I am using a PAC1720 current measuring IC to measure the current/voltage/power being delivered to a lead acid motorcycle battery that is being charged.
Everything seems to work okay until I remove the power supply to the PAC1720 chip. When Vdd to the PAC1720 is removed (i.e. my PCB power supply is off), it appears that the chip begins conducting current from its sense pins to the Vdd or GND pin (or both, not sure), which ultimately fries the chip.
This is a critical flaw because it means that if someone hooks up a battery to my product without having the power supply turned on (highly likely to happen), the product will destroy itself.
Here is a schematic of how the product is laid out:
I'm guessing it has something to do with the internal ESD strucutre of the PAC1720.
One datasheet for the PAC1720 shows a typical layout like this:
The phrases at the bottom of that picture disturb me. It says:
1: The device MUST be biased PRIOR to applying VSOURCE. Failure to do so will result in damage.
2: The unpowered bias on VDD must be greater than VDD, if VDD will be removed prior to VSOURCE.
I don't understand what they mean by "biasing the device"... sounds like they are talking about biasing the internal clamping diodes?
The second note doesn't make grammatical sense does it? I don't understand what they are trying to convey in the second note.
Now, an older datasheet shows the internal ESD structure of the PAC1720:
This isn't very helpful since it just shows a generic box labelled "Edge-Triggered ESD absorbtion circuit" between the upper clamping diode on Sense1 +/- and GND.
So, my main question is: what is happening when I disconnect Vsupply while a battery/charger is connected and the PAC1720 starts smoking?
Any way that I can solve this problem now that the product is in production?