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Original question:

ADS8402 Overheating

ADS8406: ADS8406 going into latchup

Marten Beels
Prodigy 150 points
Part Number: ADS8406

Hello,

Over the past few years, we have had issues with ADS8406 going into latchup.  This is an existing design that has been in production for several years. 

We had an independent lab perform failure analysis and were able to reproduce our symptoms.  They observed that raising the +VA supply from +5V to +6.32 V for 100 ns was enough to put the device into latchup. It matches our observations and internal tests.  After latchup, the devices draws approximately 500 mA (varies) from the +VA supply.

This seems odd, because the device has a maximum rating of +7 V on the +VA supply. 

Can you help us determine why the device does into latchup while operating within the Absolute Maximum Ratings? 

Thank you,
Marten

  • 0
    TI__Mastermind 46150 points
    Hello Marten,

    There is one other condition where ABS max specs can be exceeded with less than +7V on the +VA pin.

    +VA to +VBD; ABS max range of -0.3V to 2.55V.

    If you are using a digital supply greater than 4.5V on +VBD, then the full -0.3V to 7V range on +VA will apply. However, if +VBD is at 3.3V, then the ABS max limit on +VA will be 3.3V+2.55V = 5.85V. Exceeding this limit can cause an internal latchup.

    Please confirm the voltage on +VBD when adjusting the +VA voltage.

    Regards,
    Keith N.
    Precision ADC Applications
  • 0 in reply to Keith Nicholas
    Prodigy 150 points
    Hello Keith, thank you for the reply.

    We have +VBD at 3.3 volts. It seems that is what is happening, today I reproduced a latchup at +6 V.

    Are there any other considerations in changing +VBD to +5V?
  • 0 in reply to Marten Beels
    TI__Mastermind 46150 points
    Hi Marten,

    The ADS8406 is fully specified for +VA=5V and +VBD=3V or 5V. The only concerns will be system related; the MCU or FPGA connected to the ADS8406 digital bus will need to also work from 5V if you change +VBD to 5V operation.

    Regards,
    Keith N.
  • 0 in reply to Keith Nicholas
    Prodigy 150 points
    Yes, thank you Keith. We would need to have logic level translation with our FPGA, but it sounds like that would give us more margin on +VA before the device goes into latchup. I appreciate your input.

    Marten
  • 0 in reply to Marten Beels
    TI__Mastermind 46150 points

    Hi Marten,

    I looked at your original post, and see that there is a 10uH inductor between AGND and BDGND.  All of the ABS MAX specifications assume that AGND and BDGND are connected together using a low impedance ground near the device.  If these grounds are moving more than a few hundred mV's with respect to each other during a transient, this may cause a latch-up condition as well.

    Figure 2, page 10, for the ADS8406 EVM User's guide shows how the grounding was done for the evaluation board.

    http://www.ti.com/lit/ug/slau126a/slau126a.pdf

    Please make sure that the AGND and BDGND pins are connected together through a low impedance path near the device.

    Regards,
    Keith N.