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TPS7A4001: Considerable and Varying Voltage Drop on Output

Anthony Mitchell
Prodigy 10 points
Part Number: TPS7A4001

Hello,

We are utilizing this component as a 12V supply with inputs varying from about 13V to 30V. We noticed that as everything turns on at 13V, we only receive an output voltage of approximately 11.6V. When the input voltage reaches 14.9V, the output voltage dips down to 11.2V. As the input voltage increases to 30V, the output gradually increases to about 11.4V. We originally had a 20k resistor for R301 to set the supply to 12V, but replaced it with a 18.7k (which was in place for all the measurements taken). This did a minor impact to the output and still did not reach 12V, given that our calculation should have put us closer to 12.9V. The schematic is shown below.

Additionally, the input capacitance is not 1uF, but has a total of 133 uF for the input due to 4 parallel 33uF capacitors on VIN_2 located on a different page. The input looks clean for VIN_2 as well. Output current never exceeds 45mA, and temperature only hits about 33C on the housing. The only area of concern I can find is with the feedback pin.

Apologies if the image below is blurry. Blue is the output, and red is the feedback line. Input voltage was at 14.9V, which is where the switch from 11.6V to 11.2V occurred. We are unsure if this waveform in the picture is expected or if this could be causing our low voltage issues, but the feedback pin only seems to reach about 1.15V to 1.169V depending on the input voltage (lower input voltages closer to 1.169V, and higher input voltages closer to 1.15V) at room temperature, and that excludes the dipping that occurs.



Any assistance would be greatly appreciated. I can provide additional information on request.

Thanks.

  • 0
    TI__Genius 12040 points

    Hello,

             I am looking into this, I will get back to you by the end of the day tomorrow. Thanks!

    Regards,

    Srikanth

  • 0 in reply to Srikanth Lolla Venkata
    TI__Genius 12040 points

    Hello,

             I think your schematic looks fine. The Cap values should be enough for stability and the resistance ratio should give you a voltage of ~12.8V. Is this behavior observed across multiple units of TPS7A40? Could you replace C300 (Input Cap, Cin) with a 10uF cap and verify if the behavior improves? At this point, I suspect some partial damage at Feedback pin (unless 10uF Cin fixes it). If this behavior shows up on a new device, could you provide us the scope shots showing Vin at input pin and the input current at the time of startup using a new unit? Thanks!

    Regards,

    Srikanth  

  • 0 in reply to Srikanth Lolla Venkata
    Prodigy 10 points

    Hello. Thanks for the reply. I replaced C300 with a 10uF capacitor and it did not improve the behavior. I attempted the same change with a brand new board (10uF for C300, all other things the same) and no improvement was seen there either. The behavior was the same for the output voltage and feedback. This behavior is now consistent with 3 boards.

    Attaching the input voltage screenshot and a picture of the input current on startup. The input was 30V.

    Current:



    Voltage:

    Thanks,
    Anthony

  • 0 in reply to Anthony Mitchell
    TI__Genius 12040 points

    Hi Anthony,

                     The scope shot showing the input current is interesting. Does the ringing die down eventually? I am concerned about the bump at the 4mSec mark (from the left) where the current seems to go negative. This indicates flow of reverse current which could damage the device. Would it be possible to add a diode at the input to prevent the flow of reverse current? Could you verify Method 1 (Schottky Diode) or Method 2 (Diode before LDO) described in the following link: https://e2e.ti.com/blogs_/b/powerhouse/posts/ldo-basics-preventing-reverse-current-in-ldos on a new board and verify if the issue remains? It is interesting that the Input voltage takes around 40mSec to reach the final value, but I don't see anything in it that could cause abnormal behavior. Thanks!

    Regards,

    Srikanth  

  • 0 in reply to Srikanth Lolla Venkata
    Prodigy 10 points

    The ringing does not die down eventually (if I am understanding you correctly). I believe what is seen on the right side of the input current is expected operation, as the regulator powers the 12V portion (DVRCC, VCC - Input between -0.3 and 20V) of a NCP1034 PWM Buck Controller. Perhaps the load on the output is causing the issue? I included a zoomed in shot of what is seen on the right. No changes to the voltage seen, and the output current here is consistent with what was seen before the diode as well.

    We added the diode (Method 2, before LDO) and this does indeed eliminate the negative current seen, but the output of the regulator remains the same as before. I used a brand new regulator and installed everything before powering on.

  • 0 in reply to Anthony Mitchell
    TI__Genius 12040 points

    Hi Anthony,

                     If the timescale of the Scope shot is 20uSec, it looks like the input current ripple frequency is ~100KHz. Does this match the switching frequency of the Buck regulator? Could you verify if the LDO is functional if the load (Buck controller and any other) is removed? Thanks!

    Regards,

    Srikanth