We are using the LMR23625CQDDARQ1 part in our design where the switching frequency is 2.1MHz fixed,
We are using a 4.7uH part for improvement in EMI EMC test and we found the switching frequency is not consistant for 4.7uH part which is trying to shift to left i.e around to 1.6MHz, than it goes back to 2.1mHZ.
Please, let us know the reasomn for this shipment and also let us know how to select the proper inductor part for better performance.
Hello, Andrew
Schematics
Layout
S
Switching Waveforms
FFT with near field probe on L7
4. Inductor has saturation current of 4.6A and Peak current of 4.7A
5.Vin is 12V, Vout is 4.2V, and Iout is 400mA typical, peak is 2A.
Hi Dinesh,
I have reason to believe you are hitting the frequency foldback region of the device. Essentially the device has a on time minimum where the topside FET must be turned on before it's shut off. If the frequency is high enough (2.1MHz is pretty high), then the ton/toff ratio may be too high for outputting 400mA at 4.2V. If you add more load to the device, then this problem will most likely go away. This phenomenon is called frequency foldback mode. It's detailed here on the LMR23625 DS:
For more information, please check this technical document:
https://www.ti.com/lit/an/snva857/snva857.pdf
Thanks,
Andrew
