Electronic – Enable connected to unpowered regulator on another board

inputlogic-levelpower supply

I have an ARM board which uses a VIO of 1.8V and requires that no i/o line be driven before it is fully powered up. It signals this readiness by asserting its "SYSEN" line.

My design has SYSEN from this board connected to the enable line of a 1.8V linear regulator on a separate docking board. The 1.8V line powers (one side of) all the level shifters – thus I'm protected from damaging the ARM as 1.8V will not exist until SYSEN is asserted.

But there is a corner case which is that "what if the fuse on the docking board blows". Then the 5V line to the regulator is unpowered when SYSEN goes high (1.8V). I initially chose the Micrel MIC5247-1.8:

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but looking at the datasheet:

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the enable clearly cannot go above Vin. If Vin is zero, then EN must be zero. I would suppose there are input protection diodes in place to gnd and Vin or some such.

If I choose to use another device, say the Microchip MCP1804-1.8, then it's still not always absolutely clear what is allowed. Looking at the MCP1804 datasheet it shows protection diodes to gnd, but not Vcc. Promising. The max ratings imply that SHDN can exceed Vin but the params sort of say the opposite but in an unhelpful way:

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If I give up and just gate the enable line (eg: 74AHC1G125) at least that datasheet specifically gives static characteristics which indicate that Vi can exceed Vcc. Eg: If Vcc = 2.0V then the min for a logic high is 1.5V, there's no max given. Also leakage currents are given when Vcc=0V and Vi=5V.

(Edit: Scratch that – if Vcc is 5V and Vi is 1.8V then it doesn't do me any good to use a bus buffer of any sort. I'd have to gate it with discrete components – eg: SYSEN to a fet pulled up to 5V and then invert that logic. Which is too many components for a simple issue!).

Should I try to read between the lines on datasheets like this or is it always the case that I just have to buy the devices and test them? I'm inclined to say the MCP1804 will likely be fine but I'm not sure.

Best Answer

Interpreting the "Absolute Maximum Ratings" section of datasheets as the maximum ratings that won't damage your device, it seems like the Micrel chip would break if the edge scenario you described happens. I think the Microchip IC is more promising, because what the maximum ratings reads like to me is that the pin is tolerant to 30V regardless of Vcc.

With regards to the parameters, I think it's just defining the rules you'd want to follow for normal operation. Having a broken fuse would kind of imply that something's gone wrong, so now you're really just concerned about not breaking the chip.