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ADS1120: Short Circuit Current Maximum

ReedKacz
Expert 3462 points
Part Number: ADS1120
Other Parts Discussed in Thread: ADS1231, ADS124S08, ISOW7842

Hi experts,

I have a question regarding the ADS1120’s internal low-side switch, for bridge sensors:

If I have the ADS1120 connected to a bridge sensor, and a sensor fault occurs that shorts out the bridge sensor, is there a specification for how long the low-side power switch can tolerate sinking that short-circuit current (in my case, ~130mA) before my system is able to switch it off? I know it is specified for 30mA steady state, but I don’t see an absolute maximum rating for it, or something I can use to guess at how fast it could heat up and damage the device.

Thanks,

Reed

  • 0
    TI__Guru** 113765 points

    Hi Reed,

    Just so that I understand, the low-side switch is closed and the bridge sensor is shorted from EXC+ to EXC-, is this correct?  Was the current measured directly at 130mA? What kind of duration are we talking about for this higher current (ns, us, ms)? At some point the internal traces on the silicon die will fuse open.  It would be helpful to know what causes the short to better understand possible solutions.

    Generally the old specification for devices such as the ADS1231 (which also has a similar design with the low-side switch as the ADS1120) is 100mA momentary for the Absolute Maximum. As you noticed we don't have this specification in the ADS1120 datasheet.  Mostly because it is difficult to specify what 'momentary' really means.  For the ADS124S08 it is specified as 100mA continuous for the Absolute Maximum current through the low-side switch and the Electrical Characteristics table allowing for 75mA maximum in normal operation. The design information for ADS1231 allows for a 40mA continuous current with the low-side switch.  As I mentioned earlier the ADS1120 is similar in design and the switch resistance is characterized with 35mA.  Based on the design criteria I would say that the ADS1120 cannot withstand the same amount of current as the ADS124S08.

    Part of the problem here is having enough simulation data available to give some guidance.  Any actual device testing would be destructive and may not fully cover lot variations.  A second part of the problem is setting up specific conditions and then relaying this information properly in the datasheet. So the best approach is to avoid the over current condition in the first place when using the ADS1120. At this point it would be valuable to know the conditions that cause the short producing the high current. 

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 3462 points

    Hi Bob,


    I really appreciate the detailed response. The timing would be in the milliseconds for the 130mA short circuit (130mA comes from the ISOW7842 being the supply for the bridge and it has a SC current ~130mA). Due to this length and the fact that the ADS1231 (similar LS switch) would also not be able to handle this - we will be changing the design to use an external low side MOSFET.

    Thanks for the help!
    Reed