Which diode to use? - Page 1
Forum section: electronics / projects

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 02:08:28 pm
content: 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 RTCand 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:QuoteSource:https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/panasonic_lithium-handbook-2015_interactive.pdfpage 14Any ideas for the best diode? I was thinking about a BAT54.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 18, 2018, 02:33:14 pm
content: Does it have the be Shottky? BAT54 is Shottky I think... They tend to have higher leakage current.What about a 4148? Cheap, ubiquitous,...

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 02:33:31 pm
content: 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.)

author: GerryBags
author_rank: Frequent Contributor
timestamp: April 18, 2018, 02:35:19 pm
content: How about using a diode-connected transistor instead? They have very low reverse leakage currents.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 18, 2018, 02:37:48 pm
content: Also.. why BR? And why not the more common (and cheap) CR2032?

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 03:02:25 pm
content: Quote from: Ice-Tea on April 18, 2018, 02:33:14 pmIt needs to be schottky to avoid too much forward voltage drop.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 03:03:09 pm
content: Quote from: Ice-Tea on April 18, 2018, 02:37:48 pmBR types are better for long term, low power applications.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 18, 2018, 03:06:15 pm
content: Out of curiousity.. What chip are you using? AFAIK, uC's are pretty lenient on RTC voltage...

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 03:12:54 pm
content: Quote from: SiliconWizard on April 18, 2018, 02:33:31 pmThe 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.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 03:15:26 pm
content: Quote from: Ice-Tea on April 18, 2018, 03:06:15 pmA PCF2129 from NXP.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 18, 2018, 04:09:17 pm
content: Are yousureyou 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?

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2018, 04:31:20 pm
content: Quote from: Karel on April 18, 2018, 03:12:54 pmSo closer to 500na at 57 degrees ...What's wrong with the battery switch over feature in your RTC?

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 04:58:57 pm
content: Quote from: Karel on April 18, 2018, 03:12:54 pmThe 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.Quote from: Karel on April 18, 2018, 03:12:54 pmI 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?

author: GreggD
author_rank: Regular Contributor
timestamp: April 18, 2018, 05:19:00 pm
content: 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

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 05:28:38 pm
content: Quote from: Siwastaja on April 18, 2018, 04:09:17 pmYes, 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.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:31:05 pm
content: Quote from: Siwastaja on April 18, 2018, 04:09:17 pmThe 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 detectsthis 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...

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:33:10 pm
content: Quote from: Marco on April 18, 2018, 04:31:20 pmNothing. 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.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:38:53 pm
content: Quote from: SiliconWizard on April 18, 2018, 04:58:57 pmThanks 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 withoutconsent.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 willswitch on the I2C interface.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 18, 2018, 05:40:54 pm
content: VBAT seems to be good down to 1V8. So I doubt a Shottky is mandatory

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:41:59 pm
content: Quote from: GreggD on April 18, 2018, 05:19:00 pmI guess you are right but it's not specified. I'll do some measurements with non-schottky diodes as well.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:42:50 pm
content: Quote from: SiliconWizard on April 18, 2018, 05:28:38 pmExactly.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:44:15 pm
content: Quote from: Ice-Tea on April 18, 2018, 05:40:54 pmI'll have a look at the non-schottky diodes and do some measurements.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 05:49:58 pm
content: Quote from: Karel on April 18, 2018, 05:38:53 pmYes. 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?Quote from: Karel on April 18, 2018, 05:38:53 pmYes, 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.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.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 05:58:56 pm
content: Quote from: SiliconWizard on April 18, 2018, 05:49:58 pmYes, 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 momentand now I have to shoehorn in those components without having enough space...

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 06:06:29 pm
content: Quote from: Karel on April 18, 2018, 05:41:59 pmWell, 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.

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2018, 06:12:44 pm
content: Quote from: Karel on April 18, 2018, 05:33:10 pmWhy feed it 3.3V from what I assume is a DC-DC converter at all? SDA/SCL are allowed to be higher than Vdd and current consumption at Fscl=0 is the same as on battery backup with inactive I2C (note 3 page 60).So just connecting the Li-Po to Vdd and the battery to Vbat should allow it to function fine.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 06:34:00 pm
content: Quote from: Marco on April 18, 2018, 06:12:44 pmWhat I read at page 60 is that the I2C input thesholds (hi/lo) are relative to Vdd.So when Vdd is 4.2V (max Li-Po voltage), the high-level input threshold is 2.94V minimum.Not much room for tolerances...

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2018, 07:00:30 pm
content: Maybe pull the I2C bus up to the Li-Po voltage if the other devices can handle that?Or put an ultra low quiescent current LDO on the Li-Po.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2018, 07:06:35 pm
content: Quote from: Marco on April 18, 2018, 07:00:30 pmI got the same idea but the fist chip I checked (which is connected to the same bus) doesn't like itDidn't bother to check the others.Thanks anyway for helping.

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2018, 07:14:40 pm
content: Feeding the RTC with say a MCP1810 straight from the Li-Po would work, only run the rest of the circuitry off DC-DC.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 18, 2018, 07:28:06 pm
content: Quote from: Karel on April 18, 2018, 05:31:05 pmGreat that you are thinking about this specific use case. I'm sorry to say, but most do not; including the people designing the protection circuits! This is actuallythe most typicalfailure mode in all types of li-ion BMS and protection circuits I have seen, custom designed, discrete or ICs from the reputable big guys.So, according to my experience, it's quite likely that a broken-by-design protection chip will destroy the cell (by first allowing complete discharge to near 0%SoC, which happens already at 2.9V cutoff if your load current is very small (less than, say, 0.05C)), then applying too high quiescent current in its own control logic. And then you need to replace the device with a broken battery. You can set the RTC again at the same time.This isn't necessarily the case, some protected batteries are OK, but I have measured quiescent currents far over 50 µA in "disconnected by undervoltage" circuits. Even if the cell survives the protection chip (i.e., it over-discharges it only slightly), the same protection chip won't probably allow the charging to happen, even if it was still safe to do (for example, the cell still reads 2.9V and would be safe to charge, or it reads 2.5V and would be safe to precharge at reduced rate). The threshold voltages of the PCM chips tend to assume voltage bounceback. In these cases, you can fix the issue by manually charging the cell, bypassing the protection chip. This was a classical failure mode in many laptop battery packs. Don't remember the exact IC anymore, but you ran it down to 0% like you normally do, then forgot it just for a few weeks, and bang, the cells were at about 2.9V IIRC, so completely OK to charge, but the pack was dead and refused to charge.So, while I understand that a "protected cell" concept sounds like a simple task to allow you stop worrying, that is not always the case.No one suggested not using cell protection and management, though.Usuallyin professionally designed fixed-battery devices, it's located on your PCB however, whereas the cell itself is provided with passive protections such as fuse against short circuit (PTC, resetting bimetal, or non-resetting classic fuse).The reason for this is the assembly simplicity and part cost. Assembly simplicity is important; I have heard/read reports of cases where the "protection PCB" has been installed incorrectly, for example puncturing the cell or insulation sleeve in the case of a 18650 cell, causing a short before the protection chip. These incidents do happen because the actual cell manufacturers with high quality control and reputation factor do not tend to offer these "PCM'd" cells, but they are often provided by secondary sources, manufactured in small batches, main customers being hobbyists and clueless small companies/startups. Not saying this is always the case, reputable "complete integrated battery" manufacturers exist as well. Their products won't at least catch fire, but still, even the reputable products have failed in your "6 months on the table after complete discharge" use case.The magical "protected cell" just moves this IC from your PCB to a small PCB next to the cell, and almost always lacks crucial safety controls such as cell temperature qualification (charge rate management/prevention in low temperatures), charge overcurrent trip point with different value from discharge overcurrent trip, etc., so that the designer now simply has: 1) total lack of understanding, combined with 2) false sense of security.You may have a very good battery with a good PCM, or you may not. Just sayin'.In any case, if you were to place the cell management on your own PCB - you could even use the exact same IC - now you have the choice! - then there would be no issue of taking that 5µA current directly off the cell - possibly with a simple series resistor such as 470R to limit fault current in case of short. This 5µA would be in the same order of magnitude as the quiescent current of the PCM (or even less, if it's a shitty PCM), so even if it ended up killing the cell by overdischarge in your scenario, that would have happened anyway...You won't necessarily even need a disconnection switch. As long as your li-ion charger IC does the starting voltage&temperature qualification, as itdefinitelymust, there is nothing unsafe doing your own cutoff in the microcontroller by shutting down external circuits and going to low-current sleep at low V (of course, this is not always feasible, depending on your external circuits and if they can power down easily). But guess what? This is the normal way to design these things, andexactlythe reason why so many modern ICs provide power-down modes and boast about <1µA quiescent currents for battery operation. Worst case, you fail with the software, the cell is overdischarged, cannot be charged anymore due to charger IC qualification protection, and you need to replace it. Unless you have an over-the-air FW update mechanism, this is no different from any other bug.But I guess you won't want to do that. Maybe it's paperwork, or the classical faulty "safety belief". These arbitrary constraints do happen in projects, and I understand that. But I still hope this point of view helps...

author: Karel
author_rank: Super Contributor
timestamp: April 19, 2018, 05:52:02 am
content: Quote from: Siwastaja on April 18, 2018, 07:28:06 pmWell, I don't want to go too far offtopic but generally, I agree with you. The problem is that I'm not at liberty to discuss all detailsof the project here. What I can say is that we prefer to use Li-Po batteries that have already been tested and certified in a laboratoryfor UN38.3. We don't want to do that ourselves with the complete product. Thera are already too many other tests we are obliged to do and pay for.Second, the uC in the device observes the battery voltage and, in fact, it will shutdown before it reaches 3V. The exact level canbe set in the firmware. So, we have some more tolerance here before the battery will be damaged.Thanks for the insight anyway.

author: Karel
author_rank: Super Contributor
timestamp: April 23, 2018, 08:54:36 am
content: I decided to use two diodes BAS116H. At 10uA the forward voltage is 0.4V according to the datasheet. So, at 2.5uA it will be even less.Reverse laeakage current is very low: max 5nA@75V.Thanks to everybody.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: April 23, 2018, 09:59:13 am
content: Quote from: Karel on April 23, 2018, 08:54:36 amYou can just use two different diodes - as the reverse leakage of the diode from the coin battery is not important, use a Schottky there, and for the diode from the LiPo use a low leakage diode (BAS116, BAV199 etc).CheersAlex

author: Mr. Scram
author_rank: Super Contributor
timestamp: April 23, 2018, 10:09:09 am
content: Do I understand correctly that the diode is there to provide reverse current protection? Can't you use a FET to achieve something similar without the voltage drop? I'm probably missing something.

author: Karel
author_rank: Super Contributor
timestamp: April 23, 2018, 12:32:37 pm
content: Quote from: Mr. Scram on April 23, 2018, 10:09:09 amThere's almost no space. This extra backup battery has been added at the last moment