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TPS62147: Input filter necessary?

Matthew Findlow
Intellectual 880 points
Part Number: TPS62147

Hi, I need a buck converter for a board which has a VHF packet radio transceiver. I'm bucking from 14V to 5 V. The 5V feeds a 3.3V LDO which powers the transceiver.

The five volts also powers USB host, (500ma out), and is for this that I need the buck, otherwise I would have just used an LDO.

I have chosen TPS62147 because the data sheet says it has "very low output voltage ripple, minimizing interference with RF circuits."  Also, most of the time, the board will only be drawing 10 to 20mA and the DCS

topology can slow down its frequency, maintaining efficiency, but most importantly reducing noise that may effect the radio.

Webench design has a 1.5uH inductor, but I thought a 2.2uH would be better because it has lower ripple (perhaps less RF noise as well or not??) The higher inductance means a higher frequency. At 10 mA this is a good thing as it means less ripple out, but at 500mA it means 800kHz instead of 600kHz which could make more EMI?? 

Do you think I would benefit from an input filter ?? If I use an input filter with Fo of 800kHz would it have deleterious effects on the buck converter as it moves to lower frequencies,?

Would It be beneficial to isolate the ground planes between the buck converter and RF module with a ferrite bead??

Thanks for taking the time to help me out.

Cheers

Matt.

  • 0
    TI__Guru**** 191445 points

    Matt,

    You did not indicate your FSEL condition, high or low?  I would recommend using the values in Table 3 of the datasheet  for your output filter.  1 uH and 22 uF if FSEL is high, 2.2 uH and 47 uF for FSEL low.

    The switching converter will impress ripple voltage back on to the input voltage rail.  The ripple is proportional to the output current.  A basic input capacitor is required, 10 uF ceramic minimum.  If you need additional attenuation, you may use an additional filter.  Adding just capacitance may be sufficient.   If you use an LC filter, make sure you include an electrolytic capacitor between the inductor and the input ceramic capacitors.  Value is not critical.  I usually pick 47 uF or 100 uF.  The corner should be much lower than the switching frequency.  As an example, 1 uH and 100 uF = Fc of15.9 kHz.

    I generally do not recommend ferrites in the ground path.

  • 0 in reply to JohnTucker
    Intellectual 880 points

    Thanks John, I'm using FSEL low of 1.25Mhz, but with 2.2uH inductor at my max currrent draw of 0.5A webench says the max frequency will be 800Khz.

    I asked about the input filter, cos I read in a webinar that DCS controlled bucks didn't need input filters to attenuate EMI.  Sounded a bit fishy to me, so I posted the question. 

    For EMI filter values are you suggesting:  

    Supply+ ---- [ 100uF(MLCC) --- 1Uh ----- 100uF(electrolytic) ]   ----->  10uF(MLCC) -- IC Vin

    or just :

    Supply+ ---- [  1Uh ----- 100uF(electrolytic) ]   ----->  10uF(MLCC) -- IC Vin

    [ EMI filter inside square brackets ].

    I assume the first one where the electrolytic is just a damper for the filters high impedance at resonant frequency ??

    Thanks.

  • 0 in reply to Matthew Findlow
    TI__Guru**** 191445 points

    Matt,

    Yes option 2 is what I have seen typically.  I would not be surprised if there is additional up stream capacitance as well.  Is this your primary supply taking its input from an external  source?  If so I typically will have an electrolytic right at the input to the PCB.  If you are super concerned with EMI, you may want some HF bypass directly at the converter input.

  • 0 in reply to JohnTucker
    Intellectual 880 points

    Thanks John.  sorted.