Electrical – Buck boost converter output voltage dropping over time

boostbuckpower supply

I have a TPS61222 Buck boost voltage converter, in a circuit intended to boost a 3.7V battery source to 5V. The output voltage however, is consistently less than 5V – scoping shows a 4V signal with spikes up to 6v at about 90kHz, low duty cycle.

The current draw is 56mA nominal (into the converter). The converter powers a charge pump inverter, to generate -5V, along with a number of other devices. The feedback pin voltage should be o.5V, with about 13mV tolerance (as per data sheet) – the value it is showing is more like 0.35-0.42V.

Full schematic and PCB layout are here

Best Answer

You are trying to run a charge pump from another switching device. In general, it is best not to run this type of device from the output of this type of converter, when the capacities and switching frequencies are close. The TPS61222 relies on voltage feedback to regulate, and your LM2663 is providing a load which is changing at its switching frequency.

I would try to run the charge pump generating your negative voltage directly off VBatt as a first choice. If this is completely unacceptable, you could add a cap to the feedback point of the boost converter, although this will slow down the converter's response to changes in input voltage and output load, possibly causing overshoot and undershoot.