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

E

Eeyore

Jan 1, 1970
0
JimmyB said:
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?

An FET maybe ?

Why is the diode a problem ? This is something that's very commonly done.

Graham
 
J

JimmyB

Jan 1, 1970
0
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?
 
D

Donald

Jan 1, 1970
0
JimmyB said:
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?


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
 
F

Frank Bemelman

Jan 1, 1970
0
Eeyore said:
An FET maybe ?

Why is the diode a problem ? This is something that's very commonly done.

Often you want a bit more, controlling CE as well, to avoid corruption
of the backup ram, during power cycles. Maxim has some chips for this
purpose.
 
T

Tam/WB2TT

Jan 1, 1970
0
JimmyB said:
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?
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
 
J

Jamie

Jan 1, 1970
0
JimmyB said:
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?
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.
 
J

JimmyB

Jan 1, 1970
0
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 ...
<http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2632>

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
 
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