Other Parts Discussed in Thread: TPS61291
Hello,
We have a small, low-power device with a sub-1GHz transceiver and MCU (CC430F5xxx series), where power is supplied from a coin cell (CR2032 and/or CR2450) battery. The device sleeps most of the time and does not require a regulated output, and regulator is only enabled when a regulated rail is desired (chiefly RF communications) or when battery voltage is too low to squeeze some more battery life out of it. Whenever the regulator is not enabled, the device is powered directly from the coin battery thanks to the pass-trough capability of the TPS61221.
Whenever we enable the voltage regulation, the high inrush current causes the battery voltage to dip steeply due to the high ESR of coin batteries, which causes the output voltage to dip with it and causes a brownout event and erratic behavior. We have tried adding large bulk capacitors parallel to the battery and an 680uF battery solves this issue and a 330uF battery alleviates it to an acceptable level. However, these bulk capacitors are larger than we can accommodate in the product, therefore we would like to be able to combine it together with a method to reduce the inrush current. According to the answer to a previous question on this forum, application note SLVA307B is not applicable here due to the pass-through capability. We have done some quick tests with the soft start extension circuit proposed in SLVA307B just to see, however it drove the regulator out of stable operation.
Below are a schematics and the measurement results with and without bulk caps, and with the soft-start extension circuit:
Voltage measurements at the probe points marked on the schematic:
When a 330uF tantalum bulk capacitor is soldered parallel to the battery:
Voltage measurements at the probe points marked on the schematic:
And finally, measurements with the soft-start circuitry from SLVA307B added to the feedback loop:
Voltage measurements at the probe points marked on the schematic:
To summarize, we are looking for a method, preferably without adding or changing any ICs, to reduce the inrush current/extend the soft-start operation of TPS61221, so we can use it together with small coin cells without the need for excessively large bulk capacitors. We are more than open to any suggestions for alternative ways to approach this issue as well.
Cheers!,
İsmail
