cascaded N-mosfets

[loicalset]

A quick sanity check, because my brain is turning to mush:

Is there any problem with using two cascaded N-MOSFETs as a sort of "AND" logic, low-side cutoff for an LDO regulator? I am planning to cascade them in the ground path of the LDO. There are two conditions (batt voltage good, mode selected) that I want to act as gates for powering on a device via the LDO. If one or both gate inputs are not high, then there is no ground path for the LDO. Something tells me I am missing something big, but I can't put my finger on it at the moment. Thanks for the help!

 

[zappedagain]

If you open up the ground path for the LDO there is a good chance your LDO output will float to the input voltage resulting in a high voltage on your output.

 

[loicalset]

I love this forum. Thanks for the quick response.

If the output floats high, there is still no way for the load to draw current without a ground connection, correct? The only worry would be damaging the LDO?

 

[loicalset]

I just realized that this may still be bad for the load device. Wow, time for bed probably.

 

[IRstuff]

But, you're not floating the load, so there will be some amount of current flow.

Also, it depends on what you did with the substrate connections for the NMOS devices. If they are floating or simply tied to source, there is a possibility that the parasitic NPN could be turned on. The substrates need to be tied to the lowest potential in the circuit. That's why NMOS devices are not always a good solution for this type of application.

TTFN

FAQ731-376

 

[loicalset]

FYI, the LDO I was considering is the Texas Instruments TP715, because of the low quiescent current. Does somebody know off hand if this (p-FET) LDO would float high with its ground disconnected?

I suppose the proper course of action now would be to find an LDO with a shutdown pin.

 

[loicalset]

IRStuff - gotcha - that what was I was suspicious of but couldn't quite get my mind wrapped around. Thanks for the help.

 

[Comcokid]

Can't find the TP715. Do you mean the TPS715xx family?

There are many LDO,s made with a shutdown pin that have less than 1ua in shutdown. There are also LDO regulators optimized for low quiescent current, such as the TPS715xx which has 3.2 uA quiescent operating current.

 

[loicalset]

Oops, yes I meant the TPS715. I think I may have found a suitable low quiescent LDO with a shutdown pin (LP2980-ADJ) to substitute for this part.

Now, to locate a voltage supervisor that will go up to 7 volts input and detect a 5.2 volt crossing... but that's a topic for a separate thread.

Thanks for the help, everyone.