Really cheap very low current "regulator" - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: October 02, 2014, 05:20:12 pm
content: Hi all,I've designed a system using the STM32F4 series of microcontrollers and I'm wanting to interface my lithium battery with the VBat input of the microcontroller. According to the datasheet, the VBat input can accept voltages between 1.65v and 3.6v. The different voltages of a Lithium battery are generally disputed around the internet but going off some research information that I found for a completely discharged lithium battery having a voltage of around 3v and a completely charged battery having a voltage of 4.2v. As can plainly be seen, the voltage here doesn't match the input voltage of the VBat input.I'm trying to design a really cheap "regulator" (more so voltage dropper) that can go between the battery and the VBat input and have designed a circuit that drops just under 1v so the voltage range is within the voltage range of the VBat input. What would the best method to do this? I would be happy using a diode if I knew there wasn't any chance of it exceeding the VBat input. I'll attach an image of the circuit I've designed.Cheers!

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2014, 06:07:42 pm
content:  0.45  mV/CTEXAS INSTRUMENTS TPS73130DBVT.. IC, LDO REG 150MA 3.0V SOT23-5

author: bobcat
author_rank: Regular Contributor
timestamp: October 02, 2014, 06:16:14 pm
content: It also depends on the operating voltage and I/O requirements of devices connected to the STM32. If they can operate between at voltages between 2v and 3.2v then use a diode. Otherwise, use a regulator.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2014, 06:20:19 pm
content: Or else:Features visible "Pilot Light" operation.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2014, 06:58:03 pm
content: Or else:

author: pyrohaz
author_rank: Regular Contributor
timestamp: October 02, 2014, 07:21:04 pm
content: Quote from: Paul Price on October 02, 2014, 06:07:42 pmThank you for the help! I should've mentioned using E12 resistors and either NPN, PNP, NMOS or PMOS transistors.Quote from: bobcat on October 02, 2014, 06:16:14 pmThe VBat region only powers the RTC and backup domain (some SRAM) with a maximum current draw at anytime of ~11uA.

author: tom66
author_rank: Super Contributor
timestamp: October 02, 2014, 07:54:47 pm
content: I have to wonder. An LM317 in SOIC8 is as cheap as 8.2p in onesies...http://uk.farnell.com/jsp/search/browse.jsp?N=0&Ns=P_PRICE_FARNELL_UK%7C0&Ntk=gensearch&Ntt=LM317&Ntx=mode+matchallpartial&suppressRedirect=true&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D0%26Ntk%3Dgensearch%26Ntt%3DLM317%26Ntx%3Dmode%2Bmatchallpartial%26suggestions%3Dfalse%26ref%3Dglobalsearch%26_requestid%3D200506%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26divisionLocale%3Den_UK%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D0An op-amp + pass transistor + resistors has got to cost more than that...Add two resistors and an output capacitor (the required decoupling capacitor for Vbat will probably suffice if you put the 317 near the STM32) and you're done. Why overcomplicate it.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2014, 08:35:33 pm
content: a LM317 requires a resistor from Vset pin to Vout that uses i.e. (1.2V/240 ohm) = 5 mA ...this is a battery drainer.Also LM317 has a high dropout voltage: 3VThe simple 1-transistor 2N3904 circuit above reg. uses very little current and at VBat <3.5V, all the regulator current is delivered to the output load, no battery current wasted to power the regulator.

author: dannyf
author_rank: Super Contributor
timestamp: October 02, 2014, 08:40:43 pm
content: -As can plainly be seen, the voltage here doesn't match the input voltage of the VBat input.-if I were you, I would look to see how others have dealt with such an :issue:.the solution is far simpler than you might have thought.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2014, 09:14:19 pm
content: LM358D costs $0.10 even in small quantities at Farnell.com

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 02, 2014, 10:57:41 pm
content: Micropower LDOs aren't exactly expensive...e.g.http://www.digikey.co.uk/product-detail/en/AAT3221IGV-3.3-T1/863-1491-1-ND/4240225

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 03, 2014, 01:17:34 am
content: A series diode?

author: Andreas
author_rank: Super Contributor
timestamp: October 03, 2014, 04:38:27 am
content: Hello,I would use a NXP BF545B J-FET (or similar).Input on drain. (100nF to ground)Gate = ground.Output on source (100nF to ground).Pinch off voltage is 1.6 to 3.8 V.So you might have to select for the good ones.Quiescent current: so low that you cannot measure it.I am using similarly a BF245C as 5V regulator (ok its more or less around 4V)for a panel voltage meter with LCD drawing 10uA in total.With best regardsAndreasEDIT: I have simulated that with a MMBF4119 which gives similar results.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2014, 12:58:41 pm
content: The highest performance solutions meaning lowest quiescent current are all going to be integrated. Micropower regulators, references, and operational amplifiers intended for this type of application exist.You might be able to grade JFETs or depletion mode MOSFETs for the right gate threshold voltage.A micropower reference or micropower low dropout voltage regulator would be ideal. They can be had with quiescent currents considerably below the 11 microamps that the microcontroller uses.

author: macboy
author_rank: Super Contributor
timestamp: October 03, 2014, 02:17:31 pm
content: Your VBat input range has a greater span than the battery voltage. You should be able to get away with a dropper circuit rather than a regulator, if you don't mind VBat changing with battery voltage. You said you wanted to use E12 resistors plus NPN/PNP/NMOS/PMOS. You didn't mention diodes. So I created a diode dropper with transistors. And another with diodes.

author: ruffy91
author_rank: Regular Contributor
timestamp: October 03, 2014, 02:41:28 pm
content: You are confusing Lithium primary cell with Lithium-Ion or Lithium-Polymer secondary cell. Lithium batteries have a max. voltage of 3V (most prominent example is the CR2032). You don't want a regulator on VBat. VBat is powered by Vdd when there is power and by VBat when power is off to retain the time.

author: Paul Price
author_rank: Super Contributor
timestamp: October 04, 2014, 01:50:43 am
content: A new CR3032 offers an open circuit voltage of approx. 3.244V

author: nuno
author_rank: Frequent Contributor
timestamp: October 04, 2014, 04:05:54 am
content: If you're going to use diodes to drop voltage, you should make sure there's a minimum current through the diodes, otherwise Ohm's Law says you won't have any voltage drop.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 04, 2014, 04:12:42 am
content: Maybe he has some application that uses a lithium ion cell for some other use (e.g. maybe the entire unit is capable of being battery operated) and has it double as the RTC backup? You could use a diode connected transistor as its leakage would be extremely low.