[sdiy] uPD444 RAM battery power

[rsdio at audiobanshee.com]

Hi Adam,

On May 28, 2015, at 9:40 PM, Adam Inglis <21pointy at tpg.com.au> wrote:
> This CR-8000 I'm working on uses this CMOS static RAM. The machine has a battery compartment for 3 x 1.5 volts AA batteries to retain data.
> I would like to replace this with a CR2032 lithium battery or similar to make space for mods.
> The datasheet for the RAM says data retention voltage min = 2 volts, current 0.01 microA.
> The battery power line has diode protection from reverse voltage via the 5 volt line.
> 
> Any foreseen problems doing that?
What are the specs on that protection diode? What's the part number?

One thing I notice is that it seems you're increasing the reverse voltage on that protection diode from 0.5 V to 2.0 V (that's quadrupled). I did a quick search, and very few diodes have a maximum reverse voltage of only 2 or 3 Volts, but it's possible that you might be getting close to its limit. If there's any question, another diode with higher reverse voltage capacity would be a good idea.

Another thing is the forward voltage drop of the protection diode, assuming it's in series with the battery output. If the drop is 1.0 V or more then you won't meet the minimum voltage of the SRAM. There are LDO (Low DropOut Voltage) diodes available to solve this problem easily, though.

Brian