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THS7374: The IC dies after variable amount of time.

Chris Wilkson
Prodigy 90 points
Part Number: THS7374

Tool/software:

I'm trying to add a THS7374 to my video PCB design but it keeps failing in circuit.

The power supply is +5 VDC.  I have a 1.1uF bypass capacitance across the supply: 100nF right next to the VS+ pin and 1.0uF right next to that.

Pins 5, 6, 7, 8, and 9 and connected to GND.  Is it ok to ground the NC pins (7, 8) or must they be left floating?

Channel 4 is driven by a resistor network.  The input signal range is 0V to about 910 mV and the network's equivalent output resistance is 778.5R.  The output  drives a standard video load (150R to GND) in parallel with a divide-by-2 voltage divider.  The divider is a pair of 1k resistors to GND.

Channel 3 is driven by the divided version of CH4 output.  The input signal range is ~150 mV to to ~600mV with a driver resistance of 500R.  It drives nothing for now.  The output load is unpopulated.

Initially, I was building the PCB in sections, so Channel 1 and 2 inputs were floating on the first try.  But I've tied those off now.

Channel 1 is driven by a resistor network similar to Channel 4.  Input range is 0V to about 650mV with a source resistance of about 125R.  The output is a standard video load (150R to GND).

Channel 2 is disabled for now.  The input is tied to GND and the output load is unpopulated.

Channel 1 seems to be working still but Channel 4 always dies for some reason.  (and of course Channel 3 gets a bad input so it misbehaves as well.)

It's like the Channel 4 input goes low impedance after an indeterminate time and drags the input signal down.  If I cut the trace between the resistor network and pin 4 of the IC the signal comes back perfectly on the network side.

I've tried DC input coupling, AC coupling (0.1uF)  and AC+bias coupling (0.1uF and 3Meg pullup to +5V).  And with 3 different devices now.  But the IC always fails after some time.  It might take a few minutes or half an hour.  I'm not sure what the problem can be.  I don't think it's a thermal issue...the IC feels cool.  I'm out of ideas.  

  • 0
    TI__Mastermind 23059 points

    Hello Chris,

    Thank you for bringing this behavior to our attention as well as for sharing a detailed summary of what has occurred.

    I can certainly help investigate on this THS7374 behavior.  I have a few initial questions:

    1. Are you able to check what the current-draw of the IC is?  When we are able to check and confirm the quiescent current (IQ) of a device, it can help pinpoint if the device may be damaged or not seated correctly with pins/pads making good connection.

    2. Would you be able to share your PCB layout and/or schematic drawing?  I can message you privately for you to share if you do not want the information to be public on the forum.  Otherwise it is possible to add files or images with the 'Insert' function.

    3. What is your power supply for your 5V?  Are you using bench lab equipment, a battery, a power module board, or power circuitry on the same board as the THS7374?

    Pins 7 & 8 have no internal connection to the die, so it is okay to leave them floating or tie them to GND if you prefer.  

    Let me think about what may be the reason for this behavior.  Please share any additional information that you are able to provide.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec,

    1.  The current-draw seems to be about 10 mA.  The main IC draws ~200mA so I removed it for better accuracy so the Iccq test is with static inputs to the THS7374:  CH1 = 210 mV, CH2 = 0 V, Ch3 = 1.06 V, CH4 = 911 mV.

    2: I can share partial schematic and layout.  Please contact me through email.

    3:  Power is from a LM7805 on a different board.  Power entry to this board is bypassed by 0.1 uF (0603 MLCC) at the connector, followed by 100uF (alum elec).  At the IC there is a 1.0 uF (0805) followed by a 0.1uF (0603) right at the IC pin.  The +5V supply looks pretty clean on the oscilloscope.

    Thanks,

    Chris

  • 0 in reply to Chris Wilkson
    TI__Mastermind 23059 points

    Hello Chris,

    Thank you, I will work on a response for you.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Mastermind 23059 points

    Hello Chris,

    I have had various travel & been away from the office.

    Do you still need assistance with your evaluation of THS7374 for your video circuit on your board?

    I apologize if major changes or updates have occurred during my time away from our discussion.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec,

    The issue is not resolved.  I had to spin a new PCB with some added test circuitry around the THS7374 device plus a completely different design that doesn't use the TI device at all.  I'm expecting the test PCB revision to arrive tomorrow.  Testing will proceed over the coming days.

  • 0 in reply to Chris Wilkson
    TI__Mastermind 23059 points

    Hello Chris,

    Understood, thank you for the update.  Please let me know if you need interpretation or debugging help on the THS7374 debugging board.\

    If you would like my input on the partial schematic & layout for either the original board or revised test board, please message me on e2e privately so I can share my email.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec,

    The THS7374 is working on the new test board.  There were no changes to the schematic or layout so it had to be either a soldering issue or a bad batch of samples.  Not sure which.  If there is an opportunity for redesign in the future I will put it back in.

    Consider this issue resolved.  Thanks!