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After asking around I realized that switching regulators should be used to regulate the power going to the MSP430. I'm wondering (I'm not very informed in the topic of power electronics) how can I power the MSP430 with 3.3v or lower and a 10V load cell on the same PCB from the same power source if possible?

I've considered a combination of a switching regulator (TPS60313) with a separate battery and a linear regulator (LM317) to power the load cell with a power source of 12V or more. However that would mean that I would have to have 8 AA-batteries to run the LM317, which is a little ridiculous in terms of packaging. Also I have to consider the voltage drop off of these batteries.

Another option is to use two switching regulators: one TPS60313 for the MCU with a seperate battery (one AA battery), and another step-up switching regulator for the load cell (have not determined a specific one yet, but I have done research pointing to this list)

Just to wrap it up, my question is, which would be a better route? Pointers would be great!

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  • \$\begingroup\$ How much power will it consume and how long do you want it to run for? That will determine how many and what size batteries to use, from which I think you can choose appropriate regulators. e.g. if only 2 x AA are needed then you need a boost reg for the 10V. \$\endgroup\$ – geometrikal Apr 1 '13 at 5:13
  • \$\begingroup\$ Are there a special reasons for having separate (independent?) batteries for MCU and load cell? What's the maximum total number of AA cells in your system, which you would be comfortable with? \$\endgroup\$ – Nick Alexeev Apr 1 '13 at 5:24
  • \$\begingroup\$ I want it to run for a long time... so efficiency is an issue, and I also want it to be a small package. I don't want to have separate batteries to power both devices. Just to add constraints, it would be best to have this system run off 2 AA batteries, or a max of 4. \$\endgroup\$ – user2066639 Apr 1 '13 at 6:58
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    \$\begingroup\$ Do you have a link for the 10V loadcell - it might easily run from 3V with very little problem. I've done this after contacting the manufacturer first of course. Just because the spec says 10V it doesn't mean it has to be 10V. Also if you are switched into sleep mode for a lot of the time then you may also be able to "sleep" the loadcell. \$\endgroup\$ – Andy aka Apr 1 '13 at 16:53
  • \$\begingroup\$ omega.com/Pressure/pdf/LC304.pdf I have contacted the manufacturer, and they said it was okay also, however I have to consider the step size of the ADC. Thanks for the heads up =) \$\endgroup\$ – user2066639 Apr 1 '13 at 19:23
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A MSP430 will take a maximum of around 8-15 mA in active mode unless you're using a RF MCU or have heavy current sourcing from the I/Os. If you're programming the thing to sleep most of the time in one of the LPM modes (as you should for a battery powered application and desire long run times), most of the time the MSP430 will be consuming microamps of current sleeping. If you limit the active mode clock frequency to 1MHz or even less (maybe even 12kHz using the VLO or 32kHz) your active mode power consumption will be sub-1mA.

Use sleep mode and very low duty cycle active mode runtimes. Use low clock frequencies and perhaps low supply voltages to promote battery usage efficiency.

Size the battery mAh capacity to handle the runtime you want taking into account battery self discharge / leakage, the sleep mode vs active mode power consumption of the unit over time (mAh averaged over a day, week, month, year). Since you're using AAs you know what mAh capacity and self discharge rate to expect. You can use a 3S arrangement of AAs to get from 2.7V to 4.8V or 2S to get 1.8V to 3.2V. In the 3S case you can use a very low quiescent current LDO (possibly one with pass through capability when the battery voltage has declined to within the MSP430's operational tolerances) to power the MSP430 and still have high battery usage efficiency. For the 2S or 3S cases you'll want to use some kind of a boost converter to generate the 10V for the load cell. If the current requirement when "ON" is low enough a charge pump type may suffice, or you can use a small switcher. Look for a unit with a very low shutdown mode current (in the sub-10uA range) and an output isolated from the input during shutdown if you can use the MSP430 to control the boost converter's wake up from shutdown mode only when the load cell needs to be powered up. Check out the TI WEBENCH and Switcherpro tools on their web site to assist with optimum part selection. There are LDOs specifically tailored for use with the MSP430 which have very low quiescent current values and which can provide up to 75mA of current for the MCU.

Edit : Adding specific part information.

The TPS780x and TPS782x are low Iq LDOs specifically recommended for use with the MSP430, and there are application notes detailing their benefits in those solutions. The TPS78227 or TPS78228 will give a regulated output down to 2.7 or 2.8V respectively. There are higher and lower voltage members of the family. The TPS61041 will boost battery voltage up to the level needed for your sensor and is a reasonably compact solution. Other models with integrated LDOs and/or higher frequency operation are available if you should wish further noise reduction.

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  • \$\begingroup\$ Double line-break gives paragraph break. \$\endgroup\$ – Transistor Jul 6 '16 at 17:30
  • \$\begingroup\$ That second link is dead... \$\endgroup\$ – TDHofstetter Dec 9 '16 at 14:27
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Some general considerations.

  • Boost converters (inductive) usually need to have a boost factor of 6 or less. Otherwise, it becomes difficult to make them stable. (Some more discussion here.)
  • Does your MSP430 need a regulated voltage? It's capable of running directly of the battery boltage, even though it's going to vary as the battery discharges.

The following are all viable options. Which is better depends on the relative importance between size, cost, efficiency.

2 cells in series

1.8V to 3.2V

μC: direct from battery, or +3.3V charge pump boost, or +3.3V inductive boost
Load cell: +10V inductive boost

3 cells in series

2.7V to 4.8V

μC: +3.3V SEPIC, or +2.5V LDO
Load cell: +10V inductive boost

4 cells in series

3.2V to 6.4V

μC: +3.0V LDO
Load cell: +10V inductive boost

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