Author Topic: Which diode to use?  (Read 1342 times)

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Offline Karel

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Which diode to use?
« on: April 19, 2018, 12:08:28 am »
I'm designing a portable device containing a Li-Po battery and a very accurate Real Time Clock chip with integrated crystal.

When in service, the date and time can not be adjusted. This is a fact for this project and it can not be changed.

So, the plan is to set the date and time once after production and before taking the device into service.

The device is going to be re-checked every two years which gives the oppurtunity to check and adjust the time of the RTC.

Because we can not guarantee that the Li-Po battery will never be deeply discharged, we decided to install a coin type battery of 3V 48mAh (BR-1225).

This coin type battery alone will not be enough for the whole service life (5 years) of the device. The RTC consumes 2.5uA.

Apart from changing the battery every two years, I was thinking about the following circuit to use the Li-Po battery as the main powersupply for the RTC
and the coin type battery as a backup.

Unfortunately, this requires a diode with a low forward voltage and an ultra low reverse leakage current of less than 68 nA:

Quote
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.

Source: https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/panasonic_lithium-handbook-2015_interactive.pdf  page 14

Any ideas for the best diode? I was thinking about a BAT54.
« Last Edit: April 19, 2018, 12:11:56 am by Karel »
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Online Ice-Tea

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Re: Which diode to use?
« Reply #1 on: April 19, 2018, 12:33:14 am »
Does it have the be Shottky? BAT54 is Shottky I think... They tend to have higher leakage current.

What about a 4148? Cheap, ubiquitous,...
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Offline SiliconWizard

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Re: Which diode to use?
« Reply #2 on: April 19, 2018, 12:33:31 am »
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.)
« Last Edit: April 19, 2018, 12:39:47 am by SiliconWizard »
 

Offline GerryBags

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Re: Which diode to use?
« Reply #3 on: April 19, 2018, 12:35:19 am »
How about using a diode-connected transistor instead? They have very low reverse leakage currents.
 

Online Ice-Tea

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Re: Which diode to use?
« Reply #4 on: April 19, 2018, 12:37:48 am »
Also.. why BR? And why not the more common (and cheap) CR2032?
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Offline Karel

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Re: Which diode to use?
« Reply #5 on: April 19, 2018, 01:02:25 am »
Does it have the be Shottky? BAT54 is Shottky I think... They tend to have higher leakage current.

What about a 4148? Cheap, ubiquitous,...

It needs to be schottky to avoid too much forward voltage drop.
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Offline Karel

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Re: Which diode to use?
« Reply #6 on: April 19, 2018, 01:03:09 am »
Also.. why BR? And why not the more common (and cheap) CR2032?

BR types are better for long term, low power applications.
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Online Ice-Tea

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Re: Which diode to use?
« Reply #7 on: April 19, 2018, 01:06:15 am »
Out of curiousity.. What chip are you using? AFAIK, uC's are pretty lenient on RTC voltage...
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Offline Karel

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Re: Which diode to use?
« Reply #8 on: April 19, 2018, 01:12:54 am »
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.)

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.

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Offline Karel

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Re: Which diode to use?
« Reply #9 on: April 19, 2018, 01:15:26 am »
Out of curiousity.. What chip are you using? AFAIK, uC's are pretty lenient on RTC voltage...

A PCF2129 from NXP.
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Offline Siwastaja

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Re: Which diode to use?
« Reply #10 on: April 19, 2018, 02:09:17 am »
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?
 

Online Marco

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Re: Which diode to use?
« Reply #11 on: April 19, 2018, 02:31:20 am »
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?
 

Offline SiliconWizard

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Re: Which diode to use?
« Reply #12 on: April 19, 2018, 02:58:57 am »
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?
 

Offline GreggD

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Re: Which diode to use?
« Reply #13 on: April 19, 2018, 03:19:00 am »
At the current for the RTC the forward diode drop of a standard diode will not be 0.6V but more like 0.25-.3V
« Last Edit: April 19, 2018, 04:10:17 am by GreggD »
 

Offline SiliconWizard

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Re: Which diode to use?
« Reply #14 on: April 19, 2018, 03:28:38 am »
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.
 

Offline Karel

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Re: Which diode to use?
« Reply #15 on: April 19, 2018, 03:31:05 am »
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?

The Li-Po battery has an internal circuit that protects it from deep discharge. It disconnects the battery if the voltage is a little below 3V.
It's not a user replaceable part. Imagine the situation that the device is used till the Li-Po reaches almost 3V. The uC in the device detects
this and will warn and shutdown nicely. Then, if the user doesn't use it for 6 months, what is going to happen with the RTC...


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Offline Karel

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Re: Which diode to use?
« Reply #16 on: April 19, 2018, 03:33:10 am »
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?

Nothing. The 3V3 is only present when the device is switched on.
In that case the RTC will consume more power because it switches on the I2C interface.
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Offline Karel

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Re: Which diode to use?
« Reply #17 on: April 19, 2018, 03:38:53 am »
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?

Thanks for the advice. We never specify generic parts apart for "standard" resistors (0603 and 0402).
We always specify the manufacturer and exact manufacturer order number and they can't use something different without
consent.

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.
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the difference between theory and practice in practice.
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Online Ice-Tea

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Re: Which diode to use?
« Reply #18 on: April 19, 2018, 03:40:54 am »
VBAT seems to be good down to 1V8. So I doubt a Shottky is mandatory  8)
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Offline Karel

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Re: Which diode to use?
« Reply #19 on: April 19, 2018, 03:41:59 am »
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.
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Offline Karel

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Re: Which diode to use?
« Reply #20 on: April 19, 2018, 03:42:50 am »
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.

Exactly.
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Offline Karel

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Re: Which diode to use?
« Reply #21 on: April 19, 2018, 03:44:15 am »
VBAT seems to be good down to 1V8. So I doubt a Shottky is mandatory  8)

I'll have a look at the non-schottky diodes and do some measurements.
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Offline SiliconWizard

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Re: Which diode to use?
« Reply #22 on: April 19, 2018, 03:49:58 am »
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. :-//

Edit: I think I get it now. :popcorn:
What you actually want is to be able to use both batteries as the backup source. (Can't you really use a little beefier primary coin cell instead?)

As others have said, a classic small signal silicon diode should do it. Most have very low reverse leakage (down to a few pA for some!) and the forward voltage, at a few µA current draw, will be very low anyway.
« Last Edit: April 19, 2018, 03:57:38 am by SiliconWizard »
 

Offline Karel

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Re: Which diode to use?
« Reply #23 on: April 19, 2018, 03:58:56 am »
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. :-//

Yes, when the device is switched on, the backup battery input is not used. The RTC will use the power from the 3V3 rail when present.

The problem could be solved by not mounting the diodes and simply use a bigger coin type battery that lasts enough for 5 years @ 2.5 uA.
Problem is, there's not enough space. The requirement of the RTC and, specially, the coin type battery came at the last moment
and now I have to shoehorn in those components without having enough space...
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Offline SiliconWizard

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Re: Which diode to use?
« Reply #24 on: April 19, 2018, 04:06:29 am »
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.

Well, take a look at the BAV116T: https://www.diodes.com/products/discrete/diodes-and-rectifiers/diodes/small-signal-switching-diodes/part/BAV116T
(not even the best for this out there, but the characteristic is given in the datasheet).
You can see that Vf is typically down to 400 mV for 100 µA. What happens below is unknown but 400 mV is perfectly low enough for this application IMO. But if you try and measure what happens at 2 µA or so - you'll probably get something down to 100 mV or so - maybe even lower.

Edit: didn't see that the 400 mV point was for 150°C. So it's more like 650 mV at 25°C. Still, given the characteristic discharge curve of a Li coin cell, that should work for Vbat.
The PCF2129 is specified down to 1.8V. At 1.8+0.6 = 2.4V, you'll get most of the coin cell capacity.


And again, at approx 2 µA, Vf will be much down.
« Last Edit: April 19, 2018, 04:21:02 am by SiliconWizard »
 


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