Why does the current disappear?...

The chip's core is an array of latches running on 1.8V. When I power on the chip, the power supply draws 85mA.

A control signal controls whether data to be stored or not stored into the latch unit and it is fed from off chip. It connects to the gate of a n-mos switch inside each latch unit. Since there are so many latches for the whole chip, the control signal is buffered through a chain of inverters before it reaches individual latch.

The problem is when I pull the control signal to zero, suddenly the current from 1.8V supply is gone. When I pull load to Vdd, the current is fine. I realized when I slowly ramp down control signal to certain voltage, the current disappears. What's wrong here?

I know this board has a lot of experienced people who can just look at it tell what might be wrong. Please help me with possible suggestions..

Is the voltage at which the current disappear about 600mV?

I just checked it carefully again.

The current disappears at 1.4V. It starts to decrease when the control signal is at 1.45V and within several mV of difference, the current is completely gone. It seems the difference is the threshold voltage of the device (documented Vtlin=355mV+/-45mV).


I did the following:

at power up, current = 85mA
at control signal = 1.8V, current = 85mA
at control signal = 1.45V, current start to decrease
at control signal = 1.4V, current = 0
at control signal = back to 1.8V, current =85mA

The supply current might be provided by the control signal source (if you have an external generator for it). Since 85mA is a big current (and the circuit still works after the current disappearing - nothing burned), there might be some diode connection (ESD for example) between the signal line and the ground. Put a current meter in series with the control signal and check if there is any current there.

Yes I checked the current though the control signal line(multimeter connected in series). the current through it is 0 with all cases.

I initially thought as the current disappears from 1.8V supply, it might show up in the control signal line, but it didn't.

Since this control signal has a large fan out, a lot of the interconnects are quite long, I did manually add diodes to ground on those interconnects to correct antenna rule violation.

I'm pulling my hair out trying to find why does this happen..

If there is no current on the 1.8V supply and there is no current on the signal line, are you sure that there is current on the ground line? If not, you are powering down the chip...