Drive MSP430 low-power even lower - Part One

It’s no mistake that when you think of MSP430, Texas Instrument's flagship microcontroller, low power is one of the first things that comes to mind. After all, this is why MSP430 is such a popular choice for battery-powered applications. By limiting the current draw on your batteries you are effectively extending the battery life of your application. And given the slowing pace of advancements in Lithium-Ion battery technology, it is imperative to limit power consumption in order to achieve the optimum battery life for your application.

This is intuitive.  

But what if I told you that you could conserve 30% power or even higher by adding an additional component? That’s right: adding an additional component could actually help extend your battery life by hours. I know you must be thinking that it’s too good to be true. I assure you, though, that this is indeed possible.

That’s where a voltage regulator comes into play.

Often times in small, portable applications, it’s easy and seemingly obvious to connect the MSP430 straight to the battery. After all, MSP430 has a wide operating voltage range (1.8 to 3.6V) depending on which frequency you plan on running your core at. This is displayed in the figure below.

As an example, we can use two 1.5V alkaline coin cell batteries to power an MSP430F2274 without the need for any additional voltage regulation. The 3V provided by the batteries can power the MCU at nearly all system frequencies. However, running the system at 16MHz will require a minimum supply voltage of 3.3V.

Things start to get interesting when you look at current consumption at different supply voltage – system frequency combinations. Take a look at figures 2 and 3 below:

From a quick glance you can see that at a given frequency, if we increase the supply voltage, then current consumption rises as well. Conversely, if we hold supply voltage constant and increase operating frequency, current consumption will also rise. The main conclusion to be drawn from this observation is that there are both efficient and inefficient methods of powering your MSP430. By raising the supply voltage you could potentially be burning extra current unnecessarily.

Let’s refer back to our example. If we are powering the MSP430 running at 1MHz with 3V, we can expect the active mode current to be 390uA, as shown below. However, remember that at 1MHz, unless we are planning on programming flash memory, the operating voltage may be anywhere between 1.8 and 3.6V. If we lower the supply voltage from 3V to 2.2V, the current consumption drops to 270uA. That’s over a 30% reduction in current consumption! Think about the impact this may for battery life. 

Current consumption at various operating frequencies

Voltage regulators can help capture this efficiency. By stepping the supply down from what is being supplied from the batteries, voltage regulators lower the current being consumed by the MCU.

Next week we’ll take a look at what kinds of voltage regulators can be used to take advantage of this property. By lowering which voltage we are supplying to the MSP430, we can minimize our current consumption. However, not just any voltage regulator can be used to achieve this. Due diligence must be paid in order to optimize your application’s battery life.

Edit: Click here for part 2.

  • Hey Earl! Thanks for the input: good considerations. Check out the second part of the blog which was published earlier this week. I try to address some of the concerns you mentioned.

  • That's right! It cant't be just any voltage regulator.  Battery life (and battery capacity) are measured in A-Hrs and just by merely reducing CPU voltage the full gain in battery life cannot be realized.  The difference between Vcc and battery just gets sloughed as heat in a LDO. In a step-down converter (better) you would have to take into account the efficiency and the quiescent current draw as well.  This is very significant if there is long sleep periods. Using ultra capacitors and periodically charging from the battery results in higher efficiency but at a cost of having a higher pulse current from the battery.

    Unfortunately there is no  magic bullets,  just trade-offs.

  • Correction on #3: you would add the inverting buffer on one of the enable inputs, assuming both LDOs have the same enable logic.

  • Hey Ahmet,

    There are several ways to implement this functionality:

    1. Use a voltage-shifting LDO like TPS780330220 that can switch between 2.2 and 3.0V via toggling a pin.

    2. Use a dual-LDO like TLV7113025 and tie the outputs together. You can then toggle the EN pin for the 3.0V to switch between 2.5 and 3V.

    3. Use two separate LDOs (TPS78222 and TPS78230, for example) and have an inverting buffer like SN74LVC1G14 on one of the inputs. Then tie the enables together and switch between outputs by a simple toggle.

    This is by no means comprehensive.

    Let me know if you have any questions.

    - Aaron

  • Thanks Aaron. Is it possible switch 2.2V to 3.0 V and vice versa during operation. Regards.