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Design >> High-Power Design >> Voltage Reliability in Power-Down Mode
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Message started by Berti on Feb 17^th, 2009, 5:37am

Title: Voltage Reliability in Power-Down Mode
Post by Berti on Feb 17^th, 2009, 5:37am

Hi everybody,

I have the attached simple vt-reference circuit which uses low voltage (3.3V) transistors but it connected to a high voltage supply (4.2V). In power-down mode, the switches connected to ENA are off. Consequently, the vds of the individual transistors will depend on the leakage path which unfortunately is only poorly control over process and temperature.
Does anybody has an idea how to make sure that the transistors don't see a too high vds (e.g. by adding cascode transistors etc.?)?

Cheers

Title: Re: Voltage Reliability in Power-Down Mode
Post by NiceFeng on Apr 18^th, 2009, 11:15pm

Hi,Berti,

My two cents:

Do not completely turn off the block, but biasing it with very low current, say 10nA, thus make sure each transistor works in a somewhat predictable status with reasonable Vds.

BTW, isn't it a current generator? Can it also be used as a vt-reference circuit?

Regards,
Feng

Title: Re: Voltage Reliability in Power-Down Mode
Post by Berti on Apr 20^th, 2009, 11:42pm

Hi Feng,

Thank for the idea.

Quote:

BTW, isn't it a current generator? Can it also be used as a vt-reference circuit?

It is a current generator. I call it vt-reference as the current is given by I=Vt/R.
I think also P. Gray/R. Meyer call this circuit a vt-reference in their book.

Cheers

Title: Re: Voltage Reliability in Power-Down Mode
Post by safwatonline on Jul 6^th, 2009, 1:03pm

doesn't shorting the gate of the PMOS mirror to Vdd do it?

Title: Re: Voltage Reliability in Power-Down Mode
Post by Berti on Jul 7^th, 2009, 5:21am

I think shorting the PMOS gate to vdd doesn't avoid problems. A very small leakage current will still flow which will determine the voltages .. like a highly-ohmic resistive divider which is very sensitive to process variations.

In the meantime I added cascade devices to the circuit will are always connected to a very weak bias voltage.

Cheers

Title: Re: Voltage Reliability in Power-Down Mode
Post by safwatonline on Jul 29^th, 2009, 8:44am

U can (in addition to the PMOS switch) you can add a series switch that disconnects the gate of the diode connected MOS from its drain.