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Texas Instruments Extend Battery Life Using Load Switches and Ideal Diodes Application notes
Extend Battery Life Using Load Switches and Ideal Diodes
Andy Robles
Extending the life of a battery-operated system
enhances the experience people have with the product
and can potentially cut replacement costs. The battery
life of products can be extended by considering current
consumption, battery stress, and battery deterioration.
This document will address two different scenarios that
are often found in battery-operated systems. The first
scenario consists of a system composed of several
subsystems that can be temporarily shut down in order
to preserve battery life. For instance, a wearable
fitness tracker may not need Bluetooth® powered
throughout the majority of the day. The solution used
to turn power on or off such a subsystem must also be
optimized for current consumption in order to
maximize battery life.
CL
RL
Saving Power by Turning off Subsystems
Switching solutions must minimize current
consumption during both on and off states. Ideally,
when a particular subsystem is disabled using a
switch, there is no current consumption. However,
switch solutions have some current consumption,
typically denoted as shutdown current (ISD), in the offstate. The shutdown current is comprised of the FET
leakage and controller circuitry that remains on to
protect the FET. The TPS22916 load switch has a
shutdown current of 10 nA, which reduces the current
consumption in the off-state. There are also current
consumption considerations while in the on-state. In
load switches, this current is called quiescent current
(IQ). Quiescent current travels internally from the VIN
pin to the GND pin of the load switch in the on-state.
Table 1. Case Study
Sub system
VBAT
+
Loa d S witch
±
CL
TPS22916/TPS22917
RL
GND
Sub system
Loa d S witch
CL
RL
PFET Discrete
Solution
Other Load Switch
ISD
10 nA
2 nA
100 nA
IQ
0.51 µA
9.25 µA
1.11 µA
Capacity
Usage
1.44 µAh
22.24 µAh
4.82 µAh
2
2
0.94 mm2
Solution Size
0.55 mm
17.08 mm
Sub system
Figure 1. Turning off Subsystem Loads
The second scenario is a system with a backup power
supply circuitry. Such systems, like an electricity
meter, use an ORing configuration traditionally
implemented with diodes. This configuration switches
to the backup power supply when the main power
supply is no longer present. The ORing configuration
must power the load with minimal power dissipation to
extend the operating time of the battery. The diodes
can be replaced by ideal diodes, as seen in Figure 2,
to reduce the power dissipation by minimizing the
forward voltage drop typically seen in diodes.
Dual Ideal Diode
ORing Solution
VIN1
+
-
LM66100
CL
VBAT
RL
LM66100
Downstream Load
Another on-state current consumption factor comes
from a pulldown resistor on the enable pin of the
switch to keep it from floating. The pulldown resistor
draws current when the switch is enabled, adding to
the total on-state current consumption. The TPS22916
and TPS22917 can lower the total on-state current
consumption to 0.51 µA by incorporating a smart ON
pin pulldown resistor feature. This feature reduces the
current consumption by disconnecting the pulldown
resistor when the switch is enabled. To maximize the
battery life, the shutdown, quiescent, and pulldown
resistance currents must be as low as possible in a
system. Table 1 shows the capacity of a battery that
different switch solutions consume in a day running at
a 10% duty cycle. The solutions include the TPS22916
and TPS22917, load switches in the market, and a
discrete configuration similar to the one in the When to
Make the Switch to an Integrated Load Switch
Application Note. The TPS22916 and TPS22917
decrease the capacity usage of the battery by 70%
when compared to other load switches. In addition to
power consumption benefits, the TPS22916 reduces
the solution size by 40% due to the small package.
+ Lower Power Dissipation
+ Lower Leakage Current
+ Robust Protection
Figure 2. Backup Power Configuration
SLVAEC7 – August 2019
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Extend Battery Life Using Load Switches and Ideal Diodes Andy
Copyright © 2019, Texas Instruments Incorporated
Robles
1
www.ti.com
Battery
2.9
LM66100
2.4
Voltage (V)
Two other considerations to keep in mind are RON and
inrush currents. These switching solutions inherently
have an on-state resistance, typically denoted as RON.
This resistance does not significantly contribute to the
power consumption from the battery in low power
applications. The TPS22916 load switch features an
RON of 70 mΩ at 3.6 V input voltage. When powering a
55 mA load, this switch reduces the output voltage by
only 0.1%. This amount of drop has an insignificant
impact on battery life.
Discrete
Diode
UVLO
Threshold
tDiode
tLM66100
Operating Time
Figure 4. Voltage vs Operating Time
Figure 3. Inrush Current Control
Powering up of a system also affects the life of the
battery. During start-up, inrush current can occur due
to the load capacitance. This spike of inrush current
applies stress to the battery, decreasing its capacity.
To minimize the inrush current, load switches
implement a soft-start to control the rate at which the
switch turns on. Controlling the rate at which the
output rises minimizes the spike of current being
drawn from the battery, as seen in Figure 3. This
reduces stress on the battery and minimizes current
consumption during power-up sequences.
Battery Back-up Applications
Traditionally, the switching of power supplies is done
with an ORing configuration using two diodes. A major
drawback of this configuration is the high forward
voltage drop of a diode. The high forward voltage drop
causes a high dissipation of power which reduces the
operating time of the battery. Two LM66100 ideal
diodes can be implemented in backup power supply
applications to reduce the forward voltage drop in this
ORing configuration. The LM66100 has a much lower
voltage drop in comparison to a discrete diode solution
resulting in lower power dissipation as seen in Table 2.
Another key element to keep in mind is the
undervoltage lockout threshold of the load. With the
voltage of a battery decreasing during its lifetime, the
forward voltage drop of the discrete diode disables the
system faster than the LM66100 solution since it
reaches its minimum voltage threshold first. This
undervoltage lockout condition causes the battery to
operate for a shorter amount of time as illustrated in
Figure 4. The difference in time depends on the
battery being used and the load.
2
Extend Battery Life Using Load Switches and Ideal Diodes Andy
Another benefit of using the LM66100 ideal diode
device is having a lower reverse leakage current as
compared to a discrete diode. As seen in Table 2, the
LM66100 has a 75% lower reverse leakage current
specification. There are two cases in which reverse
leakage current can affect the battery life. In the first
case, when the main power supply is active, reverse
leakage current flowing into the backup battery can
reduce its lifetime by deteriorating its capacity. In the
second case, when the backup battery is powering the
system, reverse leakage current flowing into the main
power supply path results in undesired current
consumption. Minimizing reverse leakage current
helps extend the life of the battery.
Table 2. LM66100 vs Diode
LM66100
Discrete Diode
Power
Dissipation at 5
V, 200 mA
3.16 mW
140 mW
Max Reverse
Leakage
Current
0.5 µA
2 µA
Conclusion
Lowering current consumption and considering the
stress and deterioration of the battery is imperative to
extend the life of the battery. Reducing power
consumption with a switch, like the TPS22916 and
TPS22917 that has low shutdown and quiescent
current, keeps the battery from rapidly draining its
capacity. In ORing configurations, the LM66100 lowers
the forward voltage drop and reverse current leakage
to extend the operating time of the battery.
Table 3. Related Documents
Document Type
Title
TI Tech Note
When to Make the Switch to an Integrated
Load Switch (SLVAEC5)
TI Tech Note
Eliminate the Voltage Drop and Save Power:
An Ideal Diode (SLVAEA8)
Robles
Copyright © 2019, Texas Instruments Incorporated
SLVAEC7 – August 2019
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