Using an ICL7660 to feed -5VDC to 4116 DRAMs?

gil smith gil at baudot.net
Wed Jul 2 10:14:32 CDT 2008


Hi Ethan:

I use the 7660 all the time -- it needs two 100uF caps -- works 
great!  Can't draw a lot of current, but if the spec fits it is a great 
solution.

There is a bit of switching noise (at about the switching freq of 5 to 10 
KHz, IIRC), but it is usually not a problem.

There is some loss, which goes up as you load the device;  ie:  if you 
drive it from +5.0V and load it lightly, you will get close to -5.0V, but 
as you draw more current it will drop down to almost -4.0V.  You can 
compensate for this by driving it with about +6V instead (eg: a 78L05 with 
a diode or two stuck in the ground lead).

Another was to go is to use a real switcher like an LM2674M-5.0, but 
configured for -5 instead of +5 -- I use this chip all the time too for 
both pos and neg 5V -- very efficient.  Drive from unreg volage (eg: +12 to 
+20 or so); you also need a 100uH inductor, a schottky diode, and a few 
caps.  Pricier, but can efficiently source up to an amp.

gil smith

Who is still looking for a Cromemco SCC board, BTW folks.


At 02:05 AM 7/2/2008, you wrote:
>Hi, all,
>
>I'm in the middle of getting two bare PET boards working, one static, one
>dynamic.  I don't have great honking transformers lying around, so the
>easiest way to power them is going to be a bench-top DC supply.  One thing
>that makes this a little easier is that I don't need to worry about cassette
>motor power in this configuration - no tape drives and no tapes.
>
>The static PET is easy - it has a few TO-220 7805s on it (which were removed
>to get the very tall heat sinks off for easier packing and shipping).  I
>am not even sure I need +12VDC for that one - I don't think I do - just +5VDC
>at a few amps (also made easier because it came with no ROMs and no SRAMs,
>so I'm using one of Nicholas Welte's ROM/RAM boards that sits in the CPU
>socket).
>
>The dynamic PET, since it has soldered-in DRAMs, is a bit more complicated.
>Unless I care to remove the DRAM and fit another ROM/RAM board for the SRAM
>or adapt a 62256 in some other way, I need +12V and -5V for Vdd and Vbb on
>the 4116 chips.  My question (after all this background info) is... does
>anyone have enough knowledge of the ICL7660 chip to give an opinion on how
>well it would work for supplying -5VDC?  Looking at datasheets, the max
>draw I can find on Intel uP416s is 200uA @ -5VDC (along with all its other
>needs).  It looks like the ICL7660 will provide up to 10mA @ -5VDC, but at
>some higher amount of inefficiency than at lower draws.  16x0.200 is 3.2mA,
>well under the max draw of the ICL7660, so I think I'm OK, but I wanted to
>ask other folks about this in case I'm overlooking some important detail.
>
>I do have a real C= keyboard that made it before the last plane, and I
>plan to build either a 74LS86 or 4066-based video adapter to drive a
>standard monochrome security monitor (that I saved from being discarded
>two years ago), so keyboard and video are taken care of (plus I have a
>"C=Key" board that I probably have time to assemble to try using a
>modern keyboard with).  For a base PET, besides video and keyboard, the
>only other issue is power, thus my question.
>
>If the DRAM were socketed, I'd probably just pull it and use a 62256
>in the ROM/RAM board and be done with it, but it might just be easier
>to feed this board +5V and +12V and synthesize the -5V, unless it
>turns out that I have a bad DRAM chip - then it's time to yank them
>like a bad tooth (though I _think_ I know where I can find a few 4116
>chips on a shelf here if it comes down to just one or two chips).  If
>I were home, it would be easy - I'd pull out a board from one of my PETs,
>drop these boards in there at least for testing, then see about mounting
>them in a box of some sort.  Now, at least, the only C= stuff I have are
>these boards and a few loose parts (like 65C02s and 65C22s).
>
>One memory that's filtering up to the surface is that ISTR some
>order-of-DC-powerup requirement for 4116s, like don't apply -5V
>first or remove it last or some such.  I plan to power the ICL7660
>off of the +5V rail, but if there is a very specific order that
>really matters, I might consider running it off the +12V rail
>(with a 78L05 in-between) instead.
>
>Thanks for any input or observations on the ICL7660 and/or 4116s.
>
>-ethan
>
>--
>Ethan Dicks, A-333-S     Current South Pole Weather at  2-Jul-2008 at 08:40 Z
>South Pole Station
>PSC 468 Box 400       Temp  -83.7 F (-64.3 C)   Windchill  -114.0 F (-81.1 C)
>APO AP 96598          Wind    6.1 kts Grid 58   Barometer 678.1 mb (10700 ft)
>
>Ethan.Dicks at usap.gov            http://penguincentral.com/penguincentral.html



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