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BQ25601: EMI question

Peter Xu
Expert 5115 points
Part Number: BQ25601

Hi team,

My customer used BQ25601 as battery charger and met EMI problem. They cannot pass the EMI test in PWM mode. They found that in PWM mode, the rising edge and falling edge are too sharp. When rising time and falling time are shorter than 7ns, they cannot pass the EMI test.  When they are longer than 7ns, they can pass the EMI  test. They tried to use 1 ohm + 1.5nF/3nF between SW and GND. And they found it useless to extend the time. So I got 3 questions here: 1. Can the consistency of the chip be reflected in the rising time and falling time? (Every chip has the same rising time and falling time.) 2. What parameters are the rising/falling edge related to？3. How can hardware and software be modified to meet EMI requirements?

Best Regards,

Peter

  • 0
    TI__Guru 54141 points

    Peter,

    1. There will be some variations.

    2. Please refer to Controlling switch-node ringing in synchronous buck converters @ https://www.ti.com/lit/an/slyt465/slyt465.pdf 

    3. The following actions may be helpful:
    1)Add 1nF PMID, SYS, VBUS (opt.), BAT (opt.)
    Recommend X7R 1nF (0402) which typically has > 300M resonant frequency. Please check with vendor for impedance response.
    The cap on PMID with minimal loop with IC is the top priority.
    2)Slower SW node switching slew rate
    Resistor footprint at BTST to adjust slew rate if needed.
    Higher resistor value (slower slew rate) can adversely affect converter efficiency.
    3)Use Snubber to reduce EMI
    Snubber (resistor & capacitor) footprint at SW to control EMI source
    Components must be placed closest to SW to be effective
    4)Use shielded output inductors.
    5)The load connection is also very important. An external wider and shorter wire may improve EMI performance.

    Thanks,

    Ning.