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TPS51200 Tracking Startup Overshoot

Will Knox
Prodigy 20 points

According to Figure 17 of the datasheet, I should be able to drive EN high before REFIN and VLDOIN come up.  However, when I do this, the output (VO) overshoots to about 0.55V when REFIN crosses the ~0.4V UVLO threshold.  Note that when I raise REFIN above the UVLO threshold and then drive EN high, the output rises monotonically to its final level without overshooting.

I am using the recommended 3x10uF output capacitors, and as you can see, the input capacitors are doing their job holding the input steady.

How can I use the supply as shown in Figure 17 without getting 150mV (37.5%) of overshoot?

  • 0
    Prodigy 20 points

    I just figured out that we are putting ten times the recommended capacitance on REFOUT, which is the reference voltage for VO. VO tracks REFOUT very nicely, but REFOUT overshoots when REFIN smoothly ramps up, and that overshoot propagates to VO.

    When I only put the recommended 0.1uF of capacitance, the problem goes away.


    When EN is brought up after REFIN rises above UVLO, REFOUT still overshoots, but that isn't propagated to VO since it happens before VO comes up.