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TMP235: about TMP235 power supply ripple

Joyce Li
Intellectual 2170 points
Part Number: TMP235
Other Parts Discussed in Thread: AMC7932

Hi Dear expert,

My customer has used TMP235 in their system, the power supply of it has tied to PA supply, and the supply is has periodic ripple. See below snapshot.

The large drop period is 70ms, will drop 280mV, the small ripple period is 5ms, will drop 100mV.

I didn't find VDD ripple requirement in datasheet, would you please review this and will it impact TMP235 performance?

Thanks a lot for your reply.

BR,

Joyce Li

  • 0
    TI__Guru* 91086 points

    Dear Joyce - 

    thanks for posting! This is more than a ripple...can you share the schematic for this circuit? 

  • 0 in reply to Josh Wyatt
    TI__Intellectual 2170 points

    Dear Josh,

    Thanks a lot for your reply. Actually my customer has used TMP235 for long time and also shipping great volume. Schematic is fine.

    This issue is a new issue and the VDD of TMP235 is tied to PA(power amplifier) supply. PA is very power consuming device and to save power and in TDD(time domain division) mode, the PA power supply is turn on and off with some period. so TMP235 VDD has this "big ripple". Would you please suggest whether this will impact TMP235 performance?

    Thanks a lot!

    BR,

    Joyce

  • 0 in reply to Joyce Li
    TI__Guru* 91086 points

    Joyce - i would not agree that schematic is fine if the ripple and the dip are there. Please provide, so a suitable cap or filter can be suggested for the VDD line that feeds the TMP part and whatever else is on the rail.

  • 0 in reply to Josh Wyatt
    TI__Intellectual 2170 points

    Dear Josh,

    Thanks a lot for your reply. I have got schematic from customer, please see below:

    Would you please review the decoupling cap is enough? Do we need to add more decaps?

    Thanks a lot!

    BR,

    Joyce

  • 0 in reply to Joyce Li
    TI__Guru* 91086 points

    Joyce - 

    I meant the power supply schematic along with what was provided above. (which helps a bit here)

    1. if they are using ADE7932 ADC, then the power supply ripple captured and described in original post needs to be cleaned up for sure, that part will not take the amount of ripple indicated and also it operates off 3.3VDC. (if that is not the right part #, please advise/update) - I cannot stress enough the importance of a clean power supply rail in general, for any design. 

    2. the resistor/cap combo on the output of the TMP235 may or may not be needed. At the present, they look too heavy to me. Please ask customer to review their ADC documentation thoroughly to determine if those components are needed at all and if so, please then ask them to size them accordingly (by calculation, then check on several boards) 

  • 0 in reply to Josh Wyatt
    TI__Intellectual 2170 points

    Hi Josh,

    Thanks a lot for your reply.

    The ADC part number is AMC7932, which is 32channel DAC and embedded ADC. My customer finally re design the power supply to separate TMP235 VDD and PA VDD to ensure the power supply is clean. Before doing that, they see 9degrees temperature variation.

    We don't find very specific requirement on TMP235 VDD. I see the Delta temp/delta VDD spec, it is very low, 0.01 delta degree/delta VDD.

    Would you please share with us the power supply requirement of TMP235 if the temperature variation is within 1degree?

    Thanks alot!

    BR,

    Joyce 

  • 0 in reply to Joyce Li
    TI__Guru* 91086 points

    Dear Joyce - 

    In the TMP235 datasheet - in the Power Supply section of the Electrical Characteristics table (section 6.5, page 5), there is parameter called Line regulation. This is the change in temperature output based on change of VDD, that is correct, as you said and there are also figures 7 and 8 on page 7, which show the stability of the output with the input voltage being swept from either 2.3V or ~1.2V to 5.5V (and also over temperature) 

     https://www.ti.com/lit/ds/symlink/tmp235.pdf

  • 0 in reply to Josh Wyatt
    TI__Intellectual 2170 points

    Hi Josh,

    Thanks for your reply. I have noticed Figure 7 and 8 in datasheet, but it is not exact spec we are looking for. The VDD line regulation might be the slow VDD variation that impact temperature accuracy. The case we are facing is the VDD periodically change with 280mV drop. The VDD we are using is 5V, including 280mV drop, the VDD is still above 2V, according to Figure 8, the Vout variation should be small. So i think the datasheet didn't consider when VDD has big "ripple" like our case. I suggest customer to modify the schematic in the power supply part. They will improve VDD ripple. 

    If you have more data, you can share with us. Thanks a lot!

    BR,

    Joyce Li

  • 0 in reply to Joyce Li
    TI__Guru* 91086 points

    Joyce - 

    You are correct, I am sure, it would not be reasonable to expect or design a system with that sort of power supply issue.