I'm currently looking at hardware for a personal project/prototype.
I'm hoping to use the new mbed in a portable device, so naturally I need a power source.
In the past I've used a lithium-polymer battery, charger and dc-dc booster board. This approach seems pretty neat, until you try using the ADCs and the data is all over the place.
I'm fairly sure the culprit was ripple/noise from the dc-dc converter that was being used to convert the 2.6-4.2v from the battery into 5v for the CPU and sensors.
After quite a lot of reading around, I can see no way out of using a switching dc-dc converter while maintaining reasonable efficiency (even LDO regulators are very wasteful.
This leaves me short of a power supply, as super-capacitors are too small and NiMh are bulky and heavy. I really don't want to use alkaline batteries, they're expensive when you have to keep buying them!
I can't be the only person to have run into this problem, what's the best solution?
Is there a reason you can't use a 2S LiPo battery for 7.4v? what are the weight requirments?
Weight/size is as small as possible, I'm aiming for a hand-held design.
If I use 7.4v it will still need regulating down to 3.3v for the mbed and peripherals. I'm fairly sure the mbed's onboard regulators are going to be too inefficient as they're probably LDOs. The only alternative seems to be a switching regulator, which leaves back where I started; a power supply with lots of noise!
I'm hoping someone has come up with a clever solution, or knows a regulator that won't produce too much noise.
I've never really had to think about the efficiency as the mbed is negligible compared to the rest of my systems power draw, I feed 5v straight into the mbed the regulators don't get warm (that's all I was worried about), the 19v to 5v regulator might as well be on fire though due to the current draw by the GPS and XBee, you can get tiny 2S lipos so weight wouldn't be an issue, if no other solution comes to light it may be the only option for a clean dc line, good luck with your search
How much current are you trying to draw?
I aim for the device to take advantage of sleep/power-down facilities in the CPU and sensors so that it can sit in a low power (<10mA) mode for a number of days. The design is aimed at low power consumption to avoid bulky battery packs, so full power will likely be <150mA.
Having spent more time reading, it seems the conventional solution is a buck-boost converter or a charge pump. I'm concerned that these devices will cause significant electrical noise (as the mbed's ADCs are known to be noise susceptible), or may dissipate more power in their quiescant state than the cpu will in sleep!
This isn't an area I understand well, so I'll take all the advice I can get!
Yes boost converters and charge pumps can kick out a lot of ripple. You could try something like this Linear Tech device which has a very high switching freq which can be smoothed pretty well (<10mV ripple). The downside is that it might not be able to supply enough current. Murata also do some really good products and may be worth a look.
Just check your battery. I used a Lithium coin cell battery recently and was surprised to see that it couldn't produce much more than about 2mA of current. Unless you have a cap the size of a house brick this might be a problem!
Thanks for the pointer. I'm not at all happy about using surface mount, but I'll have a look for similar devices in a larger package.
Alternatively, I might try one of these: http://www.sparkfun.com/products/10300
Sadly Sparkfun's board has several design issues, so it would be less than ideal.
To clarify: lithium coin cells are very different from lithium rechargeables (li-ion, li-po) which can pack a reasonable punch (1Ah capacity, 500mA max current).
I'm also trying to find a LiPo/other-rechargable/small battery solution.
Did anyone actually try the 2S LiPo solution and succeed?
@Graham, did you find a way out?
also @Graham: this is a better LiPo breakout option than sparkfun's: https://www.adafruit.com/products/259
I didn't find a solution I was very happy with, I think the best I came up with was a single Lipo cell, charger and a low-drop regulator. That solution reduces the useable capacity of the battery quite a lot though. If you don't mind the ripple (ADCs really don't like it) boost converters do work, but they come with their own set of problems.
In the end I've been playing with other projects that rely less on sensitive devices and for which I've used 8 bit microcontrollers that can handle a wide operating voltage range. I miss the extra peripherals of the mbed though!
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mbed, the fastest way to prototype with ARM based microcontrollers.
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