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LM46000-Q1: LM46000-Q1 switching regulator lower EMI solution ??

Part Number: LM46000-Q1
Other Parts Discussed in Thread: LM46000PWPEVM,

1.  We are trying to meet RE102 Ground emissions spec with a LM46000-Q1 switching regulator.  Noticed a blank part location between SW and GND on the LM46000PWPEVM evaluation board.  Using a 0.012uF capacitor between SW and GND, the regulator (and product) passed the RE102 specification. 

Questions:  Is this an acceptable design modification?  How can such a capacitor affect regulator performance negatively?  We noticed heat with a 0.022uF capacitor, and no noticeable heat with 0.012uF cap. 

Details:  5V, 100mA output.  6-40V input.  Cap would be 100V device.  

2.  Using WEBENCH to simulate LM46000-Q1 configurations. 

Questions:  What are the potential problems of running 20-35 degree phase margin?  What instability can occur?  How can instability affect circuit operation?

8078.Power sup..pdf

  • G.J.

    1. The blank pad was for a shottkey diode, as seen on this schematic (pg4, close to CBOOT), but you do not need this.

    The capacitor that you've added has essentially created slew-rate control.  The cap reduces the dv/dt on the switch node which reduces HF radiated emissions.  The downside to this implementation is the reduced efficiency of the system.  You're essentially charging and discharging this capacitor at the switching frequency.  0.022uF to 6V (best case) at 400kHz gets you an extra 160mW just in that capacitor alone. Plus conducted EMI will be worse due to the added switching current going to charge the cap.

    You can use other methods to reduce radiated EMI.  You can use a snubber (RC in series, coincidentally on those same pads) or use slew rate control by placing a resistor in series with the boot cap (not too big, on the order of tens of ohms max).

    2. Low phase margin will create ringing on the output during any sort of transient (See this graph. The overshoot amplitude is dependent on other things, but the settling time is applicable).  Transients can damage components if they are extreme enough or the components are sensitive enough. Plus the margins of the components can shift the bode plot to make your phase margin worse.  We recommend at least 45 degrees phase margin.