LDO or Buck converter for low power battery operated device - Page 1
Forum section: electronics / projects

author: txescientist
author_rank: Regular Contributor
timestamp: November 28, 2015, 11:52:58 am
content: Hi,What is better to use for battery operated low power device? LDO or Buck switcher? Output voltage is 5V, input voltage range is from 10V to 5.4V and the current consumption is ~20mA. I know that the switcher is better for higher voltage differences and higher current, but form several datasheets, buck converters have ~60% efficiency at 20mA. Any suggestions are very much appreciated.

author: txescientist
author_rank: Regular Contributor
timestamp: November 29, 2015, 07:27:50 pm
content: Thank you for the reply.

author: Delta
author_rank: Super Contributor
timestamp: November 30, 2015, 10:31:15 pm
content: What battery pack are you using that delivers usuable power over such a voltage range?

author: txescientist
author_rank: Regular Contributor
timestamp: December 01, 2015, 12:05:36 am
content: I am using 6 LR6 batteries. Upper voltage range is more likely to be 9-9.6V, but for simplicity sake I chose 10V. Think will go with linear LDO regulator, because it have better voltage stability, low noise and low dropout voltage. Also, current consumption can jump from 15mA to 30-60mA sometimes, so I is more rugged solution. Battery life will be around 150h so that is good enough for a field instrument that will work 2-3h per day with rechargeable batteries. Thank you all for the suggestions.

author: Delta
author_rank: Super Contributor
timestamp: December 01, 2015, 12:10:59 am
content: Cool. I think sometimes people obsess over efficiency when it's largely irrelevant in the grander scheme of things. Keep calm and linear regulate on! 😀PS. Given that there's hardly any energy left in your pack below 6.2v, how about a standard NPN regulator for better noise and stability than the LDO?

author: mariush
author_rank: Super Contributor
timestamp: December 01, 2015, 01:54:13 am
content: Non rechargeable batteries will have a voltage of up to 1.65v when they're fresh, and with 6 of them you may have close to 9.9-10v.The voltage will quickly drop but you'll still have to be careful when you choose a LDO because some good ones have a maximum voltage at exactly 10v... you'll want to go with one rated for 12v or more.With just 20mA of power consumption, fresh batteries will stay for probably an hour or so at that voltage, but it will quickly drop to about 1.5v so then you'll have 9v input for almost all life of your product.... You have 9v in , 5v out so an efficiency of 55%.With rechargeable batteries, you'll have about 1.2v x 6 = 7.2v so we're talking about an efficiency of 70%A good switching regulator will get you over 90% efficiency, which can extend the operating life of your device for relatively minimal cost increase and small pcb cost.For example, here's a very good switching IC regulator for your product :http://uk.farnell.com/texas-instruments/tps62120dcnt/dc-dc-converter-800khz-8-sot-23/dp/1863868(ignore the picture, it's actually an 8 pin IC but relatively easy to solder) or check it out here on Mouser :http://www2.mouser.com/ProductDetail/Texas-Instruments/TPS62120DCNT/?qs=zQ2yIHQ6k3xcyGGPtl4HUQ%3D%3D(has the proper picture here)Here's the datasheet:http://www.farnell.com/datasheets/1803613.pdfIt only needs a tiny surface mounted inductor and a bunch of ceramic capacitors and resistors so there's minimal pcb usage and the IC cost is the biggest expense, but in turn you'll probably double the battery lifetime (depending on what batteries you use) :Of course, there's cheaper switching regulators but they'll have slightly lower efficiency and require larger inductors which increases the pcb space.

author: crazy horse
author_rank: Contributor
timestamp: December 01, 2015, 02:52:07 am
content: Is the cost and add complexity (and lower reliability) worth it?

author: mariush
author_rank: Super Contributor
timestamp: December 01, 2015, 03:34:27 am
content: I would guess it depends on the product's purpose and how easy it would be to replace the batteries and the profit margin on the product and other factors.For example, if the product is a fire alarm or smoke detector you may like to not replace batteries so often. Or, if it's for something that will be installed in a remote place like a gps beacon or something or some weather measurement device/data logger (like, read sensor once a minute and then idle)... or maybe he needs to make the product water resistant which would make replacing batteries very inconvenient.I just pointed out the alternative to linear regulators.One could grab a bunch of Energizer Ultimate Lithium or Varta Professional Lithium batteries instead of regular batteries ( they range between 3000-3400 mAh compared to about 2000-2200mAh for regular ones, seehttp://batteryshowdown.com/results-lo.html) ... and then the switching regulator would really make a difference in the device life.