Battery backup for RTC in AT91SAM9261

Looking at the Eval board schematic they have a 3 terminal device R1100DC121C. Ultra low power LDO

National's part might be a sub

Cheers

Thanks for the info. The Ricoh part looks interesting, but I will have to see how hard it is to get and the price.

The National part has a way higher power consumption... Quiescent Current (mA) = .14 mA

Having to add the regulator makes the RTC less useful. The total power is now 4 uA vs. 0.5 uA for a real RTC. For about the same price as the LDO, I can add a RTC chip at a much lower power consumption with a smaller battery and be done with it.

I guess none of the ARM MCUs are really perfect. The entire TI family is not 5 volt tolerant. I guess you can get away with that when you are going for ultra low power, like the MSP430 or selling FPGAs, but I am surprised that an embedded processor family would not be 5 volt tolerant.

Have a look at the Seiko S-817-A25 regulator Specs: 2.5 volt, 50 mA max, 1.2uA quiesent, 0.3V dropout available in a number of SOT packages and TO92 Can be orderd from Mouser in small quantities.

Gerhard van den Berg CSIR

The Seiko S-812C25 is actually a bit better. Specs: 2.5 volt, 50 mA max, 0.9uA quiesent, 0.3V dropout This is for the TO92 package, the dropout for the SOT packages is 0.5Volt. Also available from Mouser.

Gerhard van den Berg CSIR

Thanks for the info. The S-812 will not do the job. The required output voltage is 1.5 volts and this line only goes down to 2.0 volts. The S-817 will work down to 1.1 volt output. This is no great part. The dropout can be as high as .99 volts, at least at 10 mA, I guess it would be a lot lower at uamp currents. The regulator current can be as high as 2.5 uA which would double the current from the battery.

This is a lot better than no backup circuit, but it still irks me that they designed a circuit for a non-existent backup battery and left it to the designer to figure out how to provide the power.

What about a 1.2V ni-cad, that you'd leave on trickle charge as long as the equipment is powered up?

Or is the whole thing battery operated? If that's the case, then never mind. :-)

Good Luck! Rich

This turns out to be an adequate regulator. With a quiescent current of up to 2.5 uA, it can double the power consumption of this circuit. It will require a larger battery than I would have liked to have used, a 2032 will give me almost 5 years of backup compared to a 1632 for the RTC alone.

The S-817A12ANB regulator comes in a package as small as the SC82 which is about 2 x 2 mm. So at least size is not an issue. Cost at Mouser is only $0.30 (@3000) so cost is not an issue either. But it still irks me if for nothing else because I have to add another part to the board... >:-(

Sorry for the incorrect info. I spotted my mistake just after the second post but when I tried to reply our news server was unreachable (I could see the cashed items). The server only came back 24 hours later. I am glad you spotted the S-817A12ANB. I checked to see if the S-817B12AUA (B series without short circuit protection) had a lower quisent current, but it was the same :-(. Yes I agree, an onboard 1.2v backup regulator would have been great.

Gerhard van den Berg CSIR