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TPS786: Regulator shuts down during transient turn on

Part Number: TPS786

Dear TI,

We are using a TPS786 (SOT223 package) in numerous locations in an active design. However, during temperature testing we have observed consistent shutdown of the TPS786 that is normally off and is turned on for a measurement during circuit operation.  All the other regulators (of exactly the same schematic design) are permanently enabled and show no issue.  At temperatures above around 70C we see that when we enable the TPS786 the output drops to around 220mV.  Cooling the board does not restore output. Only turning the regulator off and on again makes a difference. We have increased output resistors such that R2 is 30K and tried a 15pF compensation capacitor on R1 - makes no difference. We have also shorted the ferrite on the input that sits between the input capacitor and the regulator (that was in place during initial testing).  If we set the regulator to be permanently enabled, then it works up to 85C (we have soaked for ~2 hrs with no issue).  

Input capacitor : GRM21BC71E106KE11L

Output capacitor : GRM21BC71E106KE11L (same as input)

The ESR of this capacitor at v.low freq is >3 ohms, which according to figure 22 on the datasheet suggests that the regulator could be operating in a region of instability? It does depend on which frequency you read off the ESR as it is lower at higher frequency.  Pretty much all ceramic 10uF capacitors have similar ESR at v. low freq.  You need to move up to 22uF to lower this value.

Any thoughts or suggestions would be gratefully received as this is a serious issue for us at the moment.

Kind regards

Mike

  • Hi Mike,

    Do you have scope shots to show this behavior, and is it possible to share the layout of this abnormal regulator? 

    Have you tried replacing the regulator that is behaving like this with a fresh one?

    ESR is constant over frequency - see the graphic below from the Digikey forums. The impedance may be higher at lower frequencies, but the impedance at the resonant frequency (i.e. the ESR) is what is important for stability. So, the ceramic caps you have chosen are fine for stability. 

    Regards,

    NIck