Hi everybody,
I have a problem and someone here might have some answer, so - in one of our company designs, there is an old voltage regulator with LM2575S (design is about 10yo and it was in production for that time, working absolutely fine). Recently, when we connected EM4095 (RFID reader), entire regulator went into oscillations (there is about 2kHz and 500mV ripple on the output voltage).
I discovered, that old LM2575 we used had about 55kHz internal osc. freq., but new batch has 64kHz. And since the RFID chip works at almost exactly second harmonic (125kHz), my theory is that this causes unstable feedback. Regulator works fine with resistive loads and RFID has no problem working with old 55kHz regulator.
Does anyone know, if they changed frequency of the internal oscillator for LM2575S? I can't even find datasheet that would say anything about frequency above 60kHz. I am going to try some things (probably notch filter), but so far, just adding capacity seems to make it worse.