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TPS630242: current limiting and output instability

frank boddeke
Prodigy 30 points
Part Number: TPS630242
Other Parts Discussed in Thread: TPS63024

hello we use a current limiter (250ma) to charge a large cap (3mF) to 5v and then use the TPS630242 to make 3.3v out of it. we choose the buckboost so we can catch large current spikes / make use of more of the large capacitance (from 5v down to 2.5v)

we now got an instability where all energy seems to be "burned" when the output is loaded with 16.5 ohm: the max current (250ma) is drawn on the 5v input (by the limiter) but the cap is only charged up to about 2v. the output is only 2.5v average (and oscillating, vp-p  about 1 v). so we got 5v x 0.25a = 1.25w going in and only get 2.5v x 0.15a = 0.38w out.

when loaded with only 33ohm we get a nice stable system with > 90% efficiency

two questions:

- what is causing this instability / burning up all energy? is it the "Output capacitor discharge function" kicking in all the time?

- if i start by loading the output with 33 ohm and later, after the big cap is "full", switch to 16.5 ohm (adding another 33ohm resister in parallel) it works fine. can i delay the "enable" of the buckboost by 100ms?

thanks

frank

  • 0
    Prodigy 30 points

    16.5 ohm load (yellow: big cap voltage, cyan: output voltage)

    33 ohm load (10  ms range)

    33 ohm load (200 ms range)

  • 0 in reply to frank boddeke
    TI__Mastermind 30630 points

    Hi  Frank,

    Could you also share your schematic here?

    Regards

    Tao

  • 0 in reply to frank boddeke
    TI__Mastermind 30630 points

    Hi  Frank,

    In your application, device start switching once Vin is higher than UVLO_ON, once start-up, device will draw a large current to charge output cap which means it will make your input voltage  decrease and UVLO_OFF triggered and device shunt down. 

    I am not sure if you can make device start after C5/6/7 full charged, such as use some pin of MAX device to control EN of TPS63024. Then, this issue will be fixed.

    Regards

    Tao

  • 0 in reply to Tao Zhang
    Prodigy 30 points

    thanks tao,

    i think an low pass filter on the input of the current limiter should do it? maybe 100k resistor charging a 5uF capacitor on the EN pin. it would be nice to use the "output capacitor discharge function" to also discharge the 5uF, maybe with a diode. if you have anything that you know works i am very interested. otherwise we will figure it out

    cheers

    frank

  • +1 in reply to frank boddeke
    TI__Mastermind 30630 points

    Hi  Frank,

    I think RC filter you mentioned will work. may be you can try with a much smaller cap first, such as 1uF.

    Discharge this cap by BUBO is not a good idea, first, EN will be clamped at a low level when Vout is 0, Vout will charged by this resister firat, it will take a long time.  Second, output discharge funtion only available when EN is low. But it is a big gap between EN_low and actual EN voltage.

    May be another better option can be a zener diode (such as 2V) connceted between Vin and EN, Then a resister in parallel with a cap connected between EN and ground.(here the resister and cap can be smaller one, may be 1k+100nF).

    Regards

    Tao

  • 0 in reply to Tao Zhang
    Prodigy 30 points

    okay, nice, so the zener is connected to the big caps / tps' vin and will start charging the "delay" cap once vin reaches 2v and will charge the cap up to 3v (5v minus 2v zener). 

    maybe use 3v zener: start charging "delay cap" at 3v and up to 2v

    can we also discharge the "delay" cap with a zener in the other direction? maybe 0.5v zener? --> charge up to 2v after vin > 3v, discharge down to 0.5v after vin < 2.5v

    does this make sense?

    thanks

    frank

  • +1 in reply to frank boddeke
    TI__Mastermind 30630 points

    Hi  Frank,

    For break down voltage of zenner, just make sure voltage of EN will higher than 1.2V in corner case.

    For discharge,  no need for another zener.

    Regards

    Tao