I was reading through the datasheet of a boost converter, TPS61023 from TI.

In the "Input Capacitor Selection section", they give the following advice:

8.2.2.5 Input Capacitor Selection

Multilayer X5R or X7R ceramic capacitors are excellent choices for the input decoupling of the step-up converter as they have extremely low ESR and are available in small footprints. Input capacitors must be located as close as possible to the device. While a 10-μF input capacitor is sufficient for most applications, larger values may be used to reduce input current ripple without limitations. Take care when using only ceramic input capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, a load step at the output can induce ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or could even damage the part. In this circumstance, place additional bulk capacitance (tantalum or aluminum electrolytic capacitor) between ceramic input capacitor and the power source to reduce ringing that can occur between the inductance of the power source leads and ceramic input capacitor.

Notably, TI advises against using only a ceramic capacitor at the input, citing that it could induce ringing when a load step is applied at the output. The recommended solution is to place an electrolytic bulk capacitor between the ceramic capacitor and the input pin of the boost converter.

I am confused why this advice is given. The claim is that is reduces ringing caused by the inductance of the input power trace, but electrolytic capacitors usually have a worse ESL/ESR than ceramics. Nowadays, ceramics can have a comparable capacitance to the electrolytics as well. I know that ceramic capacitors can have a piezoelectric effect but I have not seen that cited as an issue for SMPS input capacitors.

So, what is the reason for this manufacturer advice?

posted 4 months ago

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4mo ago by Nick Alexeev‭

Mu3‭

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