Switching main/backup battery voltage - Page 1
Forum section: electronics / projects

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 04:18:30 am
content: I have a need to design a switch for the main/backup battery for the RTC.The backup battery is a lithium cell 3.0 V nominal (3.4 maximum) voltage. And the main battery is regulated down to 1.8 V. The switch has to pick main battery when it is present, and the backup battery when the main battery is disconnected. The switching time must be short enough that decoupling capacitors must be able to handle the load during the switch.The voltage drop on the 1.8 V side should be minimal (< 0.1 V, ideally lower). The voltage drop on the 3.0 V side can be as big as needed, as long as the final output is still over 1.8 V.The current consumption is very small. Let's say 100 uA with a huge margin. The switch itself should not be a big burden on the backup battery.The optimization is for price and then the board area.Ideas?One of the options is to have a separate ~2.5 V voltage regulator and a simple diode switcher. Let's put this option aside for now.

author: krho
author_rank: Regular Contributor
timestamp: July 24, 2018, 05:04:29 am
content: I was researching this a while ago. And there is/was no cheap option. The cheapest one was take the MCU that has a VBat pin and can do this automatically

author: Mazo
author_rank: Regular Contributor
timestamp: July 24, 2018, 01:27:13 pm
content: Interesting problem,so after some initial simple discrete designs I devised the following circuit(see attached photos).Resistor values can be tweaked alot to get lower power dissipation and some low vgs(th) fets can be substituted but the main power waster is the comparator(substitute it for something more reasonable for this kind of circuit).Pulling a number from my ass I would say under 10uA consumption isn't hard with the right parts.PS: You could omit the comparator altogether but you lose precise control over the crossover voltage setting and a 1.8V drive fet or bjt will be needed instead of M1.

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 04:10:11 pm
content: Yes, this is similar to what I was thinking. I was really hoping that there is some "magical" solution that is used by everyone.The option with one more LDO and a couple diodes looks like the cheapest and smallest solution.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 24, 2018, 04:24:51 pm
content: By utilizing large current capacity, and low voltage schottkys, the forward voltage drop is actually surprisingly small.I've used 1N5820 in that fashion.The drawback is a higher leakage currents, but if the circuit is to be operated at room temperatures, you might neglect those.

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 04:29:56 pm
content: Quote from: schmitt trigger on July 24, 2018, 04:24:51 pmI see Vf around 0.5 V. And the packages for large current capacity diodes are too big.There is actually a problem with just the diodes approach. Even if main voltage present, the backup battery will be higher, so it looks like active switching is the way to go.

author: Mazo
author_rank: Regular Contributor
timestamp: July 24, 2018, 08:02:58 pm
content: The lower voltage of the main battery makes the circuit awkward somewhat.Just out of curiosity why is the main voltage regulated but the backup one not?Can the power source switching be done before the 1.8V regulator?

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 08:23:11 pm
content: Quote from: Mazo on July 24, 2018, 08:02:58 pmMain source is a big battery that is rechargeable. Backup battery is a CR2032 or similar, which should last closer to the life time of the product.Quote from: Mazo on July 24, 2018, 08:02:58 pmTarget MCU has two power domains VDD and VBAT. VDD is permanently connected to the main battery after the regulator. VBAT powers RTC and other very low power stuff. The goal is to power VBAT and VDD from the main battery when it is present, and only VBAT powered from the coin cell when the main one is not connected.It may be possible to regulate the backup battery to 1.8 V as well, but the low voltage drop switching between two 1.8 V sources is still not easy.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 24, 2018, 08:31:08 pm
content: I might be missing the point here but isn't that a fairly standard feature for RTC chips? Example:https://www.maximintegrated.com/en/products/digital/real-time-clocks/DS1339A.html

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 08:58:28 pm
content: Quote from: Ice-Tea on July 24, 2018, 08:31:08 pmDS1339A costs more than the target MCU with internal RTC. And it definitely costs more than a handful of transistors.And cheaper RTCs don't have a power source switch.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 24, 2018, 09:57:47 pm
content: Well, depends on what your volume is and how much your time is worth I suppose. You can get one around 0.5$ on Digi (so maybe a wee bit lower negotiated?) which may be a hit you'd be willing to take if volume is no more than a few k.https://www.digikey.de/product-detail/en/diodes-incorporated/PT7C43390AWEX/PT7C43390AWEX-ND/6237587

author: ataradov
author_rank: Super Contributor
timestamp: July 24, 2018, 10:03:46 pm
content: Quote from: Ice-Tea on July 24, 2018, 09:57:47 pmThis one does not have a power supply switch. Without a switch, internal RTC is easy as well. The goal is to not drain the backup battery when external power supply is present.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 25, 2018, 02:34:41 am
content: Quote from: ataradov on July 24, 2018, 04:29:56 pmThat Vf is at the rated current.At a milliamperes level you will be surprised the low voltage dropDo an experiment: wire in series a 1n5820, a 1n5817 and a 1n5711. Push 15mA thru the series. Measure with a DMM the Individual diode Vf.But they are indeed large components.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 25, 2018, 12:23:06 pm
content: Quote from: ataradov on July 24, 2018, 10:03:46 pmYeah, well, I blame Googling late. On the road now, so cant verify but check the diodes website. They have a selector guide. 50 cent is doable.