At 07:10 PM 8/7/2008, you wrote:
>Hello all, I just found a flaw in a design due to me not paying
>attention to an important spec.
>
>I'm trying to high-side switch an input from 3 AA batteries wired in
>series, and I'd like it to work down to 2.5V or lower if possible.
>
>The pfet I'm using doesn't have solid specs below 4.5V for Vgs.  Others
>will have a spec down to 2.5V, but are high Ron (up to 30 ohms) and low
>current capability (300mA).  These are marginally useful.
>
>While I'm probably going to change the design to avoid this issue
>entirely, I'm curious how this situation can be handled.
>
>It seems that the possible ways to do this are:
>
>1. use a negative voltage for the gate on the pfet, to achieve say -5V
>relative to the voltage being switched, so that it goes fully on.
>
>2. use an nfet instead, and use a voltage above the battery voltage (a
>few volts - whatever the Vgs for the nfet is) to switch it.
>
>I'm not sure which is uglier to do, neither are convenient.
>
>Ideally, I'd like to be able to switch up to 300mA (with inrush of say
>800mA) down to 2V, and have the current draw in either the off or on
>state be in the 50uA or less range.  So if some sort of voltage boost /
>invert circuit is needed, it has to be real low quiescent.
>
>Thoughts?
>Thanks!
>J
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