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TPS7A94: Output voltage is higher than programmed be reference resistor Rnr/ss

Leonid Sapozhnikov
Prodigy 50 points
Part Number: TPS7A94

Tool/software:

Hello, I have common design to use for different voltages. Some of them above 1.7V (minimum input voltage) and all output voltages observed as expected. Lowest working tested 1.8V.. However when I supply lowest voltage of 1.7V in order to to make 1V or 1.2V I can not get desirable output. I traced it to voltage at NR/SS pin used as reference.
Per data sheet current on this pin is 150uA and you select desirable voltage, however I can deduct that current on this pin higher than advertised.  I can see it has additional current source for fast start which is more potent and can supply higher current, which is probably I observe. Not sure how to dialable this feature. I have week pullup on EN_UV, and I do not control startup. Also my input voltage just under 1.7V (1.695V). Is it possible that being lower that 1.7V keep Ic in mode where it provide additional startup current and keep device with incorrect output voltage. Thank you

  • 0
    TI__Mastermind 35105 points

    Hi Leonid,

    Your FB_PG resistors are probably not set correctly.

    The FB_PG resistors set both the current limit value and also the value of Vout where the internal fast charge circuit switches over to the steady state current source.  The fast charge current source is higher than steady state current source.  Thus, during turn on the "reference voltage" is set higher than steady state and this combined with the NR/SS resistor and capacitor form an RC time constant that ramps faster to your desired Vout during turn on.  If the FB_PG resistors are not configured to not change over the LDO from fast charge to the steady state current values properly, Vout will overshoot the intended setpoint.

    FB_PG pin is compared to 0.2V internal to the LDO.  If you have 1V on the output and you wish to change over from fast charge to steady state charge at 95% of Vout (0.95V),  then you must configure the resistors to set FB_PG to be 0.2V when Vout = 0.95V.  This is a basic voltage divider problem.  The bottom setpoint resistor should be chosen to program your current limit threshold (per the datasheet, 12.4k = 100% current limit), and the top should be chosen to set the fast charge to steady state changeover voltage.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 50 points

    I see, I suspected something like this, and I know I have incorrect Rfb_good value which I selected originally for 5.3V design. Will try tomorrow and I hope it will resolve it. Thank you so much for quick response. Regards, Leonid

  • 0 in reply to Leonid Sapozhnikov
    TI__Mastermind 35105 points

    Hi Leonid,

    Thanks for the update, and I'm glad this solved the issue! Let us know if you need anything else.

    Thanks,

    Stephen