Boost Converter (TPS61322): Loop Stability with Large Capacitors

Question

I am using a TPS61322 boost converter to convert a minimum of 1.3V (single-cell alkaline) to 3.3V (using the TPS613221A version). This device needs two passives: an input inductor and an output capacitor.

^{simulate this circuit – Schematic created using CircuitLab}

I am powering an ESP8266 module. It runs at around 60mA with sudden 500mA spikes, all at 3.3V. The above example is from the datasheet, but it is meant to supply 50mA at 2.2V, not 3.3V. The datasheet always recommends small (10-22uF) ceramic capacitors.

A common recommendation is to place a large capacitor (1000uF electrolytic) between the supply and ground of the ESP8266 to smooth out the power spikes. However, I want to satisfy the need for this large capacitor and the need for these small ceramic capacitors in the same place in my schematic.

^{simulate this circuit}

The TPS61322 datasheet gives the following advice:

For the output capacitor at the VOUT pin, small ceramic capacitors are recommended. The capacitors should be placed as close as possible to the VOUT and GND pins of the device. If, for any reason, the application requires the use of large capacitors which cannot be placed close to the device, the use of a small ceramic capacitor with a capacitance value of 1 μF in parallel to the large one is recommended. This small capacitor should be placed as close as possible to the VOUT and GND pins of the device.

It's a bit ambiguous if the authors are referring to a "big" capacitor with a high capacitance value, like an electrolytic, or simply a capacitor that is far away from the pins. My understanding is that electrolytics with high capacitance values have a "slower response time". I am worried that the 1uF ceramic won't keep up with the switching of the boost converter.

My question is thus: Will a single, tiny ceramic paired with a massive electrolytic allow the boost converter to function properly?

The TI datasheet has equations for switching frequency and maximum inductor current, but the authors seem to give ballpark values for the capacitor values. L/C circuits are not my thing, and I'm hoping that I was right to select "Loop Stability" as a buzzword in the title.

Answer 1

The datasheet is quite clear, it says that basically you always need a small ceramic capacitor near the converter.

With those high frequency converters, you always need a ceramic capacitor close to the pin, Electrolytic doesn't respond well to high transient, it might work for a while but the cap lifetime will be greatly reduced.

So yes, put a ceramic capacitor, if you have the space you can put a 10uF ceramic and then the electrolytic "bulk" capacitor.

You need however to check if the converter has a trip current, as your big electrolytic will be the equivalent of a short at power-up, the converter might go in trip mode as it would think it's shorted or couldn't provide enough current to startup the initial charge, this might be solved with an inductor or better some current limiting circuit between the ceramic and electrolytic cap.