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 1. Analog Dialogue Technical Journal
 2. Articles
 3. Simple Battery Charger ICs for Any Chemistry


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 * JAN 2019
   VOL 53
 * 




SIMPLE BATTERY CHARGER ICS FOR ANY CHEMISTRY

by Steve Knoth Download PDF


BACKGROUND

It is common for many battery-powered devices to require a wide variety of
charging sources, battery chemistries, voltages, and currents. For example,
industrial, high end, feature-rich consumer, medical, and automotive battery
charger circuits demand higher voltages and currents as newer large-battery
packs are emerging for all types of battery chemistries. Furthermore, solar
panels with wide-ranging power levels are being used to power a variety of
innovative systems containing rechargeable sealed lead acid (SLA) and
lithium-based batteries. Examples include crosswalk marker lights, portable
speaker systems, trash compactors, and even marine buoy lights. Moreover, some
lead acid (LA) batteries found in solar applications are deep cycle batteries
capable of surviving prolonged, repeated charge cycles, in addition to deep
discharges. A good example of this is in deep sea marine buoys, where a 10-year
deployment life is a prerequisite. Another example is off-grid (that is,
disconnected from the electric utility company) renewable energy systems such as
solar or wind power generation, where system up-time is paramount due to
proximity access difficulties.

Even in nonsolar applications, recent market trends imply a renewed interest in
high capacity SLA battery cells. Automotive, or starting, SLA cells are
inexpensive from a cost/power output perspective and can deliver high pulse
currents for short durations, making them an excellent choice for automotive and
other vehicle starter applications. Embedded automotive applications have input
voltages >30 V, with some even higher. Consider a GPS location system used as an
antitheft deterrent; a linear charger with the typical 12 V input stepping down
to 2-in-series Li-Ion battery (7.4 V typical) and needing protection to much
higher voltages, could be valuable for this application. Deep cycle LA batteries
are another technology popular in industrial applications. They have thicker
plates than automotive batteries and are designed to be discharged to as low as
20% of their total capacity. They are normally used where power is required over
a longer time constant such as fork lifts and golf carts. Nevertheless, like
their Li-Ion counterpart, LA batteries are sensitive to overcharging, so careful
treatment during the charging cycle is very important.

Current integrated circuit (IC)-based solutions cover just a fraction of the
many possible combinations of input voltage, charge voltage, and charge current.
A cumbersome combination of ICs and discrete components has routinely been used
to cover most of the remaining, more difficult combinations and topologies. That
wasn’t until, in 2011, when Analog Devices addressed and simplified this market
application space with its popular 2-chip charging solution consisting of the
LTC4000 battery charging controller IC mated with a compatible, externally
compensated dc-to-dc converter.


SWITCHING VS. LINEAR CHARGERS

Traditional linear topology battery charger ICs were often valued for their
compact footprints, simplicity, and low cost. However, drawbacks of these linear
chargers include a limited input and battery voltage range, higher relative
current consumption, excessive power dissipation, limited charge termination
algorithms, and lower relative efficiency (efficiency ~ [VOUT/VIN] × 100%). On
the other hand, switch-mode battery chargers are also popular choices due to
their flexible topology, multichemistry charging, high charging efficiencies
(which minimize heat to enable fast charge times), and wide operating voltage
ranges. Nevertheless, some of the drawbacks of switching chargers include
relatively high cost, more complicated inductor-based designs, potential noise
generation, and larger footprint solutions. Modern LA, wireless power, energy
harvesting, solar charging, remote sensor, and embedded automotive applications
have been routinely powered by high voltage linear battery chargers for the
reasons stated above. However, an opportunity exists for a more modern
switch-mode charger that negates the associated drawbacks.


AN UNCOMPLICATED BUCK BATTERY CHARGER

Some of the tougher challenges a designer faces at the outset of a charging
solution are the wide range of input sources combined with a wide range of
possible batteries, the high capacity of the batteries needing to be charged,
and a high input voltage.

Input sources are as wide as they are variable, but some of the more complicated
ones that deal with battery charging systems are: high powered wall adapters
with voltages spanning from 5 V to 19 V and beyond, rectified 24 V ac systems,
high impedance solar panels, car, and heavy truck/Humvee batteries. Therefore,
it follows that the combination of battery chemistries possible in these
systems—lithium-based (Li-Ion, Li-Polymer, lithium-iron phosphate (LiFePO4)) and
LA-based—increases the permutations even more, thus making the design even more
daunting.

Due to IC design complexity, existing battery charging ICs are primarily limited
to step-down (or buck) or the more complex SEPIC topologies. Add solar charging
capability to this mix and you open a variety of other complexities. Finally,
some existing solutions charge multiple battery chemistries, some with onboard
termination. However, up until now, no single IC charger has provided all of the
necessary performance features to solve these issues.


NEW, FEATURE-RICH COMPACT CHARGERS

A buck IC charging solution that solves the problems discussed above would need
to possess most of the following attributes:

 * Wide input voltage range
 * Wide output voltage range to address multiple battery stacks
 * Flexibility—ability to charge multiple battery chemistries
 * Simple and autonomous operation with onboard charge termination algorithms
   (no microprocessor needed)
 * High charge current for fast charging, large, high capacity cells
 * Solar charging capability
 * Advanced packaging for improved thermal performance and space efficiency

When ADI developed the popular LTC4000 battery charging controller IC (which
works in conjunction with an externally compensated dc-to-dc converter to form a
powerful and flexible 2-chip battery charging solution) a few years ago, it
greatly simplified the existing solution, which was quite convoluted and
cumbersome. To enable PowerPathTM control, step-up/down functionality, and input
current limiting, solutions consisted of a buck-boost dc-to-dc switching
regulator or a buck-switching regulator charger controller paired with a
front-end boost controller, and a microprocessor, plus several ICs and discrete
components. Key drawbacks included limited operating voltage range, no solar
panel input capability, inability to charge all battery chemistries, and no
onboard charge termination. Fast forward to the present and now some simpler,
and much more compact, monolithic solutions are available to solve these
problems. The LTC4162 and LTC4015 buck battery chargers from Analog Devices both
provide single-chip step-down charging solutions, with varying charge current
levels and a full feature set.


THE LTC4162 BATTERY CHARGER

The LTC4162 is a highly integrated, high voltage multichemistry synchronous
monolithic step-down battery charger and PowerPath manager with onboard
telemetry functions and optional maximum power point tracking (MPPT). It
efficiently transfers power from a variety of input sources, such as wall
adapters, backplanes, and solar panels, to charge a Li-Ion/polymer, LiFePO4, or
LA battery stack while still providing power to the system load up to 35 V. The
device provides advanced system monitoring and PowerPath management, plus
battery health monitoring. While a host microcontroller is required to access
the most advanced features of the LTC4162, the use of the I2C port is optional.
The main charging features of the product can be adjusted using pin-strap
configurations and programming resistors. The device offers precision ±5% charge
current regulation up to 3.2 A, ±0.75% charge voltage regulation, and operates
over a 4.5 V to 35 V input voltage range. Applications include portable medical
instruments, USB power delivery (USB-C) devices, military equipment, industrial
handhelds, and ruggedized notebooks/tablet computers.




Figure 1. Typical application circuit for the LTC4162-L.



The LTC4162 (see Figure 1) contains an accurate 16-bit analog-to-digital
converter (ADC) that continuously monitors numerous system parameters on
command, including input voltage, input current, battery voltage, battery
current, output voltage, battery temperature, die temperature, and battery
series resistance (BSR). All system parameters can be monitored via a two-wire
I2C interface, while programmable and maskable alerts ensure that only the
information of interest causes an interrupt. The device’s active maximum power
point tracking algorithm globally sweeps an input undervoltage control loop and
selects an operating point to maximize power extraction from solar panels and
other resistive sources. Further, its built-in PowerPath topology decouples the
output voltage from the battery, thereby allowing a portable product to start up
instantly when a charging source is applied under very low battery voltage
conditions. The LTC4162’s onboard charging profiles are optimized for a variety
of battery chemistries including Li-Ion/polymer, LiFePO4, and LA. Both charge
voltage and charge current can be automatically adjusted based on battery
temperature to comply with JEITA guidelines or be customized. For LA, a
continuous temperature curve automatically adjusts the battery voltage based on
the ambient temperature. For all chemistries, an optional die junction
temperature regulation system can be engaged, preventing excess heating in space
constrained or thermally challenged applications. See Figure 2 for Li-Ion
charging efficiency performance.

Finally, the LTC4162 is housed in a 28-lead, 4 mm × 5 mm QFN package with an
exposed metal pad for excellent thermal performance. E- and I-grade devices are
guaranteed for operation from –40°C to +125°C.




Figure 2. Li-Ion charging efficiency vs. input voltage by cell count.




WHAT IF HIGHER CURRENT IS NEEDED?

The LTC4015 is also a highly integrated, high voltage, multichemistry,
synchronous step-down battery charger with onboard telemetry functions. However,
it features a controller architecture with offboard power FETs for higher charge
current capability (up to 20 A or more depending on external components chosen).
The device efficiently supplies power from an input source (wall adapter, solar
panel, etc.), to a Li-Ion/polymer, LiFePO4, or LA battery. It provides advanced
system monitoring and management functionality, including battery coulomb
counting and health monitoring. While a host microcontroller is required to
access the most advanced features of the LTC4015, the use of its I2C port is
optional. The main charging features of the product can be adjusted using
pin-strap configurations and programming resistors.




Figure 3. 12 VIN to 2-cell Li-Ion 8 A buck battery charger circuit.



The LTC4015 offers precision ±2% charge current regulation up to 20 A, ±1.25%
charge voltage regulation and operation over a 4.5 V to 35 V input voltage
range. Applications include portable medical instruments, military equipment,
battery backup applications, industrial handhelds, industrial lighting,
ruggedized notebooks/tablet computers, and remote powered communication and
telemetry systems.

The LTC4015 also contains an accurate 14-bit analog-to-digital converter (ADC),
as well as a high precision coulomb counter. The ADC continuously monitors
numerous system parameters, including input voltage, input current, battery
voltage, battery current, and reports battery temperature and battery series
resistance (BSR) on command. By monitoring these parameters, the LTC4015 can
report on the state of health of the battery, as well as its state of charge.
All system parameters can be monitored via a two-wire I2C interface, while
programmable and maskable alerts ensure that only the information of interest
causes an interruption. The LTC4015’s onboard charging profiles are optimized
for each of a variety of battery chemistries including Li-Ion/polymer, LiFePO4,
and LA. Configuration pins allow the user to select between several predefined
charge algorithms for each battery chemistry, as well as several algorithms
whose parameters can be adjusted via I2C. Both charge voltage and charge current
can be automatically adjusted based on battery temperature to comply with JEITA
guidelines, or even custom settings. See Figure 4 for lead acid charging
efficiency performance. The LTC4015 is housed in a 5 mm × 7 mm QFN package with
an exposed metal pad for excellent thermal performance.




Figure 4. Lead acid charging efficiency with the LTC4015.




SPACE SAVINGS, FLEXIBILITY, AND HIGHER POWER LEVELS

At equal power levels (for example, 3 A), because it is a monolithic device with
integrated power MOSFETs, the LTC4162 can save up to 50% of the PCB area
compared to the LTC4015. Since their feature sets are similar, the LTC4015
should be used when output currents are >3.2 A up to 20 A or more. None of the
industry competing IC battery charger solutions offer the same high level of
integration, nor can they generate the same power levels. Those that approach
the charge current (2 A to 3 A) are limited to only a single battery chemistry
(Li-Ion) or are limited in battery charge voltage (13 V maximum), and therefore
do not offer the power levels nor the flexibility of the LTC4162 or LTC4015.
Furthermore, when you consider the number of external components required for
the nearest competing monolithic battery charger solution, the LTC4162 offers up
to 40% savings in PCB area footprint, making it an even more enticing choice for
designs.


SOLAR CHARGING

There are many ways to operate a solar panel at its maximum power point (MPP).
One of the simplest methods is to connect a battery to the solar panel through a
diode. This technique relies on matching the maximum output voltage of the panel
to the relatively narrow voltage range of the battery. When available power
levels are very low (approximately less than a few tens of milliwatts), this may
be the best approach. However, power levels are not always low. Therefore, the
LTC4162 and LTC4015 utilize MPPT, a technique that finds the maximum power
voltage (MPV) of a solar panel as the amount of incident light changes. This
voltage can change drastically from 12 V to 18 V as the panel current changes
over 2 or more decades of dynamic range. The MPPT circuit algorithm finds and
tracks the panel voltage value that delivers the maximum charge current to the
battery. The MPPT function not only continuously tracks the maximum power point,
but it is also able to select the correct maximum on the power curve to increase
power harvested from the panel during partial shade conditions when multiple
peaks occur on the power curve. During periods of low light, a low power mode
allows the charger to deliver a small charge current even if there is not enough
light for the MPPT function to operate.


CONCLUSION

Analog Devices’ newest powerful and full-featured battery charging and PowerPath
manager ICs, the LTC4162 and LTC4015, simplify a very difficult high voltage and
high current charging system. These devices efficiently manage power
distribution between input sources, such as wall adapters, backplanes, solar
panels, etc., and the charging of various battery chemistries, including
Li-Ion/polymer, LiFePO4, and SLA. Their simple solution and compact footprints
enable them to achieve high performance in leading-edge applications where only
more complicated, older technology switching regulator-based topologies such as
SEPIC were once the only option. This greatly simplifies the designer’s task
when it comes to medium-to-high power battery charger circuits.


AUTHOR

Steve Knoth



Steve Knoth is a senior product marketing manager in Analog Devices’ Power
Group. He is responsible for all power management integrated circuit (PMIC)
products, low dropout (LDO) regulators, battery chargers, charge pumps, charge
pump-based LED drivers, supercapacitor chargers, and low voltage monolithic
switching regulators. Prior to rejoining Analog Devices in 2004, Steve held
various marketing and product engineering positions at Micro Power Systems,
Analog Devices, and Micrel Semiconductor. He earned his bachelor’s degree in
electrical engineering in 1988 and a master’s degree in physics in 1995, both
from San Jose State University. Steve also received an M.B.A. in technology
management from the University of Phoenix in 2000. In addition to enjoying time
with his kids, Steve is an avid music lover and can be found tinkering with
pinball and arcade games or muscle cars, and buying, selling, and collecting
vintage toys, movie, sports, and automotive memorabilia.


RELATED CONTENT


PRODUCTS

 * LTC4015
   
   Multichemistry Buck Battery Charger Controller with Digital Telemetry System

 * LTC4162-F
   
   35V/3.2A Multi-Cell LiFePO4 Step-Down Battery Charger with PowerPath and
   I2C...

 * LTC4162-L
   
   35V/3.2A Multi-Cell Lithium-Ion Step-Down Battery Charger with PowerPath
   and...

 * LTC4162-S
   
   35V/3.2A Lead-Acid Step-Down Battery Charger with PowerPath and I2C Telemetry


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    * Battery Charger IC

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