Efficient power supply for Li-Ion (3.7v - 2,000mAh) - Page 1
Forum section: electronics / beginners

author: AMyatt
author_rank: Contributor
timestamp: May 25, 2013, 05:28:40 am
content: Hello everyone,I am working on a project that needs to be extremely small and last at least 14 hours between charges.Input is a single cell Li-Ion battery rated as 3.7V 2,000mAh.Output needs to be:5v - 200mA (max), 500uA (sleep)3v3 - 116mA (max), 2.4uA (sleep)The device will sleep most of the time.Waking only the MCU (9mA) and Bluetooth 4 Low Energy module (27mA) every 8 seconds (for about 1 second).And waking everything else only if user input is provided (sleeping again after 10 seconds of no activity).My question is, what is recommended to get the required voltages at the required current?Should I:Step-up the 3.7V to 5V then down to 3.3V?Step-down the 3.7V to 3.3V then up to 5V?What configuration / components should I use to get maximum efficiency and not waste current?I appreciate any help I can get.Thanks,AnthonyAustralia (not Austria)

author: baoshi
author_rank: Regular Contributor
timestamp: May 25, 2013, 05:47:25 am
content: Hi AnthonySometime I ran into the same problem as yours.I think when you say 7.3v it is actually 3.7v. Lithium battery usually charges up to 4.2v. When discharging, the protection circuit normally cut off at 3v, or sometime lower. So you can't get 3.3v by simply step down. You'd boost 3.7 to 5v, then step down to 3.3, or buck-boost single lithium to 3.3, and boost to 5v.But I'd rather check if 3.3v is really needed. Some times you can go with 2.8 or 2.5v instead, then it becomes much simpler.Bob

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2013, 09:49:17 am
content: Wrong Baoshi, yes you can.Hint: LM2937BM-3.3VWhat you need is two tiny ics. One is a switching boost regulator converting the Li battery to 5V and the other a passive low-drop out regulator to convert that 5V output to 3.3V and you have it. The boost convertor will typically work all the way down to .8V but some models have low battery cutoff adj. pins to set whatever low battery protection level you want.At this point you have a viable two ic solution if your 3.3V circuits does some sleep enough during the 14-hour operating interval.For longer battery life, you can use a third LDO so that you can drain your LI battery down to 2.8V or whatever cutoff voltage of operation you desire and get greater efficiency:Since you can get 3.3V directly by using a LDO passive regulator, you can add a third tiny low-dropout regulator with shutdown that will directly convert the 4.15 fully charged Li ion battery output to 3.3V.Now, if the output of the first 3.3V LDO connected to the +5V output is set to a a level, say 50 millivolts lower than the third passive regulator, the LDO connected directly to the battery will supply your 3.3 circuit until the one connected to the 5V output kicks in as the LDO connected directly to the battery will kick out when the battery drains and reaches approx a level of 3.5V and battery operation will continue until the switcher's low-voltage cutoff voltage setting of your choice for the Li ion switches off the +5V switcher and everything connected to it.Single-Cell Li ion batteries do not have any low-voltage cutoff circuits built into them, but switchers that step up do, and will prevent over discharge of the battery. In MCU circuits I design to work at 5V, the MCU monitors the Li-Ion battery and automatically switches the whole circuit off to 0-current when the battery voltage falls below 2.8V.

author: AMyatt
author_rank: Contributor
timestamp: May 25, 2013, 10:09:11 am
content: Hi Bob,Thank you for your quick reply.The individual components are rated as follows:3.3V Devices:BLE112: 2.00 - 3.60VATMega328: 2.70 - 5.50V (8MHz)MAX17044: 2.50 - 4.50VMPR121: 1.71 - 3.60VSo a minimum of 2.7V and maximum of 3.6V on 3.3V rail.5V Devices:uOLED-128-G2: 4.00V - 5.50VSo a minimum of 4.0V and maximum of 5.5V on 5V rail.What would you suggest?

author: AMyatt
author_rank: Contributor
timestamp: May 25, 2013, 10:16:39 am
content: Paul,So I could use a TPS61200 Step-Up (Boost) converter, to get the 5V rail and then use a LM2937-3.3 (LDO) 3.3V Regulator?

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2013, 10:22:59 am
content: Yep, the LM2937 is the better choice.Also consider the Microchip MCP1703 3.3V, it's max output is 250mA, it is so tiny and cheap and efficient, but has .625V dropout, so it must be the one connected to the 5V supply.You can insert a Schottky diode in series with the output of the LDO connected to +5 to lower it's output voltage by approx .18V so the LDO connected to the batery dominates the regulation directly from the battery until the one connected to +5 takes over.

author: baoshi
author_rank: Regular Contributor
timestamp: May 25, 2013, 11:21:25 am
content: Hi,I think you can go with 2.8v for 3.3v rail. Use a LDO like TPS73028. Its drop-out is 120mV, pretty safe for lithium battery when battery level is low.You need a boost converter for 5V rail.Since your device will be idle for most of the time, you need to consider the efficiency of switching mode converters at light load. Most buck or boost converters only achieve its advertised efficiency at spec'ed load, unless it is specially designed for light load efficiency (PFM mode for example). You also need to consider quiescent current of the converter. Some converter has a Iq of couple of mA, way bigger than that of your circuit.Btw your 5v rail is for the OLED, I guess you can stop the 5V supply during sleep. So you do not need to consider too much about light load efficiency.BobQuote from: AMyatt on May 25, 2013, 10:09:11 am

author: mariush
author_rank: Super Contributor
timestamp: May 25, 2013, 05:49:49 pm
content: MIC2941 adjustable:http://www.micrel.com/_PDF/mic2940.pdfSet it to about 2.8-3v , it only has about 0.2v voltage drop at 200mA, maybe 0.15v at 100mA. And it will probably just output less voltage when the battery is close to empty.Check datasheet and errata for those atmega and max, but especially the atmega, it may have some bugs or quirks when it comes to working at 2.8v or close to the lowest voltage (adc's not working right, stuff like that, or may only work up to a frequency at that voltage)For the 5v device... i guess any boost ic would do.