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

[Ethan Dicks]

On Wed, Jul 02, 2008 at 06:13:22PM -0700, Scanning wrote:
> Ethan,
> 
> According to the Motorola datasheet - " All voltages referenced to Vss ( 0V)
> and Vbb ( -5 Volts ) must be applied before and removed after other supply
> voltages ".

OK.  I don't have _that_ 4116 datasheet to reference, but that does sound
familiar.

Looking carefully at the Commodore power supply schematics from zimmers.net,
I don't see anything that would bring up Vbb first or remove it last.  To
all appearances, it's all three at once.  I don't know of any voltage
priority in the Apple II, either (as an example of a contemporaneous
design to the dynamic PET), so presumably, "at the same time" suffices.
The unanswered (unanswerable?) question is then how close is close enough.

Given the replies, I think I'm going to try the ICL7660.  It means that I
can run the board off a dual supply instead of trying to find other means.
There is a remote chance that it could blow all the 4116s on a semi-common
PET 3032 motherboard and I'll have to extract the chips - not a preferred
outcome, but it seems no more risky than the factory supply.  The safest
alternative appears to be to remove the DRAM in the first place and run a
static chip on a ROM/RAM board, but that still involves a lot of work on
the board.  What is clear is that providing +5V and +12V without covering
the -5V requirement is _not_ a reasonable option.
 
> What exacerbates things is that the current spikes from all the chips are
> perfectly lined up during the RAS* cycle, but the biggest hit is on the Vdd
> line ( +12 Volts ). If they don't already have a 0.1 uFd across each chip on
> the Vdd I'd add one. I agree the 7660 should be okay on the - 5 Volts.

As originally manufactured by Commodore, the chips have 0.1 uF on both the
+12V and +5V rails.  They are so notoriously cheap that I take it as proof
that they are required.

> Also, don't think these will run at -63 C ( ha ha ha ).

Fortunately, we have this nice warm indoor space - _I_ get to walk
~10 hrs/week outside, but the classic toys stay inside where it's nice
and toasty.

Thanks for all the advice,

-ethan

-- 
Ethan Dicks, A-333-S     Current South Pole Weather at  3-Jul-2008 at 03:39 Z
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Ethan.Dicks at usap.gov            http://penguincentral.com/penguincentral.html