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About buck DC-DC Issue

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redvalorm

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Hi,buddy:
I have a project that uses buck DC-DC(2 in 1) for RF Power supply(Vin ->3.3V & Vin->1.8V;Current is about 300mA each), a strange phenomena happened:when I turn the Vin of DC-DC Supply from 3.6V to 4.2V,I found that the sensitivity of RF Part have changed from -80dBm to NG(can't measure).Why?
 

I understand, that the regulated voltages are still correct. If so, the converter is possibly generating strong interferences. You didn't tell about the used RF band, but generally using SMPS with RF could be an issue. At least an appropriate PCB layout and good filtering is required.
 

It could be a problem of PCB Layout. Whenever you have a Switching node, please make sure that the length (or Area) of the node on PCB board is as small as possible.
Otherwise it will act as antenna, which will interfere (emission EMI) with other circuitry.

One more problem could be the loop area. I dont have much knowledge of buck, but I have faced similar kind of problem with boost, where my Vout was getting effected (going down) as I increase the VIN because of the loop area. By minimizing the area of current loop, I have avoided the problem.

Isolate the ground from other circuitry.

Please follow the pcb layout mentioned in the data sheet.
 

Today i have a try again, I changed the DC-DC component,then I found the sensitivity have great improvement.New phenomena is that 3.6/4.2V(VIn) would't change the output current in good pcdboard(400mA),but in bad one,when Vin turned to 3.6V,the output current is 400mA;Vin turned to 4.2V,the output current drown to 380mA,So it's seem load effect the output current?
 

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