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LMR23630-Q1: Switching Frequency coupling into the voltage input

Part Number: LMR23630-Q1
Other Parts Discussed in Thread: LMR23630

Hello TI,

We are using the LMR23630-Q1 for one of our applications. 

A 10n and 33p Capacitor is used to filter GSM frequencies and above. This filtered 12 V signal is connected to the VIN of the LMR23630-Q1. The two 4.7u capacitors at the input were based on the design suggested by TI Workbench. 

I have found various application notes (12)  from TI regarding designing an input filter to reduce conducted EMI. 

However, where do I begin with designing the EMI filter?

How do the GSM frequency filter caps and VIN input capacitors affect the EMI filter performance? 

Any inputs would be greatly appreciated! Thank you

  • I think that the app notes you have are a good start.

    The EMI performance will depend on both your input filter and your PCB layout.

    The following link gives an example design for this device :

    www.ti.com/.../PMP15016

  • Hello Frank,

    Thank you for the prompt response. I have a few further queries I hope you could help me with.

    We are using two units of the LMR23630-Q1. The Vin remains at 12V, but one converts to 3.3V with a maximum load current of 2A and the other to 3.8V at around 3A.

    1. Is it sufficient to add one LC input filter at the Vin? Or do I have to account for the two DC/DC convertors and have one LC filter each at the input stages of each convertor?

    2. The 3.8V DC DC Convertor has a bulk capacitor of 47u with impedance at 100 kHz of 680 mOhms. This was done as per the instructions in the data sheet which mentions the need for a bulk capacitance if the Vin input is greater than 5 cm away from the input stage of the DC DC convertor (to damp voltage spike due to trace inductance). What additional precautions should be taken considering this bulk capacitance when designing the input filter.

    3. Are the simulation tools for input filter design provided by TI like the WeBench for power stage design? Is simulating the LC filter with the LMR23630 possible in LTSpice etc?

    4. What parameters apart from physical size and current rating should be taken into account when selecting the inductor?

  • One input filter should be OK for both regulators.
    The bulk input cap should be OK, along with the filter.
    You will have to check Webench for this device and see if the input filter calculator is available.
    Otherwise you can use an ordinary simulator to test the filter.