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Texas Instruments Selecting a DC/DC Converter for Maximum Battery Life in Pulsed-Load Applications Application notes
Application Report
SLVA740 – November 2018
Selecting a DC/DC Converter for Maximum Battery Life
in Pulsed-Load Applications
Milos Acanski
ABSTRACT
When designing a battery powered system, maximizing battery life is one of the most important design
goals. Battery powered systems, such as smart meters, IoT sensors or wireless medical equipment often
require a power converter to obtain fixed supply voltages for time varying loads. In order to minimize the
conversion losses it is important to look at the overall efficiency, together with the load profile. This
application report shows how to interpret efficiency under different load conditions in order to maximize the
battery life when selecting a suitable converter.
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Contents
Converter Operation Modes Under Different Load Conditions ........................................................
1.1
Operation Under Heavy Load .....................................................................................
1.2
Operation Under Light Load .......................................................................................
1.3
Operation in Shutdown Mode .....................................................................................
Case Study ...................................................................................................................
Summary ......................................................................................................................
References ...................................................................................................................
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1
Converter Operation Modes Under Different Load Conditions
In many battery powered devices, current consumption is approximately pulse-shaped with a period T, as
shown in Figure 1. Typically, there is a short activity, characterized by a high current IBAT,HI and a duty
cycle D, followed by a period of inactivity, characterized by a low current IBAT,LO. Such load profiles can be
found in wireless sensor applications for example, where measurements are periodically transmitted via
an RF transmitter. When selecting a power converter for the system, a common pitfall is to consider only
the heavy-load efficiency, neglecting the battery consumption during the period of inactivity.
With a pulse-shaped consumption profile, different operating modes of a converter can be considered.
Which ones are important depends primarily on the load profile, that is, how much time the device or load
spends in the certain mode and what is the actual load current. The exact combination of operating modes
determines the current consumption and the battery lifetime. We will here consider different operating
modes for the TPS63805, which is a buck-boost device. When looking at the power level and the
conversion efficiency, there are three different modes of operation to consider.
Figure 1. Battery Current Profile with a Pulsed Load
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Converter Operation Modes Under Different Load Conditions
1.1
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Operation Under Heavy Load
When the device operates under heavy load, the internal power stage operates in pulse-width-modulation
(PWM) mode where it is constantly switching. For the TPS63805 this is the case when the peak inductor
current is above 700 mA typically. The conversion losses depend on the particular device and the
passives around the device, in the first place the inductor. The efficiency curve for the TPS63805
operating in PWM mode is shown in Figure 2. Using the efficiency curve, it is easy to map the load current
into the battery current consumption. As it can be seen, while operating under heavy loads the device
reaches highest efficiency. However, forcing the power stage to always operate in PWM mode penalizes
the efficiency at light loads, as various losses due to constant active operation become larger than the
load power.
Figure 2. TPS63805 Efficiency vs Output Current
1.2
Operation Under Light Load
To improve efficiency at light load, power-save (PS) mode can be enabled. In PS mode, under light load
the converter operates in short bursts, frequently enough to maintain the output voltage. Within one burst
the converter operates under current higher than the load current, and therefore at higher efficiency, while
for the rest of the period the converter is inactive. This way of operation is called pulse-frequencymodulation (PFM). Using PFM at light loads, the resulting efficiency is significantly improved over PWM,
as shown in Figure 2.
An important parameter associated with PS mode is the quiescent current IQ. For TI devices, unless noted
otherwise in the datasheet, the quiescent current IQ is defined as the current drawn by the device in a noload and non-switching but enabled condition. IQ includes the current necessary for operation of all parts of
the device except for the power stage. The current for operating the power stage, which is heavily
dependent on external components, is not included in IQ. Therefore IQ is a device parameter, and not a
system parameter, since IQ is solely dependent on the device itself. For more information on IQ, see
Reference 2.
IQ is often misinterpreted as the no-load input current, which is a current drawn by the converter from the
input power supply when there is no load present on the output of the converter. The no-load input current
also includes the current for the switching power stage. Being a device parameter, IQ constitutes only a
part of the no-load input current. Nevertheless, IQ can still be used to estimate the input current under noload or light load conditions, see Reference 2. The input current consumption is determined by both IQ and
the conversion efficiency. The share of IQ in the input current becomes larger as the load current
decreases to zero, and selecting the device with lower IQ is likely to result in lower input current. Still, the
light load efficiency can be heavily influenced by the external components around the converter, and the
best way to determine it is to measure the efficiency directly. In case of the direct measurement, there are
some important aspects that need to be considered, see Reference 3 and Reference 4.
2
Selecting a DC/DC Converter for Maximum Battery Life in Pulsed-Load
Applications
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Converter Operation Modes Under Different Load Conditions
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1.3
Operation in Shutdown Mode
In some applications, it is not required for the load to be constantly on. In such cases the load can be
turned off completely by disabling the converter and putting it in shutdown mode. In shutdown mode, the
converter stops switching, all internal control circuitry is switched off, and the load is disconnected from
the input. In case of the TPS63805, the input current in shutdown mode ISD is 0.6 μA maximum. ISD is
usually much lower than IQ. Nevertheless, as is the case with IQ, ISD should also not be neglected when
dealing with load profiles with very small duty cycle D.
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Case Study
The battery currents in different operating modes can differ by several orders of magnitude, but so can the
times spent in the respective operating modes. Looking back at Figure 1, it is not uncommon to have an
application with a load duty cycle D below 10-3. If the ratio of input currents is in the same order of
magnitude as the ratio of active and inactive period, the light load current should not be neglected when
calculating battery life time.
Consider a battery powered system where a buck-boost converter is used to obtain fixed 3.3 V voltage
from a Lithium-ion battery with a nominal voltage of 3.7 V. The load current has a pulse-shaped profile as
shown in Figure 1, with corresponding values ILOAD,HI and ILOAD,LO. The resulting battery currents IBAT,HI and
IBAT,LO can be directly measured for different load currents ILOAD,HI and ILOAD,LO, or estimated via efficiency
curves and IQ. The TPS63805 is compared with a similarly rated competitor’s device which has higher
efficiency at heavy loads resulting in lower IBAT,HI, but also higher quiescent current IQ and higher shutdown
current ISD, resulting in higher IBAT,LO. The average battery current consumption can be expressed as a
function of the load duty cycle D as:
IBAT=DIBAT,HI+(1-D)IBAT,LO
(1)
The relevant device parameters are summarized in Table 1. As it can be seen, there is a difference
between the quiescent current IQ and the no-load input current. The no-load input current includes IQ, but
also the current due to losses in the power stage and the external components. In this case, using the TI
device with lower IQ resulted in lower no-load input current.
Table 1. Operating currents and efficiency for the TPS63805 and the competitors device
Device
Quiescent current IQ
No-load input current
Shutdown input current ISD
Peak efficiency
TPS63805
11 µA
17 µA
0.6 µA
95.5%
Competitor
40 µA
51 µA
1 µA
97.5%
The comparison between the TI and the competitor device for a range of load duty cycle D and different
load profiles is shown in Figure 3. At high load duty cycles D, using the competitor’s device results in up to
2% lower battery consumption, owing to its higher efficiency at heavy load. However as the D decreases,
the TI device with its lower IQ starts gaining advantage over the competitor’s device. The turning point for
ILOAD,HI = 1 A, for example, is at approximately D = 0.003, and for lower load duty cycles using the
competitor’s device results in up to 115% higher battery consumption for ILOAD,LO = 10 µA. As the load
current for inactive period ILOAD,LO becomes lower, the advantage of having lower IQ becomes more
important.
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Case Study
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Figure 3. Relative Difference in Battery Consumption vs Load Duty Cycle
If the load is turned off during the inactive period by disabling the converter, the battery current IBAT,LO
becomes the shutdown input current ISD of the converter. The same results as for low IQ apply here. For
higher load duty cycles D, using the competitor’s device results in slightly lower battery consumption. For
low load duty cycles, selecting a device with lower shutdown current results in lower battery current
consumption. The same two devices are compared and the result is shown in Figure 4. For high D the
competitor’s device has up to 2% lower battery current. For low D, the TI device gains advantage, having
lower ISD of 0.6 μA compared to the competitor’s 1 μA, and using the competitor’s device results in up to
66% higher battery consumption.
Figure 4. Relative Difference in Battery Consumption vs Load Duty Cycle
The same approach in comparing the average battery current consumption can be used in case of more
complex load profiles than the one shown in Figure 1.
4
Selecting a DC/DC Converter for Maximum Battery Life in Pulsed-Load
Applications
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Summary
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3
Summary
In many battery applications the current consumption profile of the load is pulse shaped, often with a low
load duty cycle, with a short period of activity under heavy load, followed by a long period of inactivity
under light load. When selecting a power converter for such applications, it is important to take into
account all operating modes when calculating battery consumption, especially for applications where the
load operates with a low load duty cycle. In such cases, significantly longer battery life can be achieved by
selecting a device with lower quiescent current instead of a device with higher heavy-load efficiency.
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References
1. "2-A, High-Efficient Buck-Boost Converter with Small Solution Size," TPS63805 Datasheet, SLVSDS9
2. Chris Glaser, "Iq: What It Is, What It Isn't, and How to Use It," Analog Applications Journal (2Q, 2011),
SLYT412
3. Jatan Naik, "Performing Accurate PFM Mode Efficiency Measurements," Application Report, SLVA236
4. Chris Glaser, "Accurately Measuring Efficiency of Ultralow-Iq Devices," Analog Applications Journal
(1Q, 2014), SLYT558.
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