Good day to all. I'm working on a project that runs from 2x NiMh cells (about=20 2.4Vdc). The current design uses a pair of National Semiconductor=20 LM2623 switchers to get my supply voltages of 5.5V & 7.5V. Now I need to add an auto power-off circuit that disconnects the=20 battery should the user forget to turn the system off when not in=20 use. I really need the shutdown current to be as low as possible, so=20 simply disabling the switchers and adding load disconnect switches=20 isn't a viable option (shutdown current on the LM2623 is a worst-case=20 2.5uA each). The other problem with just driving the enable inputs=20 on the LM2623 is that the enable signal must be higher than 0.7Vdd,=20 which is 5.25V for the 7.5V switcher. This is significantly higher=20 than the battery voltage. The final thing to note about the LM2623 switchers is that their=20 inrush current is extremely high when first starting up - I need to=20 set my bench supply to at higher than 6A current limit or they simply=20 won't start up (they get very, very hot instead). That means that I=20 need a MOSFET with really low RDSon. So: my plan is to use a 10f322 for monitoring for low battery voltage=20 and excessive time delay since last use and then drive a beefy=20 N-channel MOSFET that switches the ground side of the circuit. Here's the rub: I can't seem to find any decently-low priced BEEFY=20 logic-level N-channel MOSFETs that are fully enhanced with only=20 2.4Vdc applied to the gate. So: I need to boost the battery supply=20 to drive MOSFET gate. The first thing that came to mind was to have the PIC generate a=20 couple of complimentary square-wave signals, then use a pair of=20 voltage doublers to get a DC gate drive signal approaching 4.5Vdc. I've previously used CMOS buffers as high-efficiency voltage doublers=20 using standard 4000 or HC logic. These doublers are really simple:=20 you use a pair of capacitors to feed both the input and output of the=20 buffer from the same square-wave signal. Vss of the CMOS package=20 goes to battery (+), Vdd of the CMOS package goes to a reservoir=20 capacitor and is the doubled-voltage output. Use a pair of buffers=20 in the same package if you have complimentary square-waves. Parallel=20 multiple buffers if you have extras in the package. How this works is simple: when the circuit first starts up, the input=20 coupling capacitors charge and discharge through both the input=20 protection diodes and the intrinsic back-body diodes of the output=20 MOSFETs within the buffer. This raises Vdd on the buffers to the=20 point where the MOSFETs will actually start to switch. When that=20 happens, the buffer's output MOSFETs alternately shunt the upper and=20 lower back-body diodes and you get nearly a perfect rectifier. The=20 switching, of course, is controlled by the AC-coupled square wave=20 applied to the buffer's input. I don't think that I invented this, but I've been using it for a=20 really long time now. Anyway, easy as pie, sez me. The only difference is that now I'm=20 wanting to use something really tiny instead of the DIP or SOIC=20 packages that I'm used to using. Hmm . . . I remember reading about=20 some "Tiny Logic" devices. Great: they come in a SC-70 package which=20 is plenty small enough and I get two buffers in one package. Part=20 number NC7WV16P from Fairchild. Doing my usual due-diligence, I breadboard the circuit before=20 committing it to the PCB layout. Oops: doesn't work. Careful reading of the datasheet reveals the two problems: 1) The buffer inputs tolerate an input voltage that is higher than=20 Vdd. That means that they don't have clamp diodes from the input to=20 Vdd (I'm guessing that they rely upon zener clamps instead). 2) The inputs and outputs are high-impedance when Vdd=3D0. That means=20 that the MOSFET outputs don't have the intrinsic back-body diodes=20 from output pin to Vdd. I didn't even know that was possible. So: I've got to find a dual-buffer in a small package that actually=20 behaves like the old CMOS stuff that I'm used to using. Anyone have any ideas? If I can't find something suitable, then I'll abandon the capacitive=20 voltage doubler approach and instead use the PIC as a boost=20 converter. Means that I have to add another small N-channel MOSFET,=20 inductor, diode, etc but its doable. Luckily, I allocated two pins=20 for the capacitive voltage doubler, so I am able to do voltage=20 regulation within the PIC. Anyway, if anyone can suggest a readily-available small dual buffer=20 that works at 2.4V and has the normal clamping diodes present on both=20 the buffer inputs and outputs, I'd be grateful. Many thanks! dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .