As someone else noted - a Natsemi "simple switcher" or similar is very very easy to get going and probably better suited to this application. However - the FET drive totem-pole you have shown is OK BUT the gate of the top FET MUST be driven to eg 5 volts above its Source when it is turned on. As the source will be at +12 volts at this point then the gate must be driven to 17 volts (or more). For this you need a "high side" voltage source. A capacitor pump of sorts is one way of getting this :-) This is one reason to use the P channel upper FET - in that case you pull the gate down towards ground to turn the FET on so no high side drive voltage is needed. There are many app notes around showing high side supplies. One basic form works like this. - Connect capacitor Cpump side A to junction of 2 FETS. - Connect a diode, anode to +12V and cathode to Cpump side B. - Connect second diode, anode to Cpump side B, cathode to Vhigherstill. - Connect a reservoir capacitor from Vhigherstill to ground. When the FET pair output is low Cpump will charge to +12v less a diode drop. When the top FET turns on it will pull the FET junction towards +12V so the top of Cpump will be pulled to ABOVE +12v. Second diode will conduct raising Vhigherstill to about 2 x +12v. This is a low power diode pump being used to provide drive voltage for a high power diode pump :-). The first time its a bit flaky as the drive voltage is being made as the upper FET is turned on (bootstrapping) but once the top rail is at about 2 x +12v it's away. BUT - use a simple switcher :-) PS -Re someone's comment on Maxim. Real engineers will often find a cheaper solution than a Maxim part but anyone who will NEVER use Maxim is not a real engineer :-). Maxim make some very useful products. In small quantity use they are often very hard to beat. In volume use they compete with everyone else in the market and are sometimes the right solution, sometimes not. Russell McMahon _____________________________ >From other worlds - www.easttimor.com www.sudan.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-)) -----Original Message----- From: Byron A Jeff To: PICLIST@MITVMA.MIT.EDU Date: Thursday, 30 December 1999 12:17 Subject: Need another set of eyes for MOSFET capacitive switcher design >In the process of building a DC-DC converter for my car MP3 player I've >come across the need for -5 and -12 volt power. ............... >The only thing I was trying to figure out is how to put the MOSFETs in a >totem pole configuration. After researching CMOS configurations I finally >think I understand that the purpose of the P-channel MOSFET is so that the >complementary MOSFETS can be driven by the same input. But since the >characteristics of the N-channel is so much better I want to use a pair of >N-channels. So I plan to use this config: > > +12 > | > D >A-->G > S > | > |-----| | ------ > | > D >B-->G > S > | > | > gnd > >with A and B having to be complemetary. Whichever point A or B that's 5V >will conduct with the other off. Right? >One last question. Can anyone explain the difference between using a Zener and >a 7905 regulator? 7905 will regulate MUCH more closely. AND - a zener is a shunt regulator - it will dissipate the excess power to bring the voltage down to the desired level. The 7905 will turn on enough to allow the requisite energy through to bring the load up to the desired voltage. A zener is seldom a good power supply solution in small volumes unless there is a special reason to use a shunt regulator. RM