Ideas for micropower step-up converter - Page 1
Forum section: electronics / projects

author: b_force
author_rank: Super Contributor
timestamp: January 11, 2017, 09:56:20 am
content: Nowadays you can find a trillion different types of step-up/boost converters and charge pumps (inductorless)However, I can't find anything suitable for the following problem.I try to find a solution for a boost converter but within the 500uA range (100uA min to 1mA max) with a decent efficiency (=70-80% or so, higher is better)The best I can find with these currents is an efficiency of 50% or even less.The output voltage needs to be ideally around 8-12V, but worst case can be 5V (I also need a -5V at some point).The input voltage is 2-2.4V at minimum (ideally 1.2V, just a single AAA or AA battery).I've looked at the entire range from Texas Instruments, LT and On semi (and a few other ones here and there)Worst case scenario I can also develop my 'own' boost converter (discrete parts), but that is very far from ideal.Any ideas are welcome.Would a flyback/step-up transformer help a little more?Or cascading (in series) circuits?edit:To make it maybe a little bit more clear. From 1.2-2.4V to 5V is doable.But than I also need an inverter to make -5V.Haven't found a high efficient low quiescent current inverter yet.

author: senso
author_rank: Frequent Contributor
timestamp: January 11, 2017, 05:23:06 pm
content: Maybe a switched capacitor regulator given the low current needs, I think there are some models that do what you need.

author: DBecker
author_rank: Frequent Contributor
timestamp: January 11, 2017, 07:13:43 pm
content: If you already have an boost converter to +V, adding two diodes and a capacitor will generate a -V supply. (Well, a Schottky diode drop less, but close enough for most uses.)You can buy a module on eBay to do this for you. It's sold as a 5V to +12V/-12V converter. The one I bought recently was titled "New DC-DC Converter Step Up Module 5V to ±12V Power Supply Boost Module". The chip had the number ground off, but it had the common SOT-23 pinout (the same as the TPS61041 and many others).I just checked the module -- it starts up at about 1.54V, and immediately generates +12V and -12V with very low current (low enough that I don't trust my measurement).I've found that buying a $2 module like this is an inexpensive way to test a design idea. It's easy enough to replace a part or two to match your design. If it works, buy a few and see what happens with a continuous overload or short circuit output.Regarding efficiency, everyone thinks they can design high efficiency supplies at low voltages and power. Unless they encounter their first diode drop. Later they learn why gate capacitance is a key parameter. Then why power supply sequencing and back-feeding power is something they should have looked at.

author: CraigHB
author_rank: Regular Contributor
timestamp: January 11, 2017, 07:14:40 pm
content: A charge pump usually fits the bill best for very low power like that, but they're limited in output voltages, hard to find them much above 5V. You'll probably have to use a boost converter. Problem with switching converters and low power outputs is it takes an amount of power to drive the switch(s) and controller. It can be a big portion of input power at very low outputs. Since overhead is somewhat of a constant, efficiency goes up as output power goes up. You can find very small micro-power converters, but you'll probably have to live with the low efficiency. I don't think you could do better putting one together yourself. You'll still run into the same overhead.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 11, 2017, 07:31:57 pm
content: Have you tried TI's Webench?I just put in 2-2.4Vin, 8Vout, 1mA, and one of the top 5 results was a circuit based around the TPS61041. ~70% efficiency at 100uA, ~85% efficiency at 1mA.

author: b_force
author_rank: Super Contributor
timestamp: January 11, 2017, 07:57:00 pm
content: Well, after some more time searching and fooling around, I got a few candidates.One of them is the LT1617-1. When using two sections with two diodes and a cap, you can generate two equal (but inverted) outputs.Simulated efficiency is around 75% or so.After a while I realized (the pretty obvious) key factor, which is of course the quiescent current. (doh!)Unfortunately digikey doesn't have that in its search function, so it was a bit of a manual search.I also found some possibilities at Maxim, and TI, but not ideal.Diode drop voltage is indeed a significant issue, I'm aware of that.I can't use a ready made 'ebay board', because this is gonna be used in a custom made design.Which also needs to be very tiny (why do I always have these crazy ideas).Some charge pumps have an efficiency of 95%.Unfortunately, it's not always clear how that works with lower currents.(and again, if the quiescent current is already more than 50% of the load, it's pretty useless)

author: DBecker
author_rank: Frequent Contributor
timestamp: January 11, 2017, 08:00:12 pm
content: Capacitor charge pumps with mosfet switches work great for exactly doubling or exactly inverting the voltage, but they quickly lose their advantage above that. Increasing the voltage with a diode-capacitor ladder is painfully inefficient. Cascading grows the fixed and variable loss quadratically.Unless you custom order a tiny transformer, a inductive boost converter with Schottky diode is your best option. Lossy inductors, diode voltage drops, parasitic capacitance and switching losses will all be painful but ultimately unavoidable.I was suggesting the pre-made modules on eBay as a evaluation and prototyping board. If it works well enough, put something similar into your own design. The eBay board has an space-efficient layout, but you should make the switching node a little smaller in your own layout. And if you don't need the full 50mA output, you can use much smaller diodes which will lower the parasitic capacitance losses.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 11, 2017, 08:21:54 pm
content: Make your own flyback or boost. The key to low power is to get the switch to cycle based on the cap voltage.For instance when the cap voltage reaches 1.2 V switch on the flyback inductor. Switch it off when the voltage dropsto 1.1 volts. That will have bad ripple but you could post regulate this with a micropower linear regulator.Minimizing the switching frequency is the key to micropower operation. Of course the cap and inductor will have to be picked together toachieve desired function.

author: CraigHB
author_rank: Regular Contributor
timestamp: January 14, 2017, 06:15:09 pm
content: Micro-power single chip solutions usually use a synchronous design that utilizes a MOSFET rectifier in lieu of a diode. So that eliminates some of the loss, but at very low currents it's not a big difference. They also use pulse skipping and other mechanisms to reduce switching losses at low loads however that results in a penalty to ripple voltage. So you'll get all those power saving mechanisms with a micro-power switching regulator if the part is advertised with high efficiency. There's actually a lot of product development in converters like that, they have a lot of applications. You should be able find something to fit your bill, though you probably will not see something with high efficiency at very low power outputs. If your nominal power output was a little higher it would not be hard to find something, but with those very low outputs you're kind of stuck with what you get.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: January 14, 2017, 07:54:17 pm
content: My favorite one for single cell operation : LTC3525 (From the datasheet : Efficiency ~85% @ Vin 1.2V, Vout 3.3V, i 500µA)My second favorite : MCP16251 (Efficiency ~80% in the same operating conditions)