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Part Number: TPS3890
I am observing too much current draw in a TPS389033 supervisor on several boards. I am measuring about 9mA, which is enough to affect the supply being monitored. I am powering it through the TPS70933 LDO below. A diode is placed in the path of the LDO in order to provide added protection against the microcontroller trying to charge a backup battery. At the supervisor's and microcontroller's expected power draw of 250uA the voltage drop here should negligible, well under 100mV. And on most of my boards, indeed it is. (I am willing to remove this diode but it does not fix my current draw problem.)
On some boards I see closer to 2.8V at the TPS389033 VDD and SENSE pins, due to the voltage drop induced by 9mA current. I indicated the points where I observed this low voltage below. These boards are held in reset, as this is below the 3.189V VITP threshold.
Strangely, on these same boards, I measure 4ohms between the CT pin and VDD. On other boards, I do not see this, and do not see the same droop in my backup power rail. I have reflowed the part and do not think it is a solder issue. What else can I look at to try to identify why these parts are pulling my backup power supply down?
As I was reviewing your question, you mentioned that on other boards you don't see the same drop in your power rail. Can you do an AB swap between the working boards and the problematic ones? Can I have some additional information about how your microcontroller is charging your backup battery?
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In reply to Krystian Plaskota:
When used, the intended backup cell is a 3V rechargeable lithium coin cell. It is charged via this section of the reference circuit. When present, the VDD_HIGH_IN supply “is internally shorted to VDD_SNVS_IN to allow coin cell recharging if necessary.” I do not have a coin cell to maintain; I just use the main permanent internal battery and an LDO.
I had previously attempted to reflow the supervisor and did not see a change. After the A/B swap with a good board, both supervisors behaved correctly. It is possible that my attempt at reflowing the part was incomplete and did not correct a marginal solder condition.
At this point, I am satisfied to explain the bad boards’ behavior as a solder process issue and will proceed to fully reflow the supervisors on the remaining boards with high power draw. Thank you for your help.
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