BQ24765: How to improve EMI ?

Hi Jerry,

First, I recommend a low noise EMI test setup. We have found that cables from the input power source, load and/or battery to the circuit need to be short and/or twisted. Adding a small ~0.1 uF capacitor at the connection point for these cables is helpful in reducing their contribution to EMI.

Can you send a zoomed in oscilloscope shot of the PH node, without BW limit? The voltage probe needs a very short ground connection. Is there ringing on the square wave edges? If so, then the board layout may need to be improved. As with all integrated FET buck converters, the close placement of the input capacitor from DCINP to PGND is the most critical. Adding a small 0.01-0.1uF in parallel with the the DCINP bulk capacitor typically reduces EMI. Almost as critical in close placement is BOOT to PH capacitor. Next is the COUT capacitor placement and last is the inductor placement.

If the layout improvements above do not help, then adding a small, series BOOT resistor to slow down the gate drive rise time might help. This will slightly reduce efficiency.

Last resort is an RC snubber from PH to GROUND. This will also slightly reduce efficiency. The sizes of the R and C need to determined from empirical measurements of the ringing at the PH node. There are several articles on the web that explain this empirical measurement process. I wrote an article for a boost converter years ago but the steps apply to buck converters as well. www.ti.com/.../slva255.pdf