Low power, battery operated design with dual linear regulators  - Page 1
Forum section: electronics / projects

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 01:29:19 pm
content: Hello,I'm looking to build a project around the ESP32 module. It would be powered from a single LiIon cell and I want the ESP32 to be in a deep sleep state when the project is turned off so that the inbuilt RTC on the ESP32 remains active and maintains time. I plan to use 2 linear voltage regulators: First the TL1963A - 3.3V, 1.5A for powering the ESP32 + peripherals (Display, IMU, camera, etc) under normal operation and a second TPS797 - 3.3V, 50mA with a very low quiescent current (1uA) to power just the ESP when project is turned off.Here's the power switching flow I have in mind. Please refer schematic attached with the post.1- Everything is on and the 1.5A regulator (TL1963A) is active.2- User switches the power switch off, which is detected by an ESP32 GPIO.3- ESP32 performs the necessary shutdown routine (save some data to memory), shuts down power to all peripherals (using the FET), shuts down wifi and other internal hardware blocks (WiFi, Bluetooth, etc).4- ESP32 now draws less than 50mA of current so turns off the 1.5A regulator (TL1963A) and the smaller 50mA regulator (TPS797) is used. The Schottky diode will create a 0.3V drop so ESP32 Vdd will drop from 3.3V to 3V (as per my understanding). Will this voltage sag create a problem?5- The ESP32 now enters a deep sleep state ("Hibernation mode" as per datasheet) and draws about 5-10uA with the RTC and certain GPIOs active for waking the device back up when the power switch is turned ON again.I need some comments on whether something like this would work reliably? I am expecting a minimum battery life of 6 months to a year with a single 18650 cell. How can I improve the design to get the same functionality?Thank you very much for your time!

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2019, 02:28:58 pm
content: How about using LiFePO4 batteries and ditch the regulators?https://youtu.be/heD1zw3bMhw?t=4m35s(Video was made in 2016 which is why Andreas says they are not readily available.)

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2019, 02:44:23 pm
content: Another idea...How about using the TL1963A only for power the peripherals and the TPS797 only for the ESP32?The ESP32 enables the TL1963A only when it is fully awake. The two 3.3V rails don't have to be connected.

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 03:34:31 pm
content: Quote from: ledtester on September 15, 2019, 02:28:58 pmI saw that solution somewhere but the device I am making would have several peripherals in the future, all of which won't be able to run at a voltage lower than 3.3V, as the ESP. I need a stable 3.3V rail. Perhaps I can have a step up circuitry for them? But I don't like the idea of doing that.

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 03:37:10 pm
content: Quote from: ledtester on September 15, 2019, 02:44:23 pmThe TPS797 won't be able to sustain the ESP32 at all times. The ESP32 can draw up to 500mA with spikes up to 800mA during operation of WiFi and Bluetooth blocks. If I have a separate regulator of that rating for the ESP alone, I lose out on quiescent current.Also, a friend pointed out that the Schottky diode won't operate at such low uAs of current, so I need a different hardware circuitry.

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 04:42:49 pm
content: Quote from: Srijal97 on September 15, 2019, 03:37:10 pmActually, I found this diode here:https://www.mouser.com/ProductDetail/Central-Semiconductor/CFSH01-30L-TR?qs=sGAEpiMZZMtQ8nqTKtFS%2FOl9H8JU2QXlSOBgZQJH20Qh9buCivJ6mw%3D%3DThe datasheet says that at a forward current of 5-10uA, the voltage drop would be about 0.1V, and at 10-50mA the drop would be about 0.3V. But the diode DOES conduct and my ESP32 would work, right? From what I understand, the ESP32 Vdd would go from 3.3V (on the TL1963A) to 3V (on the TPS797 at 50mA) and then to 3.2V (on the TPS797 at 10uA) as I go to a deep sleep state and draw a lower current. Is that okay?

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2019, 05:44:27 pm
content: Quote from: Srijal97 on September 15, 2019, 04:42:49 pmThe regulator's output capacitor will help mitigate the transition, and you could always make it biggerWhen you say that 50mA will be drawn by the system before going into deep sleep, how much of that is due to the ESP32 and how much is due to peripherals?

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2019, 05:51:02 pm
content: I found this thread:https://esp32.com/viewtopic.php?t=4908If you look at the next to last post:https://esp32.com/viewtopic.php?t=4908#p21327you'll see that the poster's ESP32 system had a voltage dip from 3.3 to 2.9 volts and everything still worked.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 15, 2019, 05:58:38 pm
content: Quote from: ledtester on September 15, 2019, 02:28:58 pmNot a bad idea, but just a few thoughts:- LiFePO4 doesn't have as much energy density, so the OP should see whether the added weight/and possibly volume is no problem in their application,- LiFePO4 batteries have a 3.6V voltage when fully charged. That's the upper limit of most 3.3V ICs. Should be fine then without regulators, but something to check/think about,- In the same vein, unless you need a strict 3.3V, the changing voltage won't be a problem; if you need a ref. voltage, design so that it will be under 3V, and use a low-power reference IC, it will be better than a lousy LDO for a reference anyway,- Their typical cut-off voltage is around 2.75V, which is actually quite good here. If it's a bit low for your application you can use a voltage supervisor with 3V threshold or so - you wouldn't lose much capacity,- They are inherently much safer than Li-ion batteries,- They have a much better cycle life,- They are still a bit more expensive at the moment.That's certainly several points in their favor here.

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 06:07:50 pm
content: Quote from: ledtester on September 15, 2019, 05:44:27 pmThe 50mA is just for the ESP32, the peripherals are all shut down by then. And 50mA is just the maximum limit, realistically it would be around 10-30mA. I think a large capacitor would work just like you suggested and do the job!

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 06:10:12 pm
content: Quote from: ledtester on September 15, 2019, 05:51:02 pmOh, that's good. I think I'll test it out once I get the dev board in a couple of days and see what happens.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2019, 06:21:26 pm
content: Are there really no LDOs with a high current rating and a low quiescent current? I have to admit it has been a long time since I have looked for something like that, but it's not exactly that uncommon to find a use for such a device. If nothing else, it shouldn't be that difficult to make one with a P channel MOSFET and a micropower opamp.

author: mariush
author_rank: Super Contributor
timestamp: September 15, 2019, 08:03:01 pm
content: Have you looked at switching regulators to power the low current part?You seem to focus on the low quiescent bit, but you're still wasting a lot of battery with a linear regulator.Here's for example some examples of switching regulators on Digikey :ablic S-85S1P series :snt-8a package :https://www.digikey.com/product-detail/en/ablic-u-s-a-inc/S-85S1PD33-I8T1U/1662-3379-6-ND/9489617snt-6a package (cheaper, no voltage divider block) :https://www.digikey.com/product-detail/en/ablic-u-s-a-inc/S-85S1AB33-I6T1U/1662-3366-1-ND/9489567• Ultra low current consumption:260 nAquiescent current• Efficiency (when under 100 μA load):90.5%• Fast transient response: COT control• Input voltage: 2.2 V to 5.5 V• Output voltage: 0.7 V to 2.5 V, in 0.05 V step , 2.6 V to 3.9 V, in 0.1 V step• Output voltage accuracy: ±1.5% (1.0 V ≤ VOUT ≤ 3.9 V) ±15 mV (0.7 V ≤ VOUT < 1.0 V)• Switching frequency: 1.0 MHz (at PWM operationAlso has separate voltage divider block built in (you can disable it if you don't need it).. for the snt-8a package version that's around 0.5$ more expensive.Yeah, it's expensive compared to a linear regulator, but if you extend the devices life by some percentage...If you want something cheaper, maybe look at Torex XC9265A331MR-G :https://www.digikey.com/product-detail/en/torex-semiconductor-ltd/XC9265A331MR-G/893-1307-6-ND/5964917Up to 200mA, 0.8..2.1uA current for the 3.3v out versionFor the high current regulator, you can simply use something with enable pin to put that regulator to sleep when not neededSome examples... see maybe Semtech SM189SC189ZSKTRT (SOT-23-5 version) :https://www.digikey.com/product-detail/en/semtech-corporation/SC189ZSKTRT/SC189ZSKCT-ND/2182343SC189ZULTRT :https://www.digikey.com/product-detail/en/semtech-corporation/SC189ZULTRT/SC189ZULDKR-ND/2182342shutdown current typ 1uA , max 10uA, up to 1.5a switching current, around 92%efficiency - see datasheet, page 5 top right cornerADP2120ACPZ-3.3-R7 :https://www.digikey.com/product-detail/en/analog-devices-inc/ADP2120ACPZ-3.3-R7/ADP2120ACPZ-3.3-R7CT-ND/24365510.3-2uA shutdown current, up to 1.25A output current etc .. more expensive..

author: Srijal97
author_rank: Contributor
timestamp: September 15, 2019, 08:20:21 pm
content: Quote from: mariush on September 15, 2019, 08:03:01 pmI think I can go for the extra cost of the SMPS. I'll check them in detail later but they seem to be good.So for paralleling the regulator and smps output, I should stick with the Schottky diode and some capacitors like mentioned earlier?

author: mariush
author_rank: Super Contributor
timestamp: September 15, 2019, 09:06:03 pm
content: Yes, you could use diodes.Once you get the high current regulator working and turn off the low current regulator, you could short out the diode on the high current regulator using a mosfet with low Rds(on) ... so you won't lose a lot of current on that diode.Or you could use a p channel mosfet to keep the output of that high current regulator disconnected until needed.Something else you could do would be to use specialized ICs for this task, but this would add more cost compared to a diode.For example here's an ideal diode :https://www.digikey.com/product-detail/en/texas-instruments/LM66100DCKR/296-53541-1-ND/10273271Here's another:https://www.digikey.com/product-detail/en/maxim-integrated/MAX40200AUK-T/MAX40200AUK-TCT-ND/7599791There's also controllers or multiplexers which allow you to switch between 2 or several inputs... but they're expensive.

author: Srijal97
author_rank: Contributor
timestamp: September 16, 2019, 04:36:33 pm
content: Quote from: mariush on September 15, 2019, 09:06:03 pmThose ideal diodes are really nice! I see the "Dual Ideal Diode ORing of supplies" section in the LM66100 datasheet and it seems perfect for what I want to achieve.I didn't know that switching regulators are better even at such low currents, thanks for pointing it out. So I think the SC189(3.3V fixed, 1.5A) would be good for the high current part but I'm a bit doubtful of the low current section. The sleep currents would be about 5-10uA, would the switching regulator S-85S1P still be better than the linear TPS797 at that time? They mention efficiency for loads of 100uA, but I'm a 1/10th lower.Other than that, I think the TPS27081A PFET load switch would be good for turning the peripherals ON/OFF. I've attached the planned schematic for now, is it alright?

author: mariush
author_rank: Super Contributor
timestamp: September 16, 2019, 05:50:02 pm
content: for SC189 it would make sense to use it for higher currents (like 0.5-1A) as that's what it's optimized for.You can look in datasheet -https://www.semtech.com/uploads/documents/sc189.pdf- at page 5 and you see there two charts on the right side.1. Efficiency vs Load Current (Vout = 3.3v) with a curve for 4.0v and a blue curve for 5v2. Efficiency vs Load Current (Vin 5v , V out = 3.3v) with multiple curves depending on inductor chosenSo you can see in first chart that with 4v (what would be typical for a lithium battery, let's ignore the 5v curve on that chart) , with the recommended inductor (or one with same properties) they think you're gonna get around 90% efficiency at 0.1A and peaks at round 0.3A ... 0.6A at around 93-94%and then above 0.6A the efficiency will slowly go down towards 90%Below around 0.1A, the efficiency goes down significantly, but it's still gonna be around 75% at let's say 25mA (the smallest tick on the x asis is 0.1A but let's estimate)So let's say your lithium battery's voltage right now is 4v and let's compare this regulator to linear regulator at these loads :The linear regulator's efficient is same across the whole range... voltage goes in, voltage goes out .. same amount of current + tiny current consumption of ldo.Vin ... 100%Vout = ? % => ? % = Vout x 100 / Vin = Vout x 100/4 = 25 x Vin = 82.5%The switching regulator will be 75% at 25mA, 90% at 0.1 , ~93% at 0.3..0.6A , ~90% at 1.5ASo the linear regulator will only be more efficient at super low loads.If the battery voltage changes a bit, let's say it goes down to 3.6v .. the linear regulator becomes a bit more efficient efficiency is : 3.3v x 100 / 3.6 = ~91.6%The switching regulator may still be more efficient at 0.3..0.6A range, but be less efficient outside this range.However, keep in mind that the linear regulator will have a minimum voltage drop. For example, the linear regulator may have 0.1v drop at 0.1A, 0.25v at 0.3A and 0.4v at 1A or more.In this case, the regulator would output 3.3v at 0.1A, and will output 3.3v at 0.3A (because 3.6v - 0.25v = 3.35v) but at 1A, the linear regulator will only output 3.2v (because 3.6v - 0.4v = 3.2v)So at 3.6v, the linear regulator may become unusable.But let's look at that Torex regulator with a maximum 200mA output...See datasheet -https://www.torexsemi.com/file/xc9265/XC9265.pdf-Right on the first page, you will see a chart for the version of the chip with 1.8v output and you can see the curves for 2.7v, 3.6v and 4.2v - you an estimate similar curves for 3.3v output versionBut what's cool in that chart is that even at 0.1 mA the efficiency of the chip is already at over around 85-90% and stays relatively flat up to 100mA or so.So you could use this regulator for 3.3v < 100mA, and switch to the SC189 regulator from 100mA and higher.You'll have 85-90% efficiency below 100mA, and then you'd have 90% to 93-94% up to around 0.6A and then slowly go down towards 90% efficiency.A switching regulator will also be able to work with much lower input-output differential (linear regulators have to deal with that voltage drop on the internal transistor)Even at 1mA sleep power consumption, a linear regulator will be as low as ~78% efficient, while this Torex IC will be around 85-90% efficient, if they don't lie in datasheet. That extra 10% efficient is not something to sneeze at.

author: Srijal97
author_rank: Contributor
timestamp: September 16, 2019, 06:21:56 pm
content: Quote from: mariush on September 16, 2019, 05:50:02 pmYes, the SC189 is good for higher currents.For the lower currents, I just need to keep the ESP under sleep at 5-10uA. If you see the efficiency curves of the switching regulators (Page 30 of the S-85S1P datasheet), I'll be going from an efficiency of 80% to 60% from 10uA to 1uA. Meanwhile the TPS797 linear regulator is at an efficiency of 78% at full battery voltage of 4.2V, and this goes up to 90% as the battery discharges. So I think that the linear regulator is a better option here.However, the quiescent current for the switching regulator is significantly lower(300nA vs 1uA). Does this make it better?

author: mariush
author_rank: Super Contributor
timestamp: September 16, 2019, 07:55:12 pm
content: If you're sure your circuit won't use more than 50mA, then the linear regulator may make more sense. It's a less complex circuit and the output voltage will be smoother compared to the output of a switching regulator.Note though that there may be better options for linear regulators than that regulator you chose, which costs 1$For example:MIC5317 (3v) orMIC5365orMIC5366(3.3v) -https://www.digikey.com/product-detail/en/microchip-technology/MIC5366-3.3YMT-TZ/1611-MIC5366-3.3YMTTZCT-ND/5701178Less than 10 cents if you buy 25 or more. Has higher current at 30uA but has as little as 20mV dropout voltage at <1mA

author: Srijal97
author_rank: Contributor
timestamp: September 17, 2019, 04:49:18 am
content: I just need to make 2 devices, so I'm ready to pay more for better performance here. Isn't that ground pin current just too high?