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TMS320F28379D: Vref dropping when ADC is switched on

Arun Kumar
Expert 1030 points

Part Number: TMS320F28379D
Other Parts Discussed in Thread: TMDSCNCD28379D, LAUNCHXL-F28379D

Tool/software:

Hello experts ,

We are testing our custom inverter hardware which is based on the TMDSCNCD28379D and LAUNCHXL-F28379D

VREFH in our board is derieved from the 3.3v supply.

The problem we are facing is that , the moment we enable the ADC voltage at VREFHIA,B,C,D drops from 3.02v to 2.81v

We confirmed this by deleberately toggling the ADC(ADCPWDNZ bit) and checking it.

At first we through , there isnt enough by bypass capacitance as the datasheet recomends at least 1uF , for 12 bit mode operation.

We soldered a 4.7 uF cap across C418 and C426 and rechecked.

Once again the moment we enable ADC A and ADC D, the reference voltage drops to 2.81v. The supply voltage (COMA_3v3) is a constant 3.3v and there is no change in its value at all.

We don't violate the current requirments as only 130 uA is required by the VREFHI pin.

Any idea what can be causing this issue?

  • 0
    TI__Mastermind 28580 points

    Hi AK,

    What is the purpose of the 1 Ohm resistors between VREFHI and 3V3? This is likely causing the unwanted voltage drop.

    Thank you,

    Luke

  • 0 in reply to Luke Jones
    Expert 1030 points

    Hi Luke, Thanks for your reply!

    Where do you see 1 ohm resistors? R551,R552,R553 and R554 are 1 Kilo-Ohms and they form a voltage divider with R547, R548,R549 and R550 respectively.

    The 3.3v is stepped down to 3v and then fed into the VREFHI pins of the mcu.

  • 0 in reply to Arun Kumar
    TI__Mastermind 28580 points

    Hi AK,

    My mistake, I meant to say 1K Ohm.

    A voltage divider is not a reliable approach to step down the 3.3V to 3V. This depends on the input resistance of the VREFHI pins staying the same. Something changing within the device (such as enabling the ADCs) is likely to lower the input resistance of the VREFHI pins which will change the voltage observed at the pin when using a voltage divider approach.

    What is the reason to step down the 3.3V to 3V? If this is absolutely necessary, you would need a buffer to maintain this voltage at 3V.

    Thank you,

    Luke

  • 0 in reply to Luke Jones
    Expert 1030 points

    Hi Luke,

    Luke Jones said:

    What is the reason to step down the 3.3V to 3V? If this is absolutely necessary, you would need a buffer to maintain this voltage at 3V.

    Thank you,

    The 3 volt reference is necessary as we are trying to maintain compatability with some older source code., yes looking into the TRM we realized this is a very crude way to get 3v for reference, we plan to use a buffer and a 3v reference IC directly for the next HW revision.

    Luke Jones said:
    This depends on the input resistance of the VREFHI pins staying the same.

    I see, i understand, But as a temporay solution what if we reduce the voltage divider resistance a little to make sure its less than the input resistance of VREFHI, I mean instead of using a [1K | 10K] combination, we use [500 ohm | 5K] combination, reduce it by half?

  • 0 in reply to Arun Kumar
    TI__Mastermind 28580 points

    Hi AK,

    I'm not sure how you would reduce the input resistance of VREFHI by half. Could you elaborate on how this would be implemented?

    Thank you,

    Luke