Need some advice: LiPo power management - Page 1
Forum section: electronics / projects

author: homebrew
author_rank: Frequent Contributor
timestamp: August 17, 2015, 12:53:53 pm
content: Hi,I'm currently designing a wireless sensor node and I'm fine with the sensor and uC stuff but I don't feel confident with the power management. Clearly I need to go with LiPo of some sort as these nodes should be both small and durable (and rechargeable).The sensor will sleep most of the time but weak up once per hour to send data. Hence it is vital that the power management part is very efficient and has a low quiescent current as it is always on by definition...Do you have a tip for a decent power management IC?Requirements:3.3V@1mA Always on (RTC supply)3.3V@250mA Switchable System powerSingle-Cell LiPo support (300mAh)Thermistor supported (in the LiPo)5V charging voltage (Via USB)The only IC I found was the MAX8934G. However as I understand the datasheet, the thing cuts off, if the battery drops below 4V...http://www.farnell.com/datasheets/1648644.pdfAny Ideas?

author: janekm
author_rank: Unknown
timestamp: August 17, 2015, 02:50:43 pm
content: Looks like a nice part for what you're looking for (though you'd have to add another LDO for your 330mA supply as that's passed through from the input voltage). It won't cut out at 4V on the battery but at 3.3V + the LDO dropout voltage (the datasheet is a tad confusing perhaps but it's clear from the diagrams / graphs later on).TI and Linear have some similar chips that might be more suitable for your exact application (this one is a bit focused on having two input power sources).

author: Koen
author_rank: Frequent Contributor
timestamp: August 17, 2015, 04:00:09 pm
content: Linear LTC4081 combines a 300mA converter and 500mA Li-ion/Li-po charger with NTC. Use converter's PWM mode for the 250mA period and Burst mode for the 1mA period.http://cds.linear.com/docs/en/datasheet/4081fa.pdf

author: Maxlor
author_rank: Frequent Contributor
timestamp: August 17, 2015, 04:04:55 pm
content: The thing I'm missing is the undervoltage cutoff for the battery... but maybe I'm just overlooking it in the datasheet. Anyway, as far as I understand it, SYS is basically just connected through to your battery unless the chip gets external power, at which point it's 4.35V. So you probably need external regulation in any case, since the 30mA from the builtin LDO won't be enough.I did a small wireless sensor node project a couple of months ago. Maybe it's interesting to you, so let me summarize: it measures temperature, humidity and pressure once a minute, and transmits the data via WiFi every two hours. The target runtime on a single charge was 6 months, which by now I'm happy to report I achievedI'm using the following components to power the thing:- 1000mAh LiPo. No thermistor.- ST Micro STC4054 battery charging IC, connected to an USB socket. I set the maximum charging current to 450mA based on the limited heat sinking capability of my board.- Diodes Inc. PAM2305 LDO. Chosen for it's very low Iq.- Rohm BD4835 reset controller. It disables the LDO if battery voltage sinks below 3.5V so that if the sensor node is forgotten for a year or two, the battery hopefully isn't deep-discharged. I probably should have gone for the BD49 though, which is push-pull instead of open-drain. As it is, there's an 1MOhm pull-up resistor discharging an empty battery. Meh.Overall power consumption when idle is around 55uA, measured at the battery. This includes an electric imp and two always-on LCD displays.

author: ElektroQuark
author_rank: Unknown
timestamp: August 17, 2015, 04:25:15 pm
content: Dumb question.The Linear LTC4081 is a buck converter only, if a 3V output voltage is used it will stop draining the battery at about 3V, can it be used as a undervoltage cutoff?

author: Maxlor
author_rank: Frequent Contributor
timestamp: August 17, 2015, 05:54:08 pm
content: The LTC4081 has an undervoltage lockout for the boost converter at 2.5V. That's rather low!. It's a safety feature that's there to stop the battery from venting if it's deep-discharged, but if the battery voltage gets that low, the battery will already be damaged.

author: Koen
author_rank: Frequent Contributor
timestamp: August 17, 2015, 06:44:29 pm
content: Quote from: EdoNork on August 17, 2015, 04:25:15 pmNo. Add an active high voltage supervisor between the battery and the LTC4081 EN pin.

author: homebrew
author_rank: Frequent Contributor
timestamp: August 17, 2015, 09:19:55 pm
content: Quote from: Maxlor on August 17, 2015, 04:04:55 pmThanks to report from you WSN project, quite impressive!Is it a Swiss thing to do WSN stuff? What a coincidence!Your specs are really pretty close to mine, however, I would have some additional sensory requirements.Is the platform you have designed commercially available?

author: Maxlor
author_rank: Frequent Contributor
timestamp: August 17, 2015, 10:42:44 pm
content: Quote from: homebrew on August 17, 2015, 09:19:55 pmI think it's a global trendQuote from: homebrew on August 17, 2015, 09:19:55 pmNo, it was just a hobby project. I wanted to test dave's claim that those "calculator LCDs" run on next to nothing (they do), and I also wanted to play with the Electric Imp. Since the Imp has a very low power sleep mode, it seemed like a good idea to combine the two. Add some high quality sensors (like Amphenol's (now GE) ChipCap2 with it's 0.2°C absolute accuracy around room temperature) to satisfy the measurement geek in me, and you have a fun project.