Question
The current layout uses a 10uF ceramic cap on the output, but due microphonic problems, I changed to a 4u7 tantalum cap.
Since the ESR is high (5R), I added a 100nF in parallel to lower the effective ESR.
However, the circuit oscillates at ~7 MHz. The ceramic's ESR is below the recommended 50 mR from 1-72 MHz.
What is the required frequency range or how can I determine a solution that will give an acceptable production yield?
Regulation is from 6V down to 5V, with 15kR and 4k7R regulator divider resistors.
Answer
The problem is that the tantalum cap is giving a zero at about 6.8kHz, and that is causing the instability. The 100nF cap doesn’t help offset this asit’s pole doesn’t kick in until about 300kHz. The design simply needs more ceramic output cap to be stable.
I’m surprised that microphonics are an issue here. The typical frequency range that microphonics would affect are well within the loop bandwidth ofthe LDO and would be practically eliminated by the loop action.
I’d recommend just going to a 4.7uF minimum ceramic cap instead of using the tantalum.