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BQ24250: BQ24250RGET creates 300MHz EMC noise

carlosgdj07
Prodigy 240 points
Part Number: BQ24250

Dear all,

I am using BQ24250RGET as SAI chip (charging Li-Ion cell 3V7 and powering the system) in an embedded PCB which includes a ST32 uC working at 72MHz and a GPS/4G SoC from SimCOM with LTE antenna which takes power from BQ24250RGET. Here my IC connection:

BQ24250RGET works fine in terms of managing Li-Ion cell and powering the system but, when recharging the battery from 5V main power source at 400mA rated current, it is created in the PCB a 300MHz noise which is not present when the battery is not charging...

I know EMC radiations are highly dependant on PCB layout but, taking into account that this IC's fixed frequency ranges from 2.7MHz to 3.3MHz my question is... how is possible that I had a 300MHz noise?? Its a 100th harmonic and it is supposed to be very little in amplitude. Everything in the PCB is switched off during this test (SimCOM module and ST32 uC), only this chip recharging the battery..

Any idea? Any of those of you who were using this chip have experimented similar kind of noise?

Best regards!

  • 0
    TI__Guru*** 149235 points
    Carlos,

    Try adding a 0.1uF in parallel with C7 as close to IC PMID and GND pins as possible to filter switching noise.
    If that doesn't work, try adding a 1-10 ohm resistor in series with C1 to slow down the SW rise time.
    If that doesn't work, you can add an R+C snubber from SW to GND, close to IC. The RC needs to be sized based on measurements of the ringing at the SW node rising and/or falling edges. When measuring on an oscilloscope, you must disable BW limit and use a ground pigtail not a long ground wire connection. A simple method for sizing the RC is explained here for a boost converter but works the same for a buck converter: www.ti.com/.../slva255.pdf.