AN947

AN947
Power Management in Portable Applications: Charging
Lithium-Ion/Lithium-Polymer Batteries
Each application is unique, but one common theme
rings true: maximize battery capacity usage. This
theme directly relates to how energy is properly
restored to rechargeable batteries. While no single
method is ideal for all battery chemistries, an understanding of the charging characteristics of the battery,
along with the application’s requirements, is essential
when designing an appropriate and reliable batterycharging system. Each method has its associated
advantages and disadvantages, with the particular
application (and its individual requirements)
determining the best method to use.
This application note focuses on the fundamentals of
charging Lithium-Ion/Lithium-Polymer batteries. In
particular, a linear, stand-alone solution utilizing
Microchip’s MCP73841 will be explored.
BATTERY OVERVIEW
A battery is a device that converts the chemical energy
contained in its active materials directly into electric
energy by means of an electrochemical oxidationreduction (redox) reaction. This type of reaction
involves the transfer of electrons from one material to
another through an electric circuit. In a non-electrochemical redox reaction, such as rusting or burning, the
transfer of electrons occurs directly and only heat is
involved.
The operation of a battery during discharge is depicted
schematically in Figure 1. When the electrodes (positive and negative terminals of the battery) are connected to an external load, electrons flow from the
anode, which is oxidized, through the external load to
the cathode. The cathode accepts the electrons and
the cathode material is reduced. The electric circuit is
completed in the electrolyte by the flow of anions
(negative ions) and cations (positive ions) to the anode
 2004 Microchip Technology Inc.
Electron Flow
Load
–
+
Flow of Anions
Flow of Cations
Cathode
Powering today’s portable world poses many challenges for system designers. The use of batteries as a
prime power source is on the rise. As a result, a burden
has been placed on the system designer to create
sophisticated systems utilizing the battery’s full
potential.
Electrolyte
FIGURE 1:
Discharge of a Battery.
When recharging a battery, the current flow is reversed,
with oxidation occurring at the positive electrode and
reduction at the negative electrode. As the anode is, by
definition, the electrode at which oxidation occurs and
the cathode where reduction occurs, the positive electrode is now the anode and the negative electrode is
the cathode. Refer to Figure 2.
Electron Flow
–
–
DC
Supply +
Flow of Anions
Flow of Cations
+
Anode
INTRODUCTION
and cathode, respectively. By definition, the cathode
(oxidizing electrode) is the electrode that accepts
electrons from the external circuit and is reduced during the electrochemical reaction. The anode (reducing
electrode) is the electrode which gives up electrons to
the external circuit and is oxidized during the electrochemical reaction. The electrolyte (ionic conductor)
provides the medium for transfer of charge, as ions,
inside the battery between the anode and cathode.
Anode
Scott Dearborn
Microchip Technology Inc.
Cathode
Author:
Electrolyte
FIGURE 2:
Charge of a Battery.
DS00947A-page 1
AN947
The standard potential of a battery is determined by the
type of active materials contained in the battery. It can
be calculated from free-energy data or obtained experimentally. The standard potential of a battery can be
calculated from the standard electrode potentials as
follows (the oxidation potential is the negative value of
the reduction potential):
CATHODE
CURRENT
COLLECTOR
CONTAINER,
TERMINALS,
SEALS,
ETC.
ANODE
CURRENT
COLLECTOR
ELECTRO
LYTE
Anode (oxidation potential)
+ Cathode (reduction potential)
Standard Potential
CATHODE
MATERIAL
ANODE
MATERIAL
For example, in a NiCd battery:
SEPARATOR
Cathode:
2NiOOH + 2H2O + 2e ---- 2Ni(OH)2 + 2OH= 0.52V
Anode:
Cd + 2OH- ---- Cd(OH)2 + 2e = -0.81V
Standard Potential:
0.52 – (-0.81) = 1.33V
The theoretical capacity of a battery is determined by
the amount of active materials in the battery. It is
expressed as the total quantity of electricity involved in
the electrochemical reaction and is defined in terms of
coulombs (C) or ampere-hours (Ah). The ampere-hour
capacity of a battery is directly associated with the
quantity of electricity obtained from the active
materials. Theoretically, 1 gram equivalent weight of
material will deliver 96,487C or 26.8 Ah. (A gram
equivalent weight is the atomic or molecular weight of
the active material in grams divided by the number of
electrons involved in the reaction.) The capacity of a
battery can also be considered on an energy (watthour)
basis by taking both the voltage and the quantity of
electricity into consideration. This theoretical energy
value is the maximum value that can be delivered by a
specific electrochemical system.
The maximum energy that can be delivered by an
electrochemical system is based on the types of active
materials that are used (this determines the voltage)
and the amount of active materials that are used (this
determines the ampere-hour capacity). In practice,
only a fraction of the theoretical energy of the battery is
realized. This is due to the need for the electrolyte and
non-reactive components (containers, separators,
seals, etc.) that add to the weight and volume of the
battery.
DS00947A-page 2
FIGURE 3:
Components of a Battery.
The weight of the materials of construction reduces the
theoretical energy density of the battery by almost
50%, with the actual energy delivered by a practical
battery (even when discharged under conditions close
to optimum) possibly being 50% to 75% of that lowered
value.Thus, the actual energy available from a battery
under practical discharge conditions is only about 25%
to 35% of the theoretical energy of the active materials.
Referring to Figure 4, the theoretical voltage of a
battery, as defined previously, is equivalent to the opencircuit voltage of a fully charged battery. When an
electrical circuit is connected around the battery, with
current being drawn from the battery, the closed-circuit
voltage potential will be lower than the open-circuit
voltage. This is due to two factors:
1.
2.
The electrodes have “real” impedance.
The rate at which current can be drawn from the
battery is restricted by the rate at which the
chemical reaction occurs. This looks like “resistance” to the electrical circuit and can be
modeled as resistance in series with the
cathode.
The nominal voltage is the voltage at the “plateau” of
the discharge curve. For NiCd and NiMH batteries, the
nominal voltage is 1.2V. For a Lithium-Ion battery, the
nominal voltage is 3.6V. The end voltage is defined by
the system and is the potential at which the system no
longer draws current from the battery. The discharge
cut-off voltage is a secondary safety potential, below
which the battery can experience irrepairable damage.
 2004 Microchip Technology Inc.
AN947
Open-Circuit Voltage
Voltage (V)
Closed-Circuit Voltage
Working
Voltage
End Voltage
Discharge Cut-off Voltage
Unused Capacity
Time (Hrs)
FIGURE 4:
Battery Voltage Definitions.
When charging or discharging, the rate of charge or
discharge is often expressed in relation to the capacity
of the battery. This rate is known as the C-rate. The Crate equates to a charge or discharge current and is
defined as:
I = M × Cn
Where:
I =
M=
C=
n =
charge or discharge current, A
multiple or fraction of C
numerical value of rated capacity, Ah
time in hours at which C is declared
A battery discharging at a C-rate of 1 will deliver its
nominally-rated capacity in one hour. For example, if
the rated capacity is 1000 mAh, a discharge rate of 1C
corresponds to a discharge current of 1000 mA. A rate
of C/10 corresponds to a discharge current of 100 mA.
Typically, manufacturers specify the capacity of a battery at 5 hour rate, n = 5. For example, the above mentioned battery would provide 5 hours of operating time
when discharged at a constant current of 200 mA. In
theory, the battery would provide 1 hour of operating
time when discharged at a constant current of
1000 mA. In practice, however, the operating time will
be less than 1 hour due to inefficiencies in the
discharge cycle.
BATTERY TYPES
Batteries can be divided into two main categories: primary cells and secondary cells. Table 1 gives
examples of primary and secondary cells.
TABLE 1:
BATTERY TYPES
Primary Cells
Secondary Cells
Zinc Carbon
Sealed Lead Acid
Alkaline
Nickel Cadmium
Lithium
Nickel Metal-Hydride
Lithium-Ion
Primary cells produce an irreversible chemical
reaction. Zinc Carbon batteries were the first introduced. The carbon was later purified to increase the
energy capacity. These cells are more readily known as
Zinc Chloride. Alkaline batteries are commonly found
on store shelves and are widely used in disposable
applications. Silver coin cell or button cell batteries are
lithium batteries comprised of lithium metal and, since
their chemical reaction is irreversible, are categorized
as primary cells. Primary cells generally do not need
built-in intelligence. Their disposable nature means
that there is no need for recharge control, protection
circuitry or “fuel” gauging.
Secondary cells are rechargeable by passing a current
through them in the direction opposite to that of its
discharge and reversing the chemical reaction. The
most common forms of secondary cells include Sealed
Lead Acid, Nickel Cadmium, Nickel Metal-Hydride,
Lithium-Ion and Lithium-Polymer. Lead Acid batteries
are typically used in automotive applications or fixed
installations because of their large size and weight. Our
focus will be discussing Lithium-Ion. These batteries
have been emerging as the dominate chemistry in the
portable market place.
LITHIUM-ION BATTERIES
Lithium-Ion batteries are comprised of cells that employ
lithium intercalation compounds as the positive and
negative materials. The positive electrode material is
typically a metal oxide with either a layered structure
(such as lithium cobalt oxide (LiCoO2)) or a tunneled
structure (such as lithium manganese oxide (LiMn2O4))
on a current-collector of aluminum foil. The negative
electrode material is typically a graphite carbon on a
copper current-collector.
The first Lithium-Ion batteries to be marketed (and the
majority of those currently available) utilize lithium
cobalt oxide as the positive electrode. This material
offers good electrical performance, is easily prepared,
has good safety properties and is relatively insensitive
to process variation and moisture. More recently,
lower-cost (lithium manganese oxide) or higher performance materials, such as lithium nickel cobalt oxide
(LiNiXCo1-XO2), have been introduced, permitting
development of batteries with improved performance.
The first Lithium-Ion batteries employed cells with coke
negative electrode materials. As better quality graphite
became available, the industry shifted to graphite
carbons as negative electrode materials because of
their higher specific capacity, with improved life and
rate capability. Until 1990, NiCd batteries dominated
the portable, rechargeable market. Environmental
concerns led to the development of NiMH and LithiumIon batteries. Lithium is the lightest metal in the periodic
system and features the greatest electrochemical
potential.
Lithium-Polymer
 2004 Microchip Technology Inc.
DS00947A-page 3
AN947
One of the biggest advantages of Lithium-Ion batteries
is their superior energy density by weight and volume.
Additionally, Lithium-Ion batteries do not exhibit the
“memory” effect associated with the nickel-based
batteries. Lithium-Ion batteries are, therefore, low
maintenance. In other words, they do not need to be
cycled periodically in order to maintain capacity. Of the
three main types of portable, rechargeable batteries,
Lithium-Ion exhibits the lowest self-discharge.
A drawback to Lithium-Ion is that it is in its relative
infancy, resulting in higher costs. Also, Lithium-Ion
batteries lose potential capacity even when not in use.
In other words, Lithium-Ion batteries are subject to
aging. Lithium-Ion batteries have a relatively high
internal resistance, excluding them from highdischarge current applications, such as portable power
tools. The high internal resistance is compounded by
the added protection circuitry required by Lithium-Ion
battery packs.
Why do Lithium-Ion battery packs need protection
circuitry? The main reason is consumer safety.
Excessive charging of Lithium-Ion cells can result in
sudden, automatic and rapid disassembly. This is the
Achilles' heel of Lithium-Ion cells. Conversely,
excessive discharging of Lithium-Ion cells can
decompose the anode, causing copper shunts to form.
This causes permanent degradation of cell
performance.
All Lithium-Ion battery packs, including single cells,
employ protection circuitry in order to meet UL1642
and IEC Secondary Lithium Battery Standards. The
protection circuitry consists of added electronics in
series with the cell electrodes. The protection circuitry
is composed of two MOSFETs connected in either a
common drain or common source configuration. In
batteries with 2 or more cells in series, P-channel
MOSFETs are generally employed in series with the
positive electrode. In single-cell batteries, N-channel
MOSFETs are generally employed in series with the
negative electrode. This protection should be viewed
as the second, or last line, of defense. The battery
charger should ensure that this protection is not utilized
during normal operating conditions.
The operating range of a Lithium-Ion battery is between
4.2V and 2.8V. The internal safety protection circuits
are designed to inhibit operation beyond this window,
with the main goal of maintaining consumer safety.
Therefore, protection against over-charge is its primary
function. However, the protection circuitry often
protects against undervoltage and excessive current.
The primary goal of these protections is to maintain
reliability, as opposed to safety. Refer to Figure 5 for a
more detailed description.
5V
Cathode Decomposes:
Oxygen and Heat are Evolved
4.3 - Pack Protection Circuit Opens
4.35V MOSFET (Temporary Disconnect)
4.2V Upper Charge Voltage
Unstable Region
Acceptable
Operating
Range
Over Discharge
Region
Unstable Region
FIGURE 5:
Normal
Operating
Range
2.8V
End Voltage
2.0 2.5V
0V
Pack Protection Circuit Opens
MOSFET (Temporary Disconnect)
Anode Dissolves: Copper Shunts
Form (Permanent Degradation)
Lithium-Ion Voltage Range.
Lithium-Ion Charging Algorithms
So how is energy properly restored to a Lithium-Ion
battery? The preferred charge algorithm for Lithium-Ion
battery chemistries is a constant or controlled current;
constant-voltage algorithm that can be broken up into
three stages: trickle charge, fast or bulk charge and
constant voltage. Refer to Figure 6.
Stage 1: Trickle Charge - Trickle charge is employed
to restore charge to deeply depleted cells. When the
cell voltage is below approximately 2.8V, the cell is
charged with a constant current of 0.1C maximum. An
optional safety timer can be utilized to terminate the
charge if the cell voltage has not risen above the trickle
charge threshold in approximately 1 hour.
Stage 2: Fast Charge - Once the cell voltage has risen
above the trickle charge threshold, the charge current
is raised to perform fast charging. The fast charge
current should be less than 1.0C. In linear chargers, the
current is often ramped-up as the cell voltage rises in
order to minimize heat dissipation in the pass element.
An optional safety timer can be utilized to terminate the
charge if no other termination has been reached in
approximately 1.5 hours from the start of the fast
charge stage (with a fast charge current of 1C).
Stage 3: Constant Voltage - Fast charge ends, and
the Constant Voltage mode is initiated, when the cell
voltage reaches 4.2V. In order to maximize
performance, the voltage regulation tolerance should
be better than ±1%. It is not recommended to continue
to trickle charge Lithium-Ion batteries. Charging is
typically terminated by one of two methods: minimum
charge current or a timer (or a combination of the two).
The minimum current approach monitors the charge
current during the constant voltage stage and
terminates the charge when the charge current
diminishes below approximately 0.07C. The second
method determines when the constant voltage stage is
invoked. Charging continues for an additional two
hours before being terminated.
Charging in this manner replenishes a deeply depleted
battery in roughly 165 minutes.
Advanced chargers employ additional safety features.
For example, charge is suspended if the cell temperature is outside a specified window, typically 0°C to
45°C.
DS00947A-page 4
 2004 Microchip Technology Inc.
AN947
4.50
1200
Cell Voltage
1000
Cell Voltage (V)
3.50
Capacity
3.00
800
2.50
600
2.00
1.50
400
Charge Current
1.00
200
0.50
0.00
Where:
REQ = the resistance of the secondary
winding plus the reflected resistance
of the primary winding (RP/ a2)
RPTC= the resistance of the PTC, and VFD is
the forward drop of the bridge
rectifiers.
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Time (Hours)
Lithium-Ion Charge Profile.
LITHIUM-ION CHARGING
CONSIDERATIONS
In addition, transformer core loss will slightly reduce the
output voltage. Core loss is due to eddy current and
hysteresis losses, and is effected by the area and permeability of the core, as well as the length of the closed
magnetic path in the core.
The unregulated wall cube produces a typical DC
output voltage to the charger, as shown in Figure 8.
A high-performance, fast battery charger is required to
recharge any battery quickly and reliably. The following
system parameters should be considered in order to
ensure a reliable, cost-effective solution.
Input Source
Many applications use very inexpensive wall cubes as
the input supply. Figure 7 depicts the schematic of a
typical unregulated wall cube.
16
14
DC Output Voltage
FIGURE 6:
2 × V IN × a – I O × ( R EQ + RPTC ) – 2 × V FD
VO =
Charge Current / Capacity
(mA / mAh)
4.00
12
132 VAC Input
10
8
120 VAC Input
6
4
90 VAC Input
2
AC
INPUT
DC
OUTPUT
0
0
100
200
300
400
500
Output Current (mA)
N1 N2
C
PTC
a = (N1 / N2)
FIGURE 7:
Unregulated Wall Cube
Due to its unregulated nature, the output voltage is
highly dependent on the AC input voltage and the load
current being drawn by the charger.
In the United States, the AC mains input voltage can
vary from 90VRMS to 132 VRMS for a standard wall
outlet. Assuming a nominal input voltage of 120 VRMS,
the tolerance is +10%, –25%. The charger must provide proper regulation to the battery, independent of its
input voltage. The input voltage to the charger will scale
in accordance with the AC mains’ voltage and charge
current, assuming the output capacitance (C) is
sufficiently large to minimize the voltage ripple:
 2004 Microchip Technology Inc.
FIGURE 8:
Output Voltage.
Unregulated Wall Cube
Applications that charge from a car adapter can experience a similar problem. The pseudo-regulated car
adapter output, depicted in Figure 9, is highly dependent on the input voltage and output current. At light
loads, less than 20 mA, the car adapter operates in a
discontinuous conduction mode and is regulated to
approximately 8.2V. The duty cycle is dependent on the
input voltage, output voltage, inductance and load. At
outputs above 20 mA, the car adapter transitions to a
continuous conduction mode. At this time, the output is
unregulated. The duty cycle goes to the maximum,
50%. This is primarily due to the choice of the timing
capacitor and the operation of the MC33063A. The output is roughly half the voltage seen at the switch emitter
when Q1 is on. The output varies slightly with load current due to losses in the current sense resistor (Q1), the
inductor and both diodes. Larger variations are seen in
the output voltage due to the input voltage. Nominally,
the input voltage is 12V – 12.5V when the car is not running and 14V – 14.5V when powered from the alternator. This corresponds to, roughly, a one volt change in
output voltage.
DS00947A-page 5
AN947
0.27Ω
470Ω
Osc
Transient
Protection
47 uF
60.4
kΩ
220 uF
x2
Ref * 100
pF
+
-
DC
Output
10.7
kΩ
* 1.25V
Reference
FIGURE 9:
Adapter.
Output Voltage Regulation Accuracy
Pseudo-regulated Car
As is, this adapter could not be used in many
applications. A simple modification would allow the output to be regulated to 5V independent of input voltage
or output current (there will be a slight dependence on
output current because the regulation is being
performed at the anode of the diode in series with the
output). The modification can be performed by simply
changing the value of the timing capacitor from 100 pF
to 270 pF and changing the 10.7 kΩ resistor to 18 kΩ.
The pseudo-regulated car adapter produces a DC output voltage as shown in Figure 10. The simple modification mentioned increases the efficiency of the
charging system, decreasing the heat generated in the
handset.
The output voltage regulation accuracy is critical in
order to obtain the desired goal: maximize battery
capacity usage. A small decrease in output voltage
accuracy results in a large decrease in capacity.
However, the output voltage can not be set arbitrarily
high because of safety and reliability concerns.
Figures 11 and 12 depict the importance of output
voltage regulation accuracy.
1020
1000
Capacity (mAh)
S
R
Fuse
Input
Therefore, the tolerance on the fast charge current
regulation becomes extremely important to a linear
system. If the regulation tolerance is loose, pass
transistors and other components will need to be
oversized for a typical situation. In addition, if the fast
charge current is low, the complete charge cycle will be
extended. Refer to the “Design Example” section for
details regarding the effects of fast charge currentregulation tolerance.
980
960
940
920
900
4.20
8.00
7.00
VIN = 14.5V
6.00
VIN = 12.0V
5.00
4.00
VIN = 10.0V
3.00
2.00
Regulated
VIN = 10.0V - 20.0V
1.00
0.00
0
100
200
4.19
FIGURE 11:
Voltage.
4.18
4.17
4.16
Charging Voltage (V)
4.15
Capacity vs. Charging
10%
9%
300
400
500
Output Current (mA)
FIGURE 10:
Pseudo-regulated Car
Adapter Output Voltage.
Capacity Loss (%)
DC Output Voltage (V)
9.00
8%
7%
6%
5%
4%
3%
2%
1%
Fast Charge Current and Accuracy
The choice of topology for a given application may be
determined by the desired fast charge current. Many
high fast charge current or multiple cell applications
rely on a switch-mode charging solution for improved
efficiency and less heat generation.
0%
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
1.2%
Percent Undercharge (%)
FIGURE 12:
Capacity Loss vs.
Undercharge Voltage.
Linear solutions are desirable in low-to-moderate fast
charge current applications for their superior size and
cost considerations. However, a linear solution
purposely dissipates excess power in the form of heat.
DS00947A-page 6
 2004 Microchip Technology Inc.
AN947
Charge Termination Method
Primary and secondary charge termination methods
are essential for reliably charging any battery
chemistry. It can not be stressed enough that overcharging is the Achilles' heel of Lithium-Ion cells. The
primary termination method for Lithium-Ion cells is
determined by monitoring the charge current. After the
constant voltage phase of the charge cycle has been
entered, the charge current tapers off naturally. The
charge cycle is considered complete when the charge
current has diminished below the 0.1C to 0.07C rate. At
this point, the charge cycle is terminated.
Battery Discharge Current or Reverse
Leakage Current
In many applications, the charging system remains
connected to the battery in the absence of input power.
The charging system should minimize the current drain
from the battery when input power is not present. The
maximum current drain should be below a few
microamps or, ideally, below one microamp.
For a redundant secondary safety control, an elapsed
charge timer should be utilized. If the battery does not
reach full charge within a specified time, the charge
cycle should be terminated. Continuing to charge may
cause the battery to become hot, explode or ignite.
Cell Temperature Monitoring
The temperature range over which a Lithium-Ion battery can be charged is 0°C to 45°C, typically. Charging
the battery at temperatures outside of this range may
cause the battery to become hot. During a charge
cycle, the pressure inside the battery increases,
causing the battery to swell. Temperature and pressure
are directly related. As the temperature rises, the
pressure can become excessive. This can lead to a
mechanical breakdown inside the battery or venting.
Charging the battery outside of this temperature range
may also harm the performance of the battery or
reduce the battery’s life expectance.
Generally, thermistors are included in Lithium-Ion
battery packs in order to accurately measure the
battery temperature. The charger measures the
resistance value of the thermistor between the
thermistor terminal and the negative terminal. Charging
is inhibited when the resistance and, therefore, the
temperature, is outside the specified operating range.
 2004 Microchip Technology Inc.
DS00947A-page 7
AN947
DESIGN EXAMPLE
A practical design example will be presented along with
actual charge-cycle waveforms.
The design parameters are given as follows:
Input Source:
5V, +5%
Battery:
Single-cell, Lithium-Ion
Battery Capacity:
1000 mAh
Fast Charge Rate:
1C or 0.5C
Regulation Voltage:
4.2V
Primary Termination:
Imin
Secondary Termination: Timer, 6 hours
Charging Temperature:
0°C to 45°C
Microchip’s MCP73841-420I/MS was chosen as the
preferred charge management controller because it
satisfies all the given design parameters.
CONSTANT CURRENT REGULATION - FAST
CHARGE
Preconditioning ends and fast charging begins when
the battery voltage exceeds the preconditioning threshold. Fast charge regulates to a constant current (IREG )
based on the supply voltage minus the voltage at the
SENSE input (VFCS) developed by the drop across an
external sense resistor (RSENSE). Fast charge continues until either the battery voltage reaches the regulation voltage (VREG ) or the fast charge timer expires; In
which case, a fault is indicated and the charge cycle is
terminated. In this design example, VREG equals 4.2V.
CONSTANT VOLTAGE REGULATION
When the battery voltage reaches the regulation voltage (VREG ), constant voltage regulation begins. The
MCP73841 monitors the battery voltage at the VBAT
pin. This input is tied directly to the positive terminal of
the battery.
Device Overview
CHARGE CYCLE COMPLETION AND
AUTOMATIC RE-CHARGE
The MCP73841 is a highly advanced, linear chargemanagement controller. The MCP73841 utilizes an
external pass transistor (MOSFET), thereby allowing
great design flexibility and higher power (charging)
levels. Figure 13 depicts the operational flow algorithm
from charge initiation to completion and automatic
recharge.
The MCP73841 monitors the charging current during
the constant voltage regulation phase. The charge
cycle is considered complete when the charge current
has diminished below approximately 7% of the
regulation current (IREG ) or the elapsed timer has
expired.
CHARGE QUALIFICATION AND
PRECONDITIONING
Upon insertion of a battery or application of an external
supply, the MCP73841 automatically performs a series
of safety checks to qualify the charge. The input source
voltage must be above the undervoltage lockout
threshold, the Enable pin must be above the logic-high
level, with the cell temperature monitor being within the
upper and lower thresholds. These qualification parameters are continuously monitored, with any deviation
beyond these limits automatically suspending or
terminating the charge cycle.
Once the qualification parameters have been met, the
MCP73841 initiates a charge cycle. The charge status
output is pulled low throughout the charge cycle (see
Table 2 for charge status outputs). If the battery voltage
is below the preconditioning threshold (VPTH), the
MCP73841 preconditions the battery with a tricklecharge. The preconditioning current is set to approximately 10% of the fast charge regulation current. The
preconditioning trickle-charge safely replenishes
deeply depleted cells and minimizes heat dissipation in
the external pass transistor during the initial charge
cycle. If the battery voltage has not exceeded the preconditioning threshold before the preconditioning timer
has expired, a fault is indicated and the charge cycle is
terminated.
DS00947A-page 8
The MCP73841 automatically begins a new charge
cycle when the battery voltage falls below the recharge
threshold (VRTH) assuming all the qualification
parameters are met.
CHARGE STATUS OUTPUT
A status output provides information on the state of the
charge. The current-limited, open-drain output can be
used to illuminate an external LED. Table 2
summarizes the state of the output during a charge
cycle.
TABLE 2:
STATUS OUTPUTS
Charge Cycle State
Qualification
STAT1
Off
Preconditioning
On
Constant Current Fast
Charge
On
Constant Voltage
On
Charge Complete
Off
Safety Timer Fault
Flashing *
(1Hz, 50% duty cycle)
Cell Temperature Invalid
Flashing *
(1Hz, 50% duty cycle)
Disabled - Sleep mode
Off
Battery Disconnected
Off
* The flashing rate (1 Hz) is based off a timer
capacitor (CTIMER) of 0.1 µF. The rate will vary
depending on the value of the timer capacitor.
 2004 Microchip Technology Inc.
 2004 Microchip Technology Inc.
Initialize
EN High
No
STAT1 = Off
Yes
Temperature OK
Yes
VDD > VUVLO
VBAT < VOVSTOP
No
STAT1 = Flashing
Safety Timer Suspended
Charge Current = 0
No
STAT1 = Off
Yes
Preconditioning Phase
Charge Current = IPREG
Reset Safety Timer
No
VBAT > VPTH
STAT1 = On
Yes
Constant Current Phase
Charge Current = IREG
Yes
VBAT > VPTH
Constant Voltage Phase
Output Voltage = VREG
Reset Safety Timer
No
Yes
Fault
Charge Current = 0
Reset Safety Timer
Yes
No
MCP73841 Flow Algorithm.
Yes
No
Yes STAT1=Flashing
Charge Termination
Charge Current = 0
Reset Safety Timer
Yes
VBAT > VOVSTRT
Safety Timer
Expired
VDD < VUVLO
VBAT < VRTH
or EN Low
No
No
Yes STAT1 = Off
Yes
Temperature OK
No
STAT1 = Flashing
Safety Timer Suspended
Charge Current = 0
Temperature OK
No
STAT1 = Flashing
Safety Timer Suspended
Charge Current = 0
AN947
DS00947A-page 9
VDD< VUVLO
Cell Removal
or EN Low
No
STAT1 = Flashing
Safety Timer Suspended
Charge Current = 0
FIGURE 13:
VBAT > VOVSTRT
Safety Timer
Expired
No
Temperature OK
Yes
No
No
Safety Timer Expired
Yes
IOUT < ITERM
Elapsed Timer
Expired
Yes
VBAT = VREG
AN947
Circuit Design
The current sense resistor, R1, is calculated by:
Figure 14 illustrates the design circuit.
EQUATION
VFCS
R 1 = -----------I REG
Due to the low efficiency of linear charging, the most
important factors are thermal design and cost, which are
a direct function of the input voltage, output current and
thermal impedance between the external P-channel
pass transistor and the ambient cooling air. The worstcase situation is when the device has transitioned from
the preconditioning phase to the constant current phase.
In this situation, the P-channel pass transistor has to
dissipate the maximum power. A trade-off must be made
between the charge current, cost and thermal
requirements of the charger.
Where:
IREG is the desired fast charge current
A standard value 110 mΩ, 1% resistor provides a
typical fast charge current of 1000 mA and a maximum
fast charge current of 1091 mA. Worst-case power
dissipation in the sense resistor is:
EQUATION
Component Selection
2
P R1 = 110mΩ × 1091mA = 131mW
Selection of the external components is crucial to the
integrity and reliability of the charging system. The
following discussion is intended to be a guide for the
component selection process.
Two Panasonic® ERJ-3RQFR22V 220 mΩ, 1%, 1/8W
resistors in parallel are more than sufficient for this
application.
A larger-value sense resistor will decrease the fast
charge current and power dissipation in both the sense
resistor and external pass transistor, but will increase
charge cycle times. Design trade-offs must be considered to minimize space while maintaining the desired
performance. In this design example, fast charge rates
of 1C and 0.5C have been compared. For a charge rate
of 0.5C, one of the paralleled resistors was removed.
SENSE RESISTOR
The preferred fast charge current for Lithium-Ion cells
is at the 1C rate, with an absolute maximum current at
the 2C rate. For this design example, the 1000 mAh
battery pack has a preferred fast charge current of
1000 mA. Charging at this rate provides the shortest
charge cycle times without degradation to the battery
pack performance or life.
Voltage
Regulated
Wall Cube
Optional
Reverse
Blocking
Diode
R1
2 x 220 mΩ @ 1C
1 x 220 mΩ @ 0.5C
Q1
4.7 µF
4.7µF
SENSE
VDD
STAT1
EN
RT1
THREF
1
10
2
9
3
MCP73841
8
4
7
5
6
+
-
DRV
VBAT
VSS
0.22 µF
TIMER
THERM
10.5 kΩ
RT2
15.0 kΩ
FIGURE 14:
DS00947A-page 10
Battery
Pack
Design Circuit.
 2004 Microchip Technology Inc.
AN947
EXTERNAL PASS TRANSISTOR
The worst-case VGS is:
The external P-channel MOSFET is determined by the
gate-to-source threshold voltage, input voltage, output
voltage and fast charge current. The selected P-channel
MOSFET must satisfy the thermal and electrical design
requirements.
EQUATION
Thermal Considerations
The worst-case power dissipation in the external pass
transistor occurs when the input voltage is at the maximum and the device has transitioned from the preconditioning phase to the constant current phase. In this
case, the power dissipation is:
EQUATION
PQ1 = ( V DDMAX – ( VPTHMIN + V FCS ) ) × I REGMAX
Where:
VDDMAX is the maximum input voltage
IREGMAX is the maximum fast charge current
VPTHMIN is the minimum transition threshold
voltage
Maximum power dissipation in this design example
occurs when charging at the 1C rate.
EQUATION
PQ1 = ( 5.25V – ( 2.85V + 0.120 ) ) × 1091mA = 2.48W
Utilizing a Fairchild® NDS8434 or an International
Rectifier IRF7404 mounted on a 1 in2 pad of 2 oz.
copper, the junction temperature rise is 99°C, approximately. This would allow for a maximum operating
ambient temperature of 51°C.
By increasing the size of the copper pad, either a higher
ambient temperature or a lower-value sense resistor
could be utilized.
Alternatively, different package options can be utilized
for more or less power dissipation. Again, design tradeoffs should be considered to minimize size while
maintaining the desired performance.
Electrical Considerations
The gate-to-source threshold voltage and RDSON of the
external P-channel MOSFET must be considered in the
design phase.
The worst-case VGS provided by the controller occurs
when the input voltage is at the minimum and the fast
charge current regulation threshold is at the maximum.
 2004 Microchip Technology Inc.
V GS = V DRVMAX – ( V DDMIN – VFCSMAX )
Where:
VDRVMAX is the maximum sink voltage at the
VDRV output
VDDMIN is the minimum input voltage source
VFCSMAX is the maximum fast charge current
regulation threshold
Worst-case VGS with a 5V, ±5% input voltage source
and a maximum sink voltage of 1.0V is:
EQUATION
VGS = 1.0V – ( 4.75V – 120mV ) = – 3.63V
At this worst-case VGS, the RDSON of the MOSFET
must be low enough as to not impede the performance
of the charging system. The maximum allowable
RDSON at the worst-case VGS is:
EQUATION
VDDMIN – V FCSMAX – V BATMAX
RDSON = ------------------------------------------------------------------------------I REGMAX
4.75V – 120mV – 4.221V
R DSON = ------------------------------------------------------------- = 375mΩ
1091mA
The Fairchild NDS8434 and International Rectifier
IRF7404 both satisfy these requirements.
External Capacitors
The MCP73841 is stable with or without a battery load.
In order to maintain good AC stability in the Constant
Voltage mode, a minimum capacitance of 4.7 µF is
recommended to bypass the VBAT pin to VSS. This
capacitance provides compensation when there is no
battery load. In addition, the battery and interconnections appear inductive at high frequencies. These
elements are in the control feedback loop during
Constant Voltage mode. Therefore, the bypass capacitance may be necessary to compensate for the inductive nature of the battery pack.
Virtually any good quality output filter capacitor can be
used, independent of the capacitor’s minimum Effective Series Resistance (ESR) value. The actual value of
the capacitor and its associated ESR depends on the
forward transconductance, gm, and capacitance of the
external pass transistor. A 4.7 µF ceramic, tantalum, or
aluminum electrolytic capacitor at the output is usually
sufficient to ensure stability for up to a 1A output
current.
DS00947A-page 11
AN947
REVERSE-BLOCKING PROTECTION
The optional reverse-blocking protection diode,
illustrated in Figure 14, provides protection from a
faulted or shorted input, or from a reversed-polarity
input source. Without the protection diode, a faulted or
shorted input would discharge the battery pack through
the body diode of the external pass transistor.
If a reverse protection diode is incorporated in the
design, it should be chosen to handle the fast charge
current continuously at the maximum ambient temperature. In addition, the reverse-leakage current of the
diode should be kept as small as possible. A
Panasonic® MA2YD100L, 1.5A, 15V, Schottky diode
has been chosen. The forward voltage drop is 350 mV
at 1A, which is important when determining the
maximum allowable RDSON of the pass transistor. The
reverse leakage of this diode is outstanding. With a
reverse voltage of 4.0V, the leakage is less than 1 µA.
ENABLE INTERFACE
In the stand-alone configuration, the enable pin is generally tied to the input voltage. The MCP73841 automatically enters a low power mode when voltage on the
VDD input falls below the undervoltage lockout voltage
(VSTOP) reducing the battery drain current to 0.23 µA,
typically.
CHARGE STATUS INTERFACE
A status output provides information on the state-ofcharge. The current-limited, open-drain output can be
used to illuminate an external LED. Refer to Table 2 for
a summary of the state of the status output during a
charge cycle.
CELL TEMPERATURE MONITORING
The MCP73841 continuously monitors temperature by
comparing the voltage between the THERM input and
VSS with the upper and lower comparator thresholds. A
negative or positive temperature coefficient, NTC or
PTC thermistor and an external voltage divider typically
develop this voltage. The temperature-sensing circuit
has its own reference to which it performs a ratio metric
comparison. Therefore, it is immune to fluctuations in
the supply input (VDD). The temperature-sensing circuit
is removed from the system when VDD is not applied,
eliminating additional discharge of the battery pack.
The design example specifies a charging temperature
of 0°C to 45°C. A NTC thermistor with a 25°C resistance of 10 kΩ and a sensitivity index (β) of 3982 is
inside the battery pack. The thermistor has a resistance
of 33.56 kΩ at 0°C and 4.52 kΩ at 45°C. The values for
resistors RT1 and RT2 are calculated with the following
equations.
DS00947A-page 12
For NTC thermistors:
2 × RCOLD × RHOT
R T1 = ---------------------------------------------RCOLD – RHOT
2 × RCOLD × RHOT
R T2 = ---------------------------------------------RCOLD – 3 × RHOT
Where:
RCOLD and RHOT are the thermistor
resistance values at the temperature window
of interest
The calculated values for RT1 and RT2 are 10.44 kΩ
and 15.17 kΩ, respectively. Standard values of
10.5 kΩ and 15.0 kΩ provide a temperature window
within 1°C of that desired.
SAFETY TIMER
The TIMER input programs the period of the safety
timers by placing a timing capacitor (CTIMER) between
the TIMER input pin and VSS. Three safety timers are
programmed via the timing capacitor.
The preconditioning safety timer period:
EQUATION
C TIMER
tPRECON = ------------------- × 1.0Hour s
0.1µF
The fast charge safety timer period:
EQUATION
C TIMER
t FAST = ------------------- × 1.5Hours
0.1µF
And, the elapsed time termination period:
EQUATION
C TIMER
t TERM = ------------------- × 3.0Hours
0.1µF
The preconditioning timer starts after qualification and
resets when the charge cycle transitions from preconditioning to the fast charge phase. The fast charge and
elapsed timers start once the MCP73841 transitions
from preconditioning. The fast charge timer resets
when the charge cycle transitions to the constant
voltage phase. The elapsed timer will expire and terminate the charge if the sensed current does not diminish
below the termination threshold.
The design example specifies a charge termination
time of six hours. A standard value 0.22 µF ceramic
capacitor has been chosen.
 2004 Microchip Technology Inc.
AN947
Charge Cycle Waveforms
CONCLUSION
Figure 15 depicts complete charge cycles utilizing the
MCP73841 with fast charge currents at the 1C and
0.5C rates. When charging at a rate of 0.5C instead of
1C, the same steps are performed. It takes about one
hour longer to reach the end of charge. The MCP73841
scales the charge termination current proportionately
with the fast charge current. The result is an increase
of 36% in charge time, with the benefit of a 2% gain in
capacity and reduced power dissipation. The change in
termination current from 0.07C to 0.035C results in an
increase in final capacity from ~98% to ~100%. The
system designer has to make a trade-off between
charge time, power dissipation and available capacity.
Properly restoring energy using the latest battery technology for today’s portable products requires careful
consideration. An understanding of the charging characteristics of the battery, as well as the application’s
requirements is essential in order to design an
appropriate and reliable battery-charging system.
A stand-alone linear charging solution for Lithium-Ion/
Lithium-Polymer batteries was presented. The guidelines and considerations presented herein should be
taken into account when developing any battery
charging system.
REFERENCES
.
1.
2.
3.
4.
5.
6.
“Handbook of Batteries, Third Edition”, David
Linden, Thomas B. Reddy, (New York: McGrawHill, Inc., 2002)
MCP73841/2/34 Data Sheet, “Advanced Single
or Dual Cell Lithium-Ion/Lithium-Polymer
Charge Management Controllers”, DS21823,
Microchip Technology Inc., 2004
ADN008: “Charging Simplified for HighCapacity Batteries”, DS21864, Bonnie Baker,
Microchip Technology Inc., 2004
Http://www.powercellkorea.com
Http://sanyo.com/batteries/lithium_ion.cfm
Http://lgchem.com/en_products/electromaterial/
battery/battery.html
VBAT @ 1C
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
1.20
1.00
VBAT @ 0.5C
0.80
IBAT = 1C
0.60
0.40
IBAT = 0.5C
Current (Amps)
Voltage (Volts)
MCP73841 Charge Cycles, 1000 mAh Battery
0.20
0.0
50.0
100.0
150.0
0.00
200.0
Time (minutes)
FIGURE 15:
MCP73841 Charge Cycle Waveforms.
 2004 Microchip Technology Inc.
DS00947A-page 13
AN947
NOTES:
DS00947A-page 14
 2004 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
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with express written approval by Microchip. No licenses are
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property rights.
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 2004 Microchip Technology Inc.
DS00947A-page 15
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DS00947A-page 16
 2004 Microchip Technology Inc.