DC-DC Converter for small loads - Page 1
Forum section: electronics / projects

author: psycho0815
author_rank: Regular Contributor
timestamp: February 27, 2013, 09:42:20 pm
content: Hi,I'm working on a project where I'm using an attiny to measure a temperature and send it via a 433MHz Transmitter every minute.The whole thing is supposed to run off a pair of AA so powerconsumptions is an issue. I managed to get its consumption in idle mode down to around 8 µA but the DC-DC Converter I'm using (TPS61970) is using 100µA itself so that's no good at all.Do any of you know a Converter specifically made for low current efficiency? All I've looked at so far are not very efficient at all with small loads.Or is there maybe another solution for this? I can't run the uC directly from the batteries because of the DHT11 Sensor and RF module I'm using.Any help on this will be greatly appreciatedCheers,Psycho

author: mazurov
author_rank: Frequent Contributor
timestamp: February 27, 2013, 09:55:37 pm
content: What is your maximum current and also output voltage?

author: psycho0815
author_rank: Regular Contributor
timestamp: February 27, 2013, 10:31:04 pm
content: Sorry about that.my Peak current is 25mA, Output-Voltage is 5V. The awake-period lasts only about 1100ms.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 27, 2013, 10:44:46 pm
content: I'm not sure which part you're using, I couldn't find any reference online to a TPS61970.Anyway, here is one for example which has 20 uA "typical" quiescent current. Part number is TPS61092http://www.ti.com/product/tps61092Or use an inductorless, switched-cap boost circuit. That can put out over 100 mA when on, and only 4 uA typical in shutdown mode. If your CPU draws very little, you can completely shut down the boost circuit during sleep time and rely on a capacitor to hold you over. Here is a 5V charge pump part:http://www.ti.com/lit/ds/symlink/tps60151.pdf

author: mazurov
author_rank: Frequent Contributor
timestamp: February 27, 2013, 10:45:43 pm
content: TPS6122 is a good one - I used it to boost a-Si solar panels wit Iout < 10uA. You will need to play with different inductors to get rated Iq.

author: mariush
author_rank: Super Contributor
timestamp: February 27, 2013, 10:49:24 pm
content: Check the datasheet of this one and see if it would work for you:ISL9111(A)http://www.digikey.com/product-detail/en/ISL9111AEH50Z-T7A/ISL9111AEH50Z-T7ACT-ND/2816339datasheethttp://www.intersil.com/content/dam/Intersil/documents/fn76/fn7602.pdf0.8 startup voltage, 0.7v run voltage, 20uA quiescent voltage, 1uA while disabled through pin, 2 modes of operation depending on current consumption, can do up to about 240mA depending on input voltage (from graph, looks like it does up to about 80mA with only ~1v input)At 25mA current, depending on input voltage, the graph shows it can be between 80% and a bit over 90% efficient.I have it bookmarked but I didn't yet have the occasion to actually build a circuit and see real world data.Certainly looks to me like much better than TPS61092, which needs 1.8v to run... you could use a single AA battery with this one I linked to, and save some weight.

author: mazurov
author_rank: Frequent Contributor
timestamp: February 27, 2013, 10:52:34 pm
content: Another good one is bq25504. Too good for batteries to design a converter but it will go to sub uA - just for fun.

author: psycho0815
author_rank: Regular Contributor
timestamp: February 27, 2013, 10:54:26 pm
content: Quote from: JBeale on February 27, 2013, 10:44:46 pmThat's propably because the actual part number is TPS61070. I shouldn't start forum topics at 11pm..Quotethat sounds not bad. but wouldn't that mean a higher current draw while the boost circuit is running, since it has to charge the cap? doesn't that sort of equal each other out?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 28, 2013, 12:10:48 am
content: Hi group,To the original poster,Have alook at the LTC3388.http://www.linear.com/product/LTC3388This part has very low quiescent current, under 1uA, that is output up in regulation, no load.Jay_Diddy_B

author: mazurov
author_rank: Frequent Contributor
timestamp: February 28, 2013, 12:13:03 am
content: The OP runs from 2xAA, he needs op.range down to 1.8V.

author: psycho0815
author_rank: Regular Contributor
timestamp: March 01, 2013, 08:09:39 pm
content: Just as an Update:I just received a couple of NCP1402-5V Breakout-Boards I ordered before i created this topic.They give me a sleep-current of ~50µA which according to an online calculator I've found should give me about 6 months of batterylife.I can live with that. Also using Breakout-Boards makes the whole thing easier to build up. The circuit on itself is simple enough to fit on some veroboard.Anyways thanks for your help, but I'm probably going to use those Breakouts.

author: nctnico
author_rank: Super Contributor
timestamp: March 01, 2013, 09:41:58 pm
content: Another trick is to use a MOSFET to disconnect the switcher when it is not needed.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 01, 2013, 09:47:50 pm
content: Current draw specs on 1xAA type boost converters can be misleading, as they tend to power themselves from their output.The headline figure on the front page of the datasheet is usually the draw from the output, and so is multiplied by the boost ratio and losses.A while ago I spent quite a lot of time looking and found the TPS61221 gave teh best price/performance I could find.

author: Codemonkey
author_rank: Regular Contributor
timestamp: March 02, 2013, 12:25:19 pm
content: For what its worth, an easier option (depending on how long you've got) may be to use something like the NXP/Jennic JN5148 series wireless microcontrollers.They'll run direct from a couple of AA cells and the modules they sell with the device and support circuitry + pcb antenna or uFL connector operate right down to 2.3V without any extra DC-DC convertors. They have a 32 bit micro, an integrated 2.4GHz 802.15.4 transceiver with MAC & baseband and coupled with a temperature sensor will do the whole job. Sleep current for their module with a wake timer running is specified as 2.6uA. They even have an on chip temperature sensor, but it requires calibrating so is a bit more hassle than just connecting to a ready calibrated peripheral device. If you don't bother with all the ZigBee stuff and just use the 802.15.4 MAC library they supply for comms, its pretty easy to use them to make a simple system like you're doing.FWIW, my wireless thermostat controlling the boiler at home just packed in last weekend (the bit that controls the boiler, not too concerned, it was crap anyway and the sensor bit ate 4xAA cells about every 2 months), and in the space of 2 days I was able toknock upbotch a replacement system using one of their eval kits I had laying about from another project. The temp sensor/thermostat end just wakes up every 30 seconds, takes a measurement from a Sensirion SHT11 device and sends a message to the mains powered bit controlling the boiler and updates the LCD display before going back to sleep and takes about 500uA according to my fluke dvm, of which just about all the 500uA is the 128x64 graphic lcd module. Just need to do a couple of PCB's now to make it a bit more permanent!They're simple enough to get working with, you don't really need an eval board, just a couple of modules that have all the support circuitry, and some means of connecting a serial interface to them to program them (the eval kits ship with an FTDI USB-Serial TTL cable), and the whole SDK is a free download from the website (uses eclipse and gcc, no code size limits or any other bollocks).(http://www.jennic.com/products/modules/).

author: psycho0815
author_rank: Regular Contributor
timestamp: March 05, 2013, 10:13:39 am
content: Thanks, i might look into it eventually, but to be honest it seems a bit overkill for what i want to do.Also i already have all the code done, including the base-station, so for now I'm going to stick with the attiny.BTW: for those interested in the whole project (cheap homeautomation system) you can follow it's process on my blog athttp://h-reg.blogspot.deit's all in german though.