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TPS61200 startup voltage

Thomas Richter
Prodigy 10 points
Other Parts Discussed in Thread: TPS61200

Hi all,

I've done a TPS61200 based design, the schematic is the one you can see in the datasheet on page 15, the PCB layout is based on the TPS61200 EVB. I'm quite happy with my design except a strange behaviour of the startup voltage.

If I ramp up Vin from 0V to 1V the TPS61200 starts at 0.8V. Now I ramp down from 1V to 0.2V and the TPS61200 stops operating at 0.3V. If I ramp up again (from 0.2V) the TPS61200 starts at 0.5V (like the datasheet says). If I ramp up again from 0V the TPS61200 starts always at 0.8V (which is too high for my application).

Any hints on this?

Regards
Thomas

  • 0
    Genius 3295 points

    Thomas,

    What are your resistor values for the UVLO voltage divider, R3 and R4? Did you account for the UVLO pin leakage current?

    The data sheet says you need to have 100 times the UVLO leakage current flowing through the resistor divider. It also lists the maximum leakage at 0.3 uA, which is thirty times greater than the 0.01 uA it lists as nominal. So if you happen to have an IC that has the maximum leakage current, your resistors may be too large and the leakage current is distorting the threshold voltage.

    The data sheet says you should have a 250 K ohm resistor for 0.01 uA, so for 0.3 uA you need an 8.2 K ohm resistor.

    Are you just turning the input power on an off? or are you ramping up real slow in both cases? It may be you have a ground bounce when you are 0V and just turn on the power supply. Just a guess.

    What if you just tie UVLO high? Active 250 mV reference voltages are not usually very reliable in my experience. The data sheet shows a voltage swing of 235 mV to 265 mV, or 6% . It's hard to get an active reference circuit to work accurately below a bandgap voltage. You can make a voltage divider reference using a matched resistor pair that can be better than 1% over temp. Then use a comparator circuit to trip the PWMIC enable pin.


    -Chuck