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Device to isolate power rail from SRAM chip when the power is cut?

Hi,

I'm trying to design a simple SRAM memory backup for an old 8 Bit Computer. Everything is on track except for one problem..

I have a 3v Lithium battery connected to the SRAM Gnd and to SRAM Vcc through a diode. The idea of the diode is that when the main Vcc 5v comes on the battery won't get 'charged up' if you see what I mean.

Now what I want when the main power goes off is a way to stop the battery's power going back through the main Vcc line to all the ICs in the computer. Is there a proper way of doing this apart from putting a diode inline to prevent this?

Reply to
JimmyB
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Everything is on track

a diode. The idea of

up' if you see what

power going back

way of doing this

One diode or two diodes ??

Yes you are correct, the battery could be connected to every part on the vcc line.

Isolating the SRAM thru a diode to Vcc and another to VBatt, will do the trick. ( Low power Schottky diodes )

There are vendors that make chips for this very problem.

good luck

donald

Reply to
Donald

Apart from using special parts, you need two diodes: one in series with the battery and one in series with VCC. As somebody pointed out, you might also want to do something creative with CE. Else, when VCC goes off, CE and WE can both be in the ON state.

Don't be too conceren with diode voltage drop. I measured the drop across a

1N914 that is powering a small SRAM, and got about 0.22 V.

Tam

Reply to
Tam/WB2TT

Dallas semiconductor had a lot of gizmos for these kinds of issues (including smart sockets that provided all the batteries and switching); DS1210 is about right for this occasion ...

Reply to
whit3rd

Everything is on track

a diode. The idea of

up' if you see what

power going back

way of doing this

use 2 diodes. one to isolate the cell from charge, the other is to isolate back flow of the cell after it's diode.. its simple.. the first diode from the cell to Vdd (cathode to Vdd). the second diode from your power source cathode also connected to Vdd.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

Thanks for all the advice guys :) Seeing that this device is only a few $$$ I will go with the DS1210 route with the CS protection.

Cheers, Jimmy

Reply to
JimmyB

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