PMOS in parallel with NMOS
in [Embedded]
|
Prev: how to open include file in system generator
Next: AVR Assembler (Mega8) - Test if a value has changed more thana given amount
From: CBFalconer on 7 May 2008 11:44 Tom�s � h�ilidhe wrote: > .... snip ... > > Let's say I have a PMOS and an NMOS, and I connect their gates > together. I connect the microcontoller pin to the gates. > > Now, when my pin is high, the NMOS will turn on. When the pin is > low, the PMOS will turn on. > > What will happen though when I set the pin to high impedence? I'm > hoping that neither transistor will be on, but I'm concerned that > maybe there might be some sort of voltage on the microcontroller > pin (it might be behind a large resistance, but it's still a > voltage) that will switch one of them on. Just think about it. The voltage on the gate controls the conductance. This is called transconductance, and is characterized by the GM. That specified dI/dV at some point. However there is a specified gate voltage for full turn-on, and another for full turn-off. Connect two gates together and they are at the same voltage. They are high-Z inputs, so that voltage depends on leakage paths when not driven. If Vcc is less than the sum of the minimum Vons both can turn off, but there is no guarantee that is so. If Vcc is higher the odds are that both will turn on, producing a short, and destroying at least one. Read the data sheets. Think about the basic way FETs are controlled. -- [mail]: Chuck F (cbfalconer at maineline dot net) [page]: <http://cbfalconer.home.att.net> Try the download section. ** Posted from http://www.teranews.com **
From: Rocky on 7 May 2008 13:43 On May 7, 5:44 pm, CBFalconer <cbfalco...(a)yahoo.com> wrote: > Tomás Ó hÉilidhe wrote: > > ... snip ... > > > Let's say I have a PMOS and an NMOS, and I connect their gates > > together. I connect the microcontoller pin to the gates. > > > Now, when my pin is high, the NMOS will turn on. When the pin is > > low, the PMOS will turn on. > > > What will happen though when I set the pin to high impedence? I'm > > hoping that neither transistor will be on, but I'm concerned that > > maybe there might be some sort of voltage on the microcontroller > > pin (it might be behind a large resistance, but it's still a > > voltage) that will switch one of them on. > > Just think about it. The voltage on the gate controls the > conductance. This is called transconductance, and is characterized > by the GM. That specified dI/dV at some point. However there is a > specified gate voltage for full turn-on, and another for full > turn-off. Connect two gates together and they are at the same > voltage. They are high-Z inputs, so that voltage depends on > leakage paths when not driven. If Vcc is less than the sum of the > minimum Vons both can turn off, but there is no guarantee that is > so. If Vcc is higher the odds are that both will turn on, > producing a short, and destroying at least one. > > Read the data sheets. Think about the basic way FETs are > controlled. > And remember too that the microcontroller that has its output floating has more than likely got inputs on that pin which are suffering the same fate of both the hi and lo inputs turning on. Rocky
From: Jim Granville on 7 May 2008 15:42
Tom�s � h�ilidhe wrote: > Let's say I have a microcontroller pin that will have one of three > states: 5 V, 0 V or high impedence. > > Let's say I have a PMOS and an NMOS, and I connect their gates > together. I connect the microcontoller pin to the gates. > > Now, when my pin is high, the NMOS will turn on. When the pin is low, > the PMOS will turn on. > > What will happen though when I set the pin to high impedence? I'm > hoping that neither transistor will be on, but I'm concerned that > maybe there might be some sort of voltage on the microcontroller pin > (it might be behind a large resistance, but it's still a voltage) that > will switch one of them on. > > In my current design, I'm using bi-polar transistors, and it works > great. I'm thinking of switching to MOSFET's though because they let > current flow in both directions. > > The MOSFET's will be used to turn on LED's. Spice is your friend, or you can look at this data sheet: http://www.standardics.nxp.com/products/hef/datasheet/hef4069ub.pdf It shows P/N fet interaction with input swing quite nicely. -jg |