Power Management
Texas Instruments Incorporated
Using power solutions to extend battery life
in MSP430 applications
By Michael Day
Applications Manager, Portable Power Products
Figure 1. MSP430FG4618’s supply current
versus its supply voltage
Active-Mode Current (µA)
The MSP430 is the lowest-power microprocessor available
in the market. Its inherently low-power operation is ideal
for battery-powered applications where frequent battery
replacement is undesirable. This article shows two simple
but effective power solutions that further minimize MSP430
power consumption and extend battery life.
In an attempt to prolong battery life, software engineers
go to great lengths to optimize code, minimize memory
accesses, etc. Hardware engineers focus on ways to shut
down unused circuitry, ensure that all quiescent currents
and leakage paths are minimized, and maximize powersupply efficiency.
In most cases, engineers eliminate any DC/DC conversion altogether if the system’s source voltage falls within
the MSP430’s input operating range. Many MSP430 designs
do not need an input power supply because the MSP430
device family accepts extremely wide variations in input
voltage. For example, the MSP430FG4618 operates with
an input voltage of between 1.8 V and 3.6 V. Because of
this wide input range, many MSP430 designs operate
directly from a battery without additional power conversion. Examples of input sources that do not need power
conversion are dual alkaline, nickel metal hydride, and
nickel cadmium batteries, as well as primary lithium-ion
coin cells.
An often overlooked technique for extending battery life
is to add an input power supply, even if it is otherwise not
needed. Adding a power supply between the input-voltage
source and the MSP430 to increase battery life is contradictory to conventional thinking. This is because of two
things that all power supplies have in common: They have
quiescent current (Iq) at no load that sinks current from
the battery to ground; and they have less than 100% efficiency, which dissipates power in the power supply. Even
power supplies optimized for low-power and batterypowered applications have less than 100% efficiency, with
quiescent currents that continually drain battery capacity.
Typical power-supply efficiencies for an MSP430 application operating at 3.0 V from two AA batteries is 85 to 92%.
Typical Iq values range from 15 to 50 µA. Conventional
thinking says that removing this power supply and operating the application directly from the battery will extend
700
600
500
400
300
1.8
2.1
2.4
2.7
3.0
3.3
3.6
Supply Voltage (V)
battery life by an additional 8 to 15% because the effective
efficiency will then be 100%.
MSP430 supply current varies linearly with input voltage,
so operating the system with lower voltages reduces both
MSP430 input current and overall power consumption.
Figure 1 shows the variation in the MSP430FG4618’s 1-MHz
active-mode supply current (IAM) versus its supply voltage.
Operating at the lowest required input voltage minimizes
battery current, but this requires the insertion of a power
supply. Regardless of topology, this power supply will be
less than 100% efficient. A common design scenario is an
MSP430 operating from two series-connected AA alkaline
batteries that supply 3.2 V when new and 1.8 V when discharged. The designer must choose between two powersystem topologies. The first is to operate directly from the
battery voltage, which results in a higher MSP430 input
current. The second is to insert a power supply between
the battery and the MSP430. After considering the power
supply’s efficiency and quiescent current, many designers
quickly choose to operate directly from the input source.
Few designers are aware that adding a power supply can
actually provide significant improvements in battery life,
even with efficiency and quiescent-current concerns.
10
High-Performance Analog Products
www.ti.com/aaj
4Q 2009
Analog Applications Journal
Power Management
Texas Instruments Incorporated
Figure 2. Two alkaline-system configurations
Efficiency = 100%
Quiescent Current = 0 nA
3.2 V to 1.8 V
AA Alkaline
3.2 V to 1.8 V
MSP430
(a) System 1
Efficiency = 90%
Quiescent Current = 500 nA
TPS780xx
2.2 V
AA Alkaline
3.2 V to 1.8 V
MSP430
(b) System 2
Figure 3. MSP430’s operating current versus
battery voltage
Active-Mode Current (µA)
Designers must deviate from conventional thinking that
efficiency is the most important figure of merit in a power
system. In a battery-powered system, battery current drain
is the main concern. The examples in Figure 2 help make
this point. System 1 in Figure 2a operates directly from
two AA alkaline batteries. An equivalent power supply in
this example has 100% efficiency and 0-µA quiescent
current. All power delivered from the battery is available
to the MSP430. For System 2 in Figure 2b, a TPS780xx
LDO has been inserted. The LDO’s efficiency is defined by
VOUT /VIN, which averages to approximately 90% over the
entire voltage range of the batteries. The LDO also draws
500 nA of quiescent current from the battery. When only
efficiency and quiescent current are considered, System 1
clearly wins. However, System 2 draws less current from
the batteries, which extends the system’s operating time.
Figure 3 compares the two systems’ battery currents.
When the battery voltage is above 2.2 V, System 1 consumes more battery current because the MSP430 operating
current is a linear function of input voltage. System 2
consumes a constant current because the LDO maintains a
constant 2.2 V at the MSP430. As the battery voltage drops
to 2.2 V and below, the two MSP430s consume the same
current. System 2 consumes an additional 500 nA due to
the TPS780xx’s quiescent current (Iq). When the input
voltage is above 2.2 V, System 2’s reduced battery current
results in longer system run time.
Two lab experiments were conducted with an
MSP430FG4618 operating at 5 MHz while powered by two
AAA alkaline batteries. These experiments were set up to
correspond with the two systems in the previous example.
In this second example, System 1, with the MSP430 powered directly from the batteries, operated for 223 hours
before shutting down. System 2, which used a TPS780xx
to drop the MSP430 operating voltage to 2.2 V, operated
for 298 hours before shutting down. The addition of the
TPS780xx LDO, which operates at 90% efficiency with
these operating conditions, extended battery life by 30%.
An often underutilized method to extend battery life is
dynamic voltage scaling (DVS). With DVS, the input supply is reduced if the MSP430 is operated at a lower clock
speed or placed into a low-power mode. The examples
presented earlier demonstrated that operating with lower
input voltages reduces current consumption and extends
battery life. For example, an MSP430 system operating
with a 7-MHz maximum clock frequency may require the
input voltage to be 3.3 V. If the clock speed is reduced to
4.6 MHz, the MSP430 requires only a 2.0-V input voltage.
If the MSP430 is placed into low-power mode, the
required input voltage is only 1.8 V.
600
Iq
500
400
m
ste
Sy
1C
t
en
urr
System 2 Current
300
1.8
2.2
2.6
3.0 3.2
Battery Voltage (V)
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4Q 2009
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High-Performance Analog Products
Power Management
Texas Instruments Incorporated
Figure 4 shows a battery-powered system that uses the
TPS780xx to implement DVS to save battery power. The
TPS780xx, which is an LDO with an ultralow quiescent
current of 500 nA, contains a digital input (VSET) that
connects directly to the MSP430. The MSP430 pulls this
pin high to set VOUT at 2.2 V and pulls it low to set VOUT at
3.3 V. This configuration allows the MSP430 to adjust its
input voltage as its operating conditions change.
Even if the MSP430 is operated only at 7 MHz when
active and placed into low-power mode when not active,
DVS can significantly extend battery life. In low-power
mode 3 (LPM3), the MSP430FG4618’s operating currents
at inputs of 3.3 V and 2.2 V are 2.13 µA and 1.3 µA,
respectively. With the TPS780xx’s 0.5-µA quiescent current added, the battery currents are 2.63 µA and 1.8 µA,
respectively. DVS reduces battery current by 26% under
these conditions. This reduction of LPM3 battery current
is critical for systems that spend a significant amount of
time in sleep mode.
When designing an MSP430 power system, an engineer
should pay close attention to selecting the proper operating voltage. Minimizing the nominal operating voltage and
implementing DVS when possible will provide significant
improvements in a system’s run time.
Related Web sites
power.ti.com
www.ti.com/msp430
www.ti.com/sc/device/MSP430FG4618
www.ti.com/sc/device/TPS78001
Figure 4. MSP430 with DVS implemented
TPS780xx
VOUT
VSET = 0, VOUT = 3.3 V
VSET = 1, VOUT = 2.2 V
VSET
GPIO
VCC
MSP430
Li-lon
3.0 V to 4.2 V
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High-Performance Analog Products
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Analog Applications Journal
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