MCU + Analog Series: Driving Low-Power Even Lower – Part 2

Other Parts Discussed in Post: TPS782
Last week we discussed a particularly interesting property of MSP430: although the supply voltage range of MSP430 is fairly wide (1.8 – 3.6V), power consumption can vary depending on the specific voltage being supplied to the MCU. In other words, upping your supply voltage from 1.8V to 3.6V can also significantly increase the current drawn from your battery. This is something that we’d like to avoid if possible as it only leads to batteries being depleted faster and, ultimately, frustration on the part of the user.
 
That’s where a voltage regulator can help. By lowering the supply voltage, we are effectively limiting the current draw as well.
 
There are considerations, however, that should be taken whilst choosing the correct voltage regulator. For one thing, it’s important to know when to use an LDO vs. a DC/DC converter. Although the high efficiency properties of a switcher, aka DC/DC converter, make it an appealing option, it’s important to consider the duty cycle of your application, or how frequently you expect your MSP430 to be sleeping. The reason for this is that when the MSP430 is in low-power mode, it draws far less current from the battery than when it is active. And it’s at these lighter loads that the efficiency of typical buck converters, aka step-down DC/DC converters, begins to degrade. Take a look at the efficiency curve of TPS62122, a step-down converter geared for light loads:

Although the switcher performs remarkably well at these lighter loads, its performance begins to waiver once the output current drops into the uA realm. When supplying less than a 100uA, we can expect the efficiency to be far below 70%.
 
Let’s compare that to an LDO. Unlike a DC/DC converter, the efficiency of an LDO does not vary a whole lot with output current. Normally it can be simplified to the following:

For our example, we could say that under most conditions, our efficiency can be expected to be:

However, that equation breaks down once the output current becomes increasingly light. When this happens, we must also factor in the quiescent current of the LDO when considering power dissipation. To do this, we must use the following equation:

Let’s say we are using TPS78222 to supply 10uA to the MSP430 when it is in low power mode. As TPS782 has a quiescent current of 420nA, we should expect our efficiency to be 70.3%. We witnessed a drop in efficiency because we had to consider the quiescent current being supplied to our LDO. However, since that quiescent current was so small, it barely affected the high efficiency that we desired. It is of utmost importance to have as low quiescent current as possible when you expect your MSP430 to be in low power modes for extended periods of time.
 
As mentioned before, picking the most efficient option depends on the frequency which you expect your MSP430 to be in low-power mode.  Wearable applications like fitness bands or smart watches are not always active and may have very long periods of idle time where the MSP430 may go into low power mode. When this happens, current consumption drops as evidenced by one of the low-power modes in the below figure:

If the application spends the vast majority of its time in this mode, the more efficient option is typically the Low Iq LDO. However, if the application has a higher duty cycle, switchers will generally provide for a longer battery life.
 
Backtracking just a bit, TI has recently released a step down DC/DC converter that rivals an LDO in its ability to maintain great efficiency at light loads. The TPS62740 is a 360nA Iq switcher that maintains 90% efficiency even when sourcing 10uA of current to the load. This is a great part in that it can maintain such high efficiency during both active and low-power modes. The tradeoff, of course, is its larger solution size and the resulting electromagnetic radiation which could affect the application.
  
When dealing with small wearable applications, it’s important to keep components as small as possible. Here, depending on the size of the application, an LDO may be preferred over a buck converter since it has less external components and forgoes an inductor.
  
By now it should apparent that the choice between a Low Iq LDO or a DC/DC converter is contingent on the specifics of the application. What is certain, however, is that adding a voltage regulator is key to extending the battery life of your application. It may not be intuitive but lowering operating voltages supplied to your MSP430 limits the current draw on your batteries.
  
Fortunately, both the 5 and 6 series of MSP430 take advantage of this with the integration of a PMM, or Power Management Module. As shown below, depending on the system frequency, one of four different core voltages is required to power the MSP430F643x:

Depending on the core voltage required, you can digitally program an internal voltage regulator to engage one of these core voltages. By choosing the lowest core voltage possible, you effectively save power by means of the same logic stated above, i.e. lower operating voltages require less current to the core. This is great in that no external voltage regulator may be necessary to enjoy the benefits of reduced power consumption. The only limitation, however, is when you have a battery voltage that is higher than the maximum allowed supply voltage to the MSP430, which is 3.6V. In this case, voltage regulation will be required to step down the battery voltage to somewhere within the supply range of the MSP430. This is common with Lithium-Ion batteries which typically provide 4.2V. A Low Iq LDO or a highly efficient buck converter can step down that voltage whilst keeping power consumption to a minimum.
 
When it comes to low power computing, MSP430 leads the pack. Its low-power modes allow it to conserve vast amounts of power. However, we should also remember that the supply voltage also determines how much current will be consumed by the MCU. By lowering the battery voltage to a lower level by means of a Low Iq LDO or efficient DC/DC converter, we can effectively lengthen our battery life. Fortunately, new MSP430 series utilize this property by means of an internal PMM. However, sometimes it’s still necessary to step down voltages so that they’re within the acceptable supply range. And, remember, adding to something doesn’t always make it bigger.
 

Want to know more about TI Analog? Read more posts in the series:

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