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TL494：TL494 DTC/C1/C2 Connection capacitance

jacky.chen
Prodigy 50 points
Part Number: TL494

hi the team,

We have used lines like the following,

Its IC2 PIN4 DTC and PIN8 C1, PIN9 C2 are connected with capacitors(C9,C10),

We don't know why the capacitor is connected like this?

  • 0
    TI__Mastermind 27745 points

    Jacky,

    I'm not sure whyC9 and C10 are connected into the DTC? This makes no sense since the DTC is a DC signal capacitively "coupled" through C9 and C10? If you have waveforms to share, that would help?

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 50 points

    hi Steven,

    The waveform look like this,

    C1(Green)/C2(Blue)/DTC(yellow),

    I think it is Prevent out of control when C1 and C2 open at the same time,

    How about you think?

  • 0 in reply to jacky.chen
    TI__Mastermind 27745 points

    Jacky

    Thanks for sharing the waveforms. Looks good and I agree.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 50 points

    HI Steven,

    I have received one  abnormal TL494 PCBA,

    and I changing it to a MLCC capacitor,

    then C1, C2 singal look like good,

    but the DTC is not very stable,

    what will affect the DTC?

    c1 : orange / c2 : green / DTC : Blue /Vref : red

  • 0 in reply to jacky.chen
    TI__Mastermind 27745 points

    Jacky

    The DTC is a comparator input and is high impedance (<10 μA). I can't think of anything that would result in the waveform you are showing. My suggestions for debug would be:

    1. DTC limits are 0V<DTC<3.3V. Please verify that DTC voltage is always within these recommended limits
    2. Since DTC voltage is divided down from VREF, check VREF to verify it is stable or following the same pattern at DTC?
    3. Break the connection from VREF and try applying bias to R12 from an external voltage source and see if the same DTC signal appears?
    4. verify the PCB and solder connections under a microscope
    5. Check the leakage current of C7 and voltage rating. Make sure C7 is NPO type MLCC with voltage rating of at least 10 V
    6. Swap the TL494 to a new IC and test again under similar conditions

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 50 points

    hi steven,

    we have try the high tempture test,

    1.Look like the C1,C2 Switch to effect the output Freq (24.56~100HZ),at original tantalum

    2.we are change tantalum to MLCC => look like is ok

    3.then we are change MLCC to Origial tantalum => the C1,C2 Signal look like better then Origial tantalum,output Freq(39.5~49.23HZ)

    I don't know what could be causing the difference(when Origial tantalum => MLCC => Origial tantalum )?

    item 5 What effect does leakage current?Freq?

  • 0 in reply to jacky.chen
    TI__Mastermind 27745 points

    Jacky,

    I'm not sure? Is this happening on one board only, or can you verify this to be the case on multiple boards? 

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 50 points

    hi steve,

    There are currently 3 pieces of this phenomenon,


    I tried to touch the tantalum on yesterday, and found that the output frequency will change to 90hz

  • 0 in reply to jacky.chen
    TI__Mastermind 27745 points

    Jacky,

    Based on what you are observing, all signs are pointing to sensitivity/differences between capacitor types. From here, I would suggest that you stick with the ceramic X7R which I recommended and seems to be giving you the desired performance or else you will need to deep dive into the electrical performance differences between these two capacitor types. You need to decide if this is worth the effort based on the time spent and how valuable the information is that comes back as a result of your effort? A deep dive comparison could include measuring the impedance characteristics of each on a network analyzer, comparing temperature and tolerance variation (data sheets), comparing measured ESR, SRF, capacitor value vs DC bias effect and leakage current between each...these are just a few of the most important differentiating parameters that come to mind. 

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 50 points

    hi steven,

    very thank you to reply this problem.