LDO drops fast. This drop varies from many tens to hundreds of us. This causes a reverse current in the TPS7A49 discharging its Cout.

    Questions:
    1) Can the reverse current or the current in the feedback pin cause the defect in load-regulation and DC-voltage-shift?
    2) what is the maximum reverse current that the TPS7A49 can handle safely?
    3) what is the maximum current the FB pin can handle when it is driven below -0.3V during fast shutdown?
  • Hi ,

    TPS7A49 was not designed, tested or characterized for reverse current applications. As such we do not have a maximum specification on reverse current. In the event that reverse current is expected, we recommend external measures be taken such as an external diode to protect the LDO from reverse current.

    In an event when the input voltage dips below the output voltage, the output voltage is still a positive voltage. As a result, the voltage at the FB pin should not go negative. The reverse current would be through the pass element through parasitic elements.

    Very Respectfully,
    Ryan
  • Hi Ryan,

    Thanks for the clear statement.

    However, I would prefer some extra hints in the datasheet of TPS7A49 in addition to the absolute maximum ratings. For example, I would like to see some optional protection measures in the typical application diagram, figure 29 of the datasheet. That would prevent our problem, and likely that of some other entries in this forum about the TPS7A49.

    Regards,

    D. v. Mierlo