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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.
Note that the part on the board is TPS786 and not as per schematic, the part in the schematic was not available at the time of build
Any thoughts or suggestions would be gratefully received as this is a serious issue for us at the moment.
Kind regards
Mike
Hi Mike,
Could you provide us scope shots showing VIN, VEN and VOUT at this high temperature when the device is turned on? Could you confirm if this issue has been observed across multiple units? The selected ceramic caps look fine to me and should not cause instability. Is capacitor C223, shown in the schematic, in place in the actual setup? Thanks!
Regards,
Srikanth
Also, yes, this issue has been observed across multiple units and is quite consistent behaviour.
Hi Mike,
Does this scope shot correspond to a unit that is not providing an output voltage at 70ºC? The scale of the traces in the scope shot image are not clear to me, so I am not able to read the value of the voltages at the OUT, IN and EN pins. It is also possible that the device is hitting thermal shutdown while starting up at 70ºC. Could you provide a screen shot with better resolution that also shows the input current? What is the load current on the units that start at 70ºC? Finally, C223 being in place means that the effective feedforward capacitance is in excess of 10uF, which is quite a lot. We typically do not add more than 100nF, could you uninstall this cap and verify if the issue remains? Thanks!
Regards,
Srikanth