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TMS320F28035: RC values for low-pass filtering

John Pan
Intellectual 990 points
Part Number: TMS320F28035
Other Parts Discussed in Thread: DRV8343-Q1

Hi,

I have a question on RC values for TMS320F28035 MCU low-pass filtering used in DRV8343S-Q1 EVM board. The current sensing pins, ISEN_A, ISEN_B, ISEN_C use R=56 ohms and C=2200pF, which leads to very fast time constant, about 120 ns. I check the datasheet of TMS320F28035, ADC conversion time is 217 ns in Table 3-1. Assuming 1% error, 5 times RC equals to 600 ns, which is larger than the ADC conversion time of 217 ns. I am wondering whether the C of 2200 pF is too large in this EVM board design. Please advise.

Another question is which frequency the DRV8343S-Q1 EVM design try to filter? For 45KHz PWM, it seems to me that above 45KHz frequency should be filtered. If this is the case, RC should be about 1/(5*f_sw) = 4.5 us. From this perspective, both R and C should be larger. My understanding is that C value should be larger than C_SAMPLE inside the MCU so that C_SAMPLE can be fully charged. Does the MCU has a maximum RC value limit? If yes, what is it?

Thus, I am confused. I will appreciate your insight in RC filter design for ADC.

Thanks,

John

  • 0
    TI__Guru 62975 points

    John,

    Since that is a DRV8343-Q1 EVM, you might be able to get more clarity with respect to the EVM design decisions directly from the Motor drivers forum.

    According to the design files that I found for DRV8343S-Q1, the MCU is an MSP430x device, not F28035.  If you confirm that MSP430x is used on your EVM, you can also post your ADC-related query to the MSP430 forum.

    If the device really is F28035, I recommend this ACQPS approximation section in the F2806x TRM.  This information will be added to the F28035 TRM when it is refreshed in the future.  There are high and low bandwidth approaches for matching the RC filtering to the ACQPS sample window.  I assume that the EVM is following the high bandwidth approach.

    -Tommy