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TPS780330220DDCR - Output Voltage Incorrect

Hunter Long61
Prodigy 225 points
Other Parts Discussed in Thread: TPS780

Hello,

I am having some trouble with a TPS780 adjustable regulator. I am trying to regulate 5V down to 1.5V, but my constructed circuit is regulating it to 2.220V instead. I calculated that I should use a 1M resistor for R2 and a 1.5K in series with a 232K for R1. The datasheet also says that Vfb should be 1.216V, but I am measuring it to be 1.770V.

  • 0
    TI__Intellectual 960 points

    Hello Hunter,

    Your resistance divider looks to be fine.  

    It looks like you may have a leakage downstream of the LDO output, which could be driving the 2.22V. This also would cause a 1.77V FB voltage. (sometimes flux residues are enough)

    Another source of the problem could be, that your LDO is positioned in the regulation loop of the switcher. There it's possible, that you are exceeding the Vin max of the LDO.

    Further I strongly suggest to use a local input cap as well as positioning the output cap closer to the output pin. (see section 10.2 on page 23 in the datasheet for reference)

    You can test the leakage voltage by removing the load and ensuring the PCB is clean of flux.

    Please give me a feedback on this.

    Best regards,
    Nicolás

  • 0 in reply to Nicolás Román
    Prodigy 225 points

    Hi Nicolás,

    Thanks for the input! It is very helpful. So here is the rest of my circuit:

    The load is a 16:1 analog switch that puts different resistors in series with the regulator's 1.5V output. It is intended to calibrate a ammeter, the DC-DC converter is only used to float the 1.5V with respect to AGND.

    I measured the input to the TPS780 to be 4.279V and the O-scope shows no ripple on this voltage.

    -Hunter

  • 0 in reply to Hunter Long61
    TI__Intellectual 960 points
    Hi Hunter,

    Thank you for sharing the schematics. I suppose, that A1FeFe1 has no supply connected.

    Your schematics looks correct. Did you remove the load to test the LDO only?

    All the references of our LDOs get tested before they leave the production site. The reference is controlling the FB voltage, which leave two possibilities. Either the output get pulled up from somewhere else or the part was damaged.

    Best regards,
    Nicolás

    PS.: If you find the reason please let me know or verify the answer so we can close this thread. Thank you.
  • 0
    TI__Mastermind 22610 points

    Need your confirmation of the exact part number you are using.

    The TPS78001DDC is the adjustable version. To get a Vout of 1.5V, with R2=1 Meg, you want R1= 233.5 k

    • R1 = ((( 1.50V / 1.216V ) - 1) x 1 Meg) = ( 0.23355 x 1 Meg ) = 233.5 k

    So that seems OK as far as it goes.

    However, the TPS780330220DDC in this thread title is NOT adjustable. Vout is either 3.30V or 2.20V, depending on the voltage at device pin 4 (aka Vset). Driving the Vset pin >1.2 V (you measured 1.77V) will select the preset LOW output voltage (i.e. 2.20V).