Since Lithium primary batteries are not rechargeable, use of a reverse IC current blocking diode and a protective IC
resistor in series is required where there is the possibility of charging in the equipment circuit.
Use a diode with a low leak current as the reverse current blocking diode. To maintain the characteristics of a coin
type Lithium battery, the total charging amount of the battery during its total usage period must be kept within 3%
of the nominal capacity of the battery.
For example, assuming that a BR-1225 (48mAh) will be used in a memory back-up power supply for 5 years, charging
by the leak current of the reverse current blocking diode should be no greater than 3mAh, thus:
3mAh / usage period (5 years x 365 days x 24 hours) = 68nA.
other words, a leak current blocking diode whose reverse current is not greater must be selected.
Does it have the be Shottky? BAT54 is Shottky I think... They tend to have higher leakage current.
What about a 4148? Cheap, ubiquitous,...
Also.. why BR? And why not the more common (and cheap) CR2032?
As you mentioned, Schottky diodes have relatively high reverse leakage current and you're going to get bitten by this.
The BAT54 (depending on the vendor) has a reverse current of up to several µA (2 µA typ. for the NXP ones).
There are ideal diodes ICs, but most of them target higher average current and have still highish reverse leakage current and a quiescent current to add up.
You could take a look at the RB578VYM100 (Rohm): with a typical Ir of 0.2 µA, it may still be higher than you'd like but it's one of the lowest on the market.
(As a side note, an RTC running for this long without adjustment will drift significantly. For instance, a typical average 100 ppm drift over 2 years (which would not be swiss-watch class, but reasonable) will give you 1.7 hours drift.)
Out of curiousity.. What chip are you using? AFAIK, uC's are pretty lenient on RTC voltage...
I tested one from On Semiconductor on the workbench and it has a reverse leakage current of 47nA @ 2.2V (27degr. C).
The BAT54 from STM has a max forward leakage current of 1uA @ 30V.
I tested one from On Semiconductor on the workbench and it has a reverse leakage current of 47nA @ 2.2V (27degr. C).
The RTC has a drift of max 5 ppm (5 minutes drift in 2 years) which is good enough for the application.
Are you sure you need the backup cell? At the point your li-ion cell has been deep discharged to 1.8V, it's dead and needs a replacement.
Are you sure you need the backup cell? At the point your li-ion cell has been deep discharged to 1.8V, it's dead and needs a replacement. This scenario shouldn't happen, but if it does, it requires maintenance interaction anyway, so maybe set the RTC again at that point?
Also, 5µA is probably not a huge problem for the li-ion cell (i.e., won't cause accidental deep discharge), unless the cell is very tiny.
Or is the li-ion cell going to be a user-replaceable part? So that user might remove it for longer than a small&cheap supercap can keep the RTC running, and you need to keep RTC going during that no-cell-connected time?
I tested one from On Semiconductor on the workbench and it has a reverse leakage current of 47nA @ 2.2V (27degr. C).So closer to 500na at 57 degrees ...
What's wrong with the battery switch over feature in your RTC?
The BAT54 from STM has a max forward leakage current of 1uA @ 30V.
I tested one from On Semiconductor on the workbench and it has a reverse leakage current of 47nA @ 2.2V (27degr. C).
The Rohm RB578VYM100 has a guaranteed Ir of 0.2 µA.
I would not use any widely-implemented part reference such as the BAT54 (that have widely different specs as you have seen) that would require a very specific manufacturer to get the specs you target. This is disaster waiting to happen down the line. Any change in source due to any factor (supply issues, change made for costs reasons that you don't have control over...) would cause some batches of your product not too meet their requirements. Not good.
Look at the example of the problem Dave had with the µCurrent design and a particular opamp. And this kind of "equivalent" part change happens ALL the time, except in very controlled industries.The RTC has a drift of max 5 ppm (5 minutes drift in 2 years) which is good enough for the application.
I took a look at the PCF2129 specs. Those are impressive indeed. It has an integrated TCXO, which explains it.
This IC has a built-in switch over circuit for backup, why don't you use it?
At the current for the RTC the forward diode drop of a standard diode will not be 0.6V but more like 0.25-.03V
Are you sure you need the backup cell? At the point your li-ion cell has been deep discharged to 1.8V, it's dead and needs a replacement.
Yes, except that decent li-ion batteries contain integrated PCMs (protection circuits) with a cut-off voltage of around 2.9V usually, to protect the battery. Going below this threshold will drastically shorten the battery life and when it gets very low, it may even prevent it from recharging altogether, that's why PCMs do this.
So when the cell voltage drops below the cut-off voltage, the PCM will just shut the output down, and your circuit suddenly loses power. Thus the operating range below 2.9V-3V is normally not usable.
And using cells without PCMs is suicidal.
VBAT seems to be good down to 1V8. So I doubt a Shottky is mandatory
The built-in switch-over circuit is ment to be used to switch between the normal powerrail (3V3) and the battery backup.
The 3V3 is only present when the device is switched on. In that case the RTC will consume more power because it will
switch on the I2C interface.
The built-in switch-over circuit is ment to be used to switch between the normal powerrail (3V3) and the battery backup.
Yes. Isn't this what you want? Maybe we haven't fully understood your requirements. Is the backup battery not used just for backing up the RTC?The 3V3 is only present when the device is switched on. In that case the RTC will consume more power because it will
switch on the I2C interface.
Yes, but in this case the backup battery would not be used?
There's probably something we have all missed here.
At the current for the RTC the forward diode drop of a standard diode will not be 0.6V but more like 0.25-.03V
I guess you are right but it's not specified. I'll do some measurements with non-schottky diodes as well.