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AD8279 op amp randomly gets hot but keeps working

Category: Hardware
Product Number: AD8279

We use the AD8279 to measure battery cell voltages in a pack with 4 cells in series. We use the part in gain = 1/2 to get the four cell voltages down to a range we can put into our micro. Each cell is about 3.3V, so Cell4+ which goes to the AD8279 VS+ is about 13.2V.

So the input signals are DC: the inputs to the AD8279 are tied to the cells directly (traces on the board, then leads to the cells). The output goes to a micro:

  • we have a clamp so we don't blow up the micro
  • we have a cap on that circuit to hold the signal while the micro samples.

Circuit below:

  • "Cell1-" (INA- pin2 on U7) is tied to GND
  • "Cell2- same as Cell1+", etc
  • "Cell4+" is VBAT
  • VC1 - VC4 go directly to inputs on the micro for A/D.

We have a problem where randomly U7 (never U8) gets hot (50 degC on the board), and draws lots of current:

  • for eg: 150mA into VS+
  • for eg: 75mA into (or out of not sure) pin 3.

During the "failure" the output voltages remains correct, and we do not see oscillation on the outputs:

  • the scope is 100MHz (Tektronix TDS2012B)
  • bandwidth limit is off
  • (But - maybe it's really hard to see?)

If we disconnect the leads and reconnect them, the part starts working again. Looking online, we see a couple of possible failure modes:

  • latch up: don't really understand this or how it would happen
  • oscillations: maybe our effective capacitive load is too high?

We did not include the .1uF decoupling caps suggested in the datasheet near the part. But, we have since added them to our test boards, and the failure still happens the same.

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  • Hello,

    May I know what is the expected voltage range of each cell?

    Additionally, I am trying to understand the power supply specifications; is it indeed 13.2V?

    Further, I would like to seek clarification on the term "failure" that was mentioned. Given that the output voltage appears to be correct without any oscillations, what criteria are being used to determine a failure?

    The questions will help us understand your setup. 

    Thanks,

    Mae

  • Hi:

    - Voltage range of each cell is about 3.1 to 3.6

    - The power supply is from Cell4+, so 4 x the cell voltage. So, 12.4 to 14.4V

    - The failure is that the chip gets hot, and sinks or sources excessive current, even while the voltage on all pins is within reasonable range.

    Thanks!

  • Does it happen during charging or discharging of the battery? The opamp inputs are directly coupled to the cells, there is no current limit. If there is a spike from the charger, it could lead to a latch up. If the spike is very short, i.e. with high frequency spectral components, tha battery would not absorb it.

    Some time ago I had a similar problem with a voltage monitor of a charging circuit. Removing the battery from the chargin cradle sometimes triggered a latchup, which was fatal for the opamp. A few hundred ohms in series with the opamp input solved the problem.

  • Hi  ,

    I agree with  that connecting the op amp inputs directly to the cell will make the op amp heats. I recommend connecting a resistor (you could use a small resistance) in the opamp to avoid current spike.

    Thanks,

    Mae

  • Hi Edelbastler: thanks for the reply.

    Was your problem with a part like the AD8279 that has the built-in resistors for preset gains (we're using .5)? Or was it a normal op amp?

    Thx

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  • It was a opamp with input common mode voltages beyond the rails. When the charged LiIon battery was in the top off region, very short spikes of the charger IC triggered a latchup of the opamp, which ended in "holy smoke".

    Latchup can also occur in the input ESD protection structures, depending on their implementation. You could remove the opamp from the circuit and check the forward voltage of all ESD diodes. Usually the voltage values are very similar. If there is some beginning defect of one ESD diode, this can usually easy be seen in differences in the forward voltages.