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TLV700: Problem with TLV70033DDCR

Shlomi Yehezkia
Mastermind 6150 points
Part Number: TLV700

Hi Team,

I have problem with TLV70033DDCR:

Input +4.4V

Output shall be 3.3V

Output capacitor 100nF

In part of devices there is good stable output +3.3V

But on part of devices output is about +3.8V and on oscilloscope I see impulses.

Replacement of output capacitor does not solve problem.

Replacement of TLV70033DDCR solves problem.

Please advise what can be the reason of this issue.

Thanks,

Shlomi

 

 

  • 0
    TI__Guru 55940 points

    Hi Shlomi,

    In order for us to help debug, please provide a multichannel scopeshot showing Vin, Vout, Ven, and if possible Iout.  In particular we are interested in seeing the impulses that you mention.

    Traditional linear regulators such as TLV700 will not sink current; therefore, if the output voltage overshoots due to a transient or is biased higher than its regulation voltage, it is up to the load to pull the output back within regulation.  From your description, it sounds like you may be seeing the transient response of the LDO to the impulses.  If your load current is light enough, the output capacitors may not be being discharged back to regulation before the next impulse occurs.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Mastermind 6150 points

    Hi Ryan,

    I do not know what is higher enough.

    Output current in this mode about 3-50mA. Is it enough?

    I can capture only output static.

    Voltage appears when I connect USB cable. There is +4.4V on input. This is output.

    Blue line is zero.

    With previous batch everything was ok.

    With this batch with 100 nF capacitor problem occurs always

    With 1uF seldom

    With 4.7 uF on three devices it does not happen, but still under investigation.

    Thanks,

    Shlomi

    TLV700.docx

     

  • 0 in reply to Shlomi Yehezkia
    TI__Guru 55940 points

    Hi Shlomi,

    From your description it sounds like the TLV700 is not stable in your application.  When you are using a 0.1 uF capacitor, you are not meeting the minimum effective capacitance required to compensate the LDO.  As you increase your output capacitor's nominal value, you are adequately compensating the internal loops and are able to achieve a stable output.

    Please refer to Chapter 2 of our LDO Basics Blog for more information on Capacitor vs Capacitance:

    http://www.ti.com/lit/ml/slyy151a/slyy151a.pdf

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Mastermind 6150 points

    I agree, but with 1uF I faced the same issue. Seldom but anyway. Only with 4.7uF it looks stable and it is strange

  • 0 in reply to Shlomi Yehezkia
    TI__Guru 55940 points

    Hi Shlomi,

    You will need to look into the derating of your chosen capacitors under your application conditions.  Just because a capacitor states 1 uF does not mean that you will have 1 uF of capacitance under your use case.  Your statements about increasing the capacitor value reducing and solving the issue indicates that your specific application requires the higher capacitor values.  This will be due to how your selected capacitors derate and the transient response to the impulses you mentioned in your first post.

    Unfortunately without scopeshots showing the impulses mentioned in your first post, we are not able to further help you debug.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Mastermind 6150 points

    The input of this LDO is connector USB +5V. When I connect cable there are many spikes and I cannot capture it.

    Now with 4.7uF it works good.

    But it is very strange, that previous batch worked properly even with 100nF and new batch without any change needs 4.7uF.