Other Parts Discussed in Thread: BQ24195, TPS61232
Dear Sir,
I found and issue on a power supply that includes the battery manager BQ24195 (Vin 12V, Vout 3.7V) as primary power supply and the DC DC TPS63021(Vin 3.7V, Vout 3.3V) as local power supply connected in series.
The issue is concerning the component startup: the TPS63021 startup current force some limitation in BQ24195 output soft start. I found out that the input current limit is fixed to 100 mA till the output voltage does not reach 2.2V, while the expected limit current we set by external resistor is higher (2.4A OR set by I2C register). The TPS63021 has a soft start feature that is not controllable, the input current is limited till the output voltage is lower than 2.2V. The lock condition is verified when the BQ24195 is stuck to 1.8 V with current limit and the TPS63021 has already finished the soft start and it sinks current without limitations. The event is not regular, it is random.
In order to solve the issue, we used the EN pin of TPS63021 to arrange the power supply timing, by inserting an analogue delay RC. In these way I have carried the BQ24195 out of the limitation zone and then power on the TPS63021.
Actually, in our design and production board we have a 100nF connected to the TPS63021 EN pin and powered by VINA pin (maximum allowed .022uF). From TPS63021 datasheet I assume that there is a resistance between VIN and VINA pin, so the best way I can arrange the circuit is increasing the capacitance on EN pin, but I have the limitation of VINA (otherwise, I should rework the PCB or run a redesign).
I tested this configuration out of specs with a 10 uF capacitance and the circuit works well and the power supply timing is correct.
Actually I would know what kind of risks are connected to higher output capacitance on VINA pin .
Thanks for support
