I'm working on a new design which included the use of 2 SN74AVC4T774. They work fine under normal operating conditions, however when I power down VCCB 1.8V -> GND the chips consume 2mA from VCCA which is at 3V. Normally around 25uA or so is being pulled from 3V when VCCB is 1.8V.
If I remove these chips from my board, the extra 2mA disappears so I know that the issue is related to these chips.
To clarify the problem scenario,
I have a modem which is being powered by a 3.8V step-up regulator, the modem provides a 1.8V reference. I use the 1.8V reference to supply VCCB and I supply VCCA with 3V from an LDO which also powers an MSP430.
I would like to shut down the modem to save power, in doing so I turn off the 3.8V regulator which causes the 1.8V reference to go to GND (I've checked this 1.8V with an oscilloscope and it does as I expect it to).
When I turn off the modem I would expect the current consumption of my board to be at 2mA, but instead it is closer to 4mA+.
I've attached a schematic in PDF format to hopefully give you a better idea of what is going on.
/cfs-file/__key/communityserver-discussions-components-files/151/2337.Schematic.pdf
Things I've tried already,
Jumper OE to GND so that the chips are always on. I removed U15 and R49 in doing this.
Removed all of the 1M pull-down resistors (I didn't want floating inputs, but It was pointed out to me that maybe this was causing an issue).
Placed a 6.8K pull down resistor on the 1.8V line just to make sure it was at ground and not floating after turning the modem off.
In software I have made all the lines leading to the chips from the MSP430 outputs at 3V, outputs at 0V, all inputs, all inputs with MSP430 internal pullups enabled. This doesn't seem to change the consumption at all. Obviously I can't control the modem side, but they are all at GND when I check them with an oscilloscope.
I've also been talking to an FAE from U-Blox the company which provides the modem, as they used this exact chip in their system integration manual but so far we haven't been able to solve the problem.