Backup battery for AT91SAM9261
in [Embedded]
|
Prev: RTOS brands
Next: Low cost GPIB-USB controller
From: rickman on 5 Mar 2006 14:04 Looking at the data sheet for the AT91SAM9261, it says the back up supply voltage range is 1.08 to 1.32 volts with a current of 2.5 microAmps. This could be supplied by a watch battery except I have no idea how to regulate the voltage to this range without draining the battery from the regulator quiessent current. I don't recall seeing any circuits that could provide a regulated voltage and not draw at least 10 times this much current. What was Atmel thinking when they drew up these plans for a battery backed RTC? Or am I missing something obvious, like a 1.2 volt, low self drain battery that is perfect for this job?
From: Didi on 5 Mar 2006 14:19 rickman wrote: > Looking at the data sheet for the AT91SAM9261, it says the back up > supply voltage range is 1.08 to 1.32 volts with a current of 2.5 > microAmps. This could be supplied by a watch battery except I have no > idea how to regulate the voltage to this range without draining the > battery from the regulator quiessent current. I don't recall seeing > any circuits that could provide a regulated voltage and not draw at > least 10 times this much current. > > What was Atmel thinking when they drew up these plans for a battery > backed RTC? > > Or am I missing something obvious, like a 1.2 volt, low self drain > battery that is perfect for this job? Perhaps one of these very high capacitance capacitors, charge it to 1.3 V, 1F @ 2.5 uA will lose 0.25 V for > 10 days, I believe I have seen capacitors much larger than that. Making a regulator which cosumes 1 uA or so will be no rocket science either. Dimiter ------------------------------------------------------ Dimiter Popoff Transgalactic Instruments http://www.tgi-sci.com ------------------------------------------------------
From: Gene S. Berkowitz on 5 Mar 2006 16:52 In article <1141585448.583257.226770(a)i39g2000cwa.googlegroups.com>, spamgoeshere4(a)yahoo.com says... > Looking at the data sheet for the AT91SAM9261, it says the back up > supply voltage range is 1.08 to 1.32 volts with a current of 2.5 > microAmps. This could be supplied by a watch battery except I have no > idea how to regulate the voltage to this range without draining the > battery from the regulator quiessent current. I don't recall seeing > any circuits that could provide a regulated voltage and not draw at > least 10 times this much current. > > What was Atmel thinking when they drew up these plans for a battery > backed RTC? > > Or am I missing something obvious, like a 1.2 volt, low self drain > battery that is perfect for this job? Possibly it was originally designed for use with mercuric oxide batteries, which have a no-load voltage of 1.35 volts. Mercury-formula batteries are no longer legal in the USA, and this has been a problem for (vintage) camera buffs. The typical solution is to take a 1.5V alkaline cell and add a diode to drop the voltage, but this probably won't work well with a 2.5 uA load. Of course, a rechargeable NiMH or NiCd cell is 1.2V... You could trickle charge it when "main" power is available, and run from it for the RTC function when "off". A fully charged 500 mAH cell would run a 2.5 uA load for 200,000 hours, or 22 years, if not for the ~1% self- discharge. You would certainly get a few months before you would need to supply a recharge... --Gene
From: rickman on 5 Mar 2006 18:04 Gene S. Berkowitz wrote: > In article <1141585448.583257.226770(a)i39g2000cwa.googlegroups.com>, > spamgoeshere4(a)yahoo.com says... > > Looking at the data sheet for the AT91SAM9261, it says the back up > > supply voltage range is 1.08 to 1.32 volts with a current of 2.5 > > microAmps. This could be supplied by a watch battery except I have no > > idea how to regulate the voltage to this range without draining the > > battery from the regulator quiessent current. I don't recall seeing > > any circuits that could provide a regulated voltage and not draw at > > least 10 times this much current. > > > > What was Atmel thinking when they drew up these plans for a battery > > backed RTC? > > > > Or am I missing something obvious, like a 1.2 volt, low self drain > > battery that is perfect for this job? > > Possibly it was originally designed for use with mercuric oxide > batteries, which have a no-load voltage of 1.35 volts. > Mercury-formula batteries are no longer legal in the USA, and this > has been a problem for (vintage) camera buffs. The typical solution > is to take a 1.5V alkaline cell and add a diode to drop the voltage, > but this probably won't work well with a 2.5 uA load. > > Of course, a rechargeable NiMH or NiCd cell is 1.2V... You could > trickle charge it when "main" power is available, and run from it for > the RTC function when "off". A fully charged 500 mAH cell would run a > 2.5 uA load for 200,000 hours, or 22 years, if not for the ~1% self- > discharge. You would certainly get a few months before you would need > to supply a recharge... Isn't that a pretty large battery for a clock backup? But the self discharge is the real issue. I just find it odd that they would design the power circuit for a real time clock to be anything other than compaitble with 1.5 volt Silver Oxide found in watches or 3 volt lithium cells found in pretty much all the other stuff that needs a backup battery. If it needs an LDO to work with standard batteries, then shove that inside the chip. Isn't integration what these chips are all about?
From: Ulf Samuelsson on 5 Mar 2006 18:29
> Isn't that a pretty large battery for a clock backup? But the self > discharge is the real issue. I just find it odd that they would > design the power circuit for a real time clock to be anything other > than compaitble with 1.5 volt Silver Oxide found in watches or 3 volt > lithium cells found in pretty much all the other stuff that needs a > backup battery. If it needs an LDO to work with standard batteries, > then shove that inside the chip. Isn't integration what these chips > are all about? Since it is in a 0,13u process, the core voltage is limited to 1.2V. The integrated LDO has 30-40uA inherent power consumption. The external voltage regulator has 1 uA power consumption. Different processes you know. -- Best Regards, Ulf Samuelsson ulf(a)a-t-m-e-l.com This message is intended to be my own personal view and it may or may not be shared by my employer Atmel Nordic AB |