in our card we've used a TPS63700 converter to generate a -15V output voltage starting from an internal 5.25Vdc.
We've already produced 100 cards in the past, without any problem.
On the last production batch, without any hardware change in the DC/DC or in the load, we've seen that sometimes the converter doesn't complete the power-on sequence and the output voltage follows the attached waveform.Purple line is the -15Vdc output, while the red line is the input current.
Yellow trace shows the voltage on IN pin (pin 5) of TPS63700
Current scale is 1Amps=>0.33Volts, therefore the peak input current is around 0.98A and the current pulse duration is 10-12ms.
Standing to our test, we've seen that the output voltage reaches -16.4V, then the switching stops and the output voltage falls down exponentially.
We've repeated the test many times and we've seen that 90% of power-up procedures are successfull, but sometimes the converter does.t complete the power-on sequence.
The static load of the converter is negligible (<20mA@-15V), therefore we think that this behaviour could be related to the internal current limiting logic.
Total capacitive load is 66uF, plus DC/DC Cout (composed by 5 x 4.7uF).
If we reduced the external load from 66 to 44uF, the power-on sequence is always completed successfully.
Do you think the cause of the problem is I>I_LIM=860mA?
Could you provide any suggestion in order to solve the problem on the produced batch?
In attachment you can find our circuit: