AN1088

AN1088
Selecting the Right Battery System For Cost-Sensitive
Portable Applications While Maintaining Excellent Quality
INTRODUCTION
Portable electronic devices have played an important
role in a person’s daily digital life and have changed the
way people live and work. Commonly seen portable
electronic devices are Cellular Phone, Media Players,
Digital Camera, Digital Camcorder, Handheld GPS,
Digital Reader and PDA. With the emerging technologies that are available today, portable electronic
designers are trying to integrate more features into
thinner and smaller form-factors while maximizing the
battery life.
Batteries are the main power source for portable
electronic devices, and selecting a right battery system
for an unique application is one of the important factors
in the portable electronic design process. It involves
selecting a battery chemistry and charge management
control circuitry. The battery life indicates the length a
product can be used under portable mode. Longer
battery life can simply make a portable device standout
in the market automatically. This can usually be
achieved by reducing system power consumption and
implementing an advanced battery technology.
When it comes to production, reliability, safety, low-cost
and easy installation are the important elements while
maintaining good quality. Each battery chemistry has
its advantage over another. This application note is
intended to assist portable electronic product designers
and engineers in selecting the right chemistry for
today’s low cost portable applications with design
simplicity. The solutions are ideal for use in space-limited and cost-sensitive applications that can also
accelerate the product time-to-market rate.
DESCRIPTION
BATTERY CHEMISTRIES
There are three key attributes in a battery:
1.
2.
3.
Energy Density (Size & Weight)
Charge/Discharge Cycles (Life Cycle)
Capacity (Operational duration without AC
Adapter presence)
Like the most engineering works, the key attributes do
not exist in the same technology. There is always a
trade-off between them. In today’s portable world, the
product life cycle is very short. Thus, the battery life
cycle is a minimal concern for customers and manufacturers. The operating duration, package size and overall system weight become the most important factors
when selecting the battery chemistry for a portable
application.
TABLE 1:
Chemistry
Alkaline
BATTERY COMPARISONS 1 [8]
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Vo pe
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Vo
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)
lta ge
ge
(V
)
Brian Chu
Microchip Technology Inc.
145
400
1.2
1.6
0.9
NA
SLA
30-40
50-80
2.0
2.25
1.75
2.8
NiCd
40-80 100-150
1.2
1.3
0.9
1.6
NiMH
60-100 160-230
1.2
1.3
0.9
1.5
Li-Ion
110-130 210-320
3.6
4.2
2.8
4.2
TABLE 2:
Chemistry
BATTERY COMPARISONS 2 [8]
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p e e lfr M Di
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(% g e
R ter
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(m ge
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Author:
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Low
This application note shows characteristics of some
popular battery chemistries for portable applications
and fully integrated low cost single-cell Lithium-Ion/
Lithium Polymer battery charge management
solutions.
Alkaline
0.3
SLA
2-8
NiCd
15-20
3.5-300
1500
<10C -20-+60 Low
References to documents that treat these subjects in
more depth and breadth have been included in the
“Reference” section.
NiMH
20-25
10-400
800
<3C
Li-Ion
6-10
50-500
1000
<2C -20-+60 High
© 2007 Microchip Technology Inc.
100-300
1
0.25C -20-+55
2.5-25 50-500 <15C -20-+50 Low
0-+60
Med
DS01088A-page 1
AN1088
Batteries usually occupy a considerable space and
weight in today’s portable devices. The energy density
for each chemistry dominates the size and weight for
the battery pack. Table 1 indicates that Li-Ion (LithiumIon) has advantages in both energy density weight and
energy density volume among other available battery
technologies.
Each battery chemistry is briefly reviewed below:
Alkaline
Alkaline batteries are not rechargeable, but are
commonly seen as a portable power source because
it’s low self-discharge rate and always ready to use off
the shelf. Therefore, it is included in the Table 1 and
Table 2 as reference against secondary (rechargeable)
batteries. Rechargeable Alkaline batteries are
available, but they are not very practical and reliable to
use in a system due to its fast degradation after a few
charge cycles.
SLA (Sealed Lead Acid)
SLA batteries are mature and inexpensive battery
solutions, and have an advantage in low self discharge
rate. However, it is not an ideal candidate for portable
applications due to it’s low energy density, low charge/
discharge cycles and it is not environmentally friendly.
NiCd (Nickel-Cadmium)
NiCd batteries have the best charge/discharge cycles
among rechargeable batteries (Table 1) and are good
substitutes to Alkaline batteries because they employ
the same basic voltage profile. NiCd batteries are
required to be exercised periodically due to the
memory effect. It is a very low-cost rechargeable
solution because of the matured battery technology
and simple charge algorithm.
NiMH (Nickel-Metal Hydride)
NiMH batteries are considered improved version of
NiCd batteries that provide higher energy density and
environmentally friendly material. Both NiMH and NiCd
batteries have high self discharge rate (Table 2) and
are subject to memory effect. Although NiMH and NiCd
batteries share similar charge algorithm, NiMH
batteries require a more complex design due to the
heat that NiMH batteries generate during charging and
the difficult −ΔV/Δt detection.
Li-Ion (Lithium-Ion)
Li-Ion batteries have advantages in high energy density, low maintenance requirement, relatively low self
discharge rate, and higher voltage per cell. (Table 1
and Table 2) The major drawbacks of Li-Ion batteries
are higher initial cost and aging effect. Li-Ion batteries
age over time regardless of the usage. Protection
DS01088A-page 2
circuitry is required for Li-Ion battery to prevent over
voltage during charge cycle and under voltage during
discharge cycle.
Li-Polymer (Lithium Polymer)
Li-Polymer batteries should be recognized as Li-Ion
Polymer batteries. It is designed as an improved
version of Li-Ion with flexible form-factors and very low
profile. It is perfect for miniature applications, such as
Bluetooth headsets or MP3 players. It has similar
characteristics as Li-Ion and can be charged with same
algorithm. It is a different technology compared to LiIon, but will be discussed as Li-Ion in this application
note.
SELECTING THE RIGHT BATTERY
SYSTEM FOR COST-SENSITIVE
APPLICATIONS
In some high-end portable devices, the performances
and compactness of batteries are the most important
attributes when designers select the right battery
system. Performances include battery run time,
charge/discharge cycles, self discharge rate and
safety. Battery run time, weight and compactness are
based on the energy density and cell capacity.
Most recent portable electronic devices are cost-sensitive with fashion in design. Even high-end devices will
face lower cost during a manufacture cycle. Selecting
the right battery system that can satisfy manufacturers
and customers becomes a nightmare for designers and
engineers. The battery system includes a battery pack
and a charge management controller. With highly
integrated charge management controller and design
simplicity, the portable electronic device designers can
reduce design time and speed up time to market for
new product development.
Based on the discussions above, NiMH and Li-Ion are
the most popular battery chemistries that meet today’s
portable applications.
NiMH or Li-Ion?
Table 3 depicts the critical metrics between Li-Ion and
NiMH.
TABLE 3:
CRITICAL METRICS
Li-Ion
NiMH
Nominal Voltage
3.6V
1.2V
Cycle Life
1000
800
Memory Effect
No
Yes
Cost ($/Wh)[4]
2.5
1.3
Energy Density:
Volume (Wh/L)
210-320
160-230
Energy Density:
Weight (Wh/kg)
110-130
60-100
© 2007 Microchip Technology Inc.
AN1088
Besides the cost, the Li-Ion batteries have significant
advantages over the NiMH batteries. The 3.6V nominal
voltage also makes Li-Ion a perfect supply voltage to
most portable devices. Cell balancing can be an
important issue when more than one battery cell is
required for the system. For NiMH batteries to supply
3.6V, 3-cell NiMH is usually needed to maintain the
voltage. A single-cell Li-Ion battery supplies the same
voltage while taking less space and without worrying
about cell balancing.
No memory effect and maintenance free (e.g. no power
cycling to prolong the battery’s life) also drive Li-Ion as
a good candidate for portable applications. Although,
NiMH has improved the memory effect issue compared
to NiCd, it still could have premature termination from
deceptive peaks during early charge cycle. Premature
termination ends charge before a battery is fully
charged. Consumers can charge Li-Ion battery
operated handheld devices at any time during normal
operation because the memory effect is not an issue
with Li-Ion batteries.
Mass production and extensive R&D from battery
manufacturers have scaled down the cost between
NiMH and Li-Ion batteries. This has led many portable
device designers/engineers to favor Li-Ion over NiMH
in many portable applications.
Charge Algorithm
Appropriate Charge Algorithm for the selected battery
chemistry can effect the life, reliability and safety of a
battery. Different chemistries have different charge profiles and different battery manufacturers have different
recommendations when it comes to restoring energy
(charge) back to batteries.
The C-rate is the rated capacity for battery charge/discharge current. The rated capacity for a battery is the
total amount of current it can produce or store. For
example, 1C charge rate for a battery rated at 500 mAh
is approximately 500 mA per hour.
CHARGING NIMH BATTERIES
Charging NiMH batteries can be simple or complicated.
The simple and low cost solution is to charge batteries
at a low constant current (e.g. 0.1C or 0.2C). However,
it takes a long time to completely charge and can easily
overcharge the NiMH batteries. A timer is usually
implemented for charge termination. Minimum 10
hours is required if a battery is charged at 0.1C. Overcharge may occur without proper end of charge
detection and can reduce the life of batteries (charge/
discharge cycles).
−ΔV/Δt (the rate of voltage decrease) charge
termination has improved the charge algorithm and
allows fast charge until charge termination is reached.
False voltage drop termination can happen from
voltage fluctuations and noise that are caused by the
charger and the battery.
© 2007 Microchip Technology Inc.
−ΔT/Δt (the rate of temperature decrease) charge
termination may increase the design cost, but can
increase the battery life cycle.
To improve the battery life and maintain capacity, a
combination of all methods should be applied to the
charge algorithm. Figure 1 depicts the complete NiMH
charge algorithm.
CHARGE NIMH BATTERIES
Trickle
Charge
Fast
Charge
-ΔV
0.8V
Battery
Voltage
0
Charge
Current
Charge
Termination
1.0C
0.2C
0.05C
0
ΔT
Δt
Battery
Temperature
0
Time
FIGURE 1:
NiMH Charge Algorithm [8].
Stage 1: Trickle Charge - NiMH charge algorithm
starts restoring energy to battery cell at 0.1C or 0.2C
trickle charge until the battery reaches the minimum
working voltage for fast charge. It can be either 0.8V or
0.9V per cell.
Stage 2: Fast Charge - Fast charge restores the battery cell at a constant current rate of 1C. The charge
efficiency has a noticeable improvement at fast charge
rate compare to slow charging rate. It will continuously
charge at 1C until one of the termination requirements
is satisfied.
Stage 3: Charge Termination - The charge cycle goes
to the termination stage when either −ΔV/Δt or −ΔT/Δt
is detected. A duration of small charge current
(~0.05C) can fill up the battery cell to maximum
capacity.
Integrated solutions are available to charge NiMH
batteries, but the cost is usually high and may not be
very flexible to set battery voltage, −ΔV/Δt, −ΔT/Δt,
charge rate and timer.
With the broad range of Microchip’s PIC® microcontroller product line, the microcontroller can be sized for the
job. In many applications, a microcontroller is already
resident. By adding the Microchip’s analog high-speed
PWM (Pulse Width Modulator) MCP1630 family, a
power train can be easily added to the design. [6] The
cost of using this solution is relatively low and can
easily program all parameters compared to the total
integrated solutions.
DS01088A-page 3
AN1088
CHARGING LI-ION BATTERIES
Unlike NiMH, the preferred charge algorithm for Lithium-Ion / Lithium-Ion Polymer batteries is a CC-CV
(constant or controlled current; constant voltage) algorithm that can be broken up into four stages. Figure 2
depicts this charge algorithm.
CHARGE LI-ION BATTERIES
Trickle
Charge
Fast
Charge
4.2V
Battery
Voltage
Constant
Voltage
Charge
Charge
Termination
4.2V
2.8V
0
Charge
Current
1.0C
0.1C
0.07C
0
0
Time
Li-Ion Charge Algorithm [8].
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 not be more than 1.0C. 1.0C is used in
this example. 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 capacity,
the voltage regulation tolerance should be better than
±1%.
Stage 4: Charge Termination - 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. It is not recommended to continue to trickle
charge Lithium-Ion batteries.
DS01088A-page 4
When the cost between NiMH and Li-Ion batteries is no
longer an issue, the only concern remaining is the cost
to implement a charging circuit to portable devices.
Advanced semiconductor technology makes it possible
to provide fully integrated Li-Ion / Li-Polymer battery
charge management controller in one small package
with a completive price.
After detailed review and consideration between NiMH
and Li-Ion, the Li-Ion battery system is the most reliable
solution that is chosen for the low cost portable
devices.
LI-ION / LI-POLYMER CHARGE
MANAGEMENT SOLUTIONS
Battery
Temperature
FIGURE 2:
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. [7] [10]
Two complete Li-Ion / Li-Polymer battery charge
management design examples that utilize Microchip’s
MCP73831 and MCP73812 are proposed for designing
a new low-cost portable devices or the cost of an
alternative for an existing product.
Example 1: Design Low-Cost Li-ion / LiPolymer Battery Charge Management
With MCP73831 [10]
DEVICE OVERVIEW
The MCP73831 device is a highly advanced linear
charge management controller for use in space-limited
and cost-sensitive applications. The MCP73831 is
available in an 8-Lead, 2 mm x 3 mm DFN package or
a 5-Lead, SOT-23 package. Along with its small
physical size, the low number of external components
required make the MCP73831 ideally suited for
portable applications. For applications charging from a
USB port, the MCP73831 adheres to all the
specifications governing the USB power bus.
The MCP73831 employs a constant-current / constantvoltage charge algorithm with selectable preconditioning and charge termination. The constant voltage
regulation is fixed with four available options: 4.20V,
4.35V, 4.40V or 4.50V, to accommodate new, emerging
battery charging requirements. The constant current
value is set with one external resistor.
The MCP73831 device limits the charge current based
on die temperature during high power or high ambient
conditions. This thermal regulation optimizes the
charge cycle time while maintaining device reliability.
Several options are available for the preconditioning
threshold, preconditioning current value, charge
termination value and automatic recharge threshold.
© 2007 Microchip Technology Inc.
AN1088
The preconditioning value and charge termination
value are set as a ratio, or percentage, of the programmed constant current value. Preconditioning can
be disabled.
The MCP73831 is fully specified over the ambient temperature range of -40°C to +85°C. Figure 3 depicts the
operational flow algorithm from charge initiation to
completion and automatic recharge.
VBAT < VPTH
When the voltage at the VBAT pin rises above the
preconditioning threshold, the MCP73831 enters the
Constant-Current or Fast Charge mode.
PRECONDITIONING
MODE
Charge Current = IPREG
STAT = Low
FAST CHARGE
MODE
Charge Current = IREG
STAT = Low
FAST CHARGE: CONSTANT-CURRENT MODE
VBAT > VPTH
VBAT < VPTH
CONSTANT VOLTAGE
MODE
Charge Voltage = VREG
STAT = Low
CHARGE COMPLETE
MODE
No Charge Current
STAT = High (MCP73831)
STAT = Hi-Z (MCP73832)
FIGURE 3:
An internal under voltage lockout (UVLO) circuit
monitors the input voltage and keeps the charger in
shutdown mode until the input supply rises above the
UVLO threshold. For a charge cycle to begin, all UVLO
conditions must be met and a battery or output load
must be present. A charge current programming
resistor must be connected from PROG to VSS.
If the voltage at the VBAT pin is less than the preconditioning threshold, the MCP73831 enter a preconditioning or Trickle Charge mode. The preconditioning
threshold is factory set. In this mode, the MCP73831
supplies a percentage of the charge current (established with the value of the resistor connected to the
PROG pin) to the battery. The percentage or ratio of the
current is factory set.
SHUTDOWN MODE
VDD < VUVLO
VDD < VBAT
or
PROG > 200 kW
STAT = Hi-Z
VBAT > VPTH
CHARGE QUALIFICATION AND
PRECONDITIONING TRICKLE CHARGE
During the Constant-Current mode, the programmed
charge current is supplied to the battery or load. The
charge current is established using a single resistor
from PROG to VSS. Constant-Current mode is maintained until the voltage at the VBAT pin reaches the regulation voltage, VREG.
PROGRAM CURRENT REGULATION
Fast charge current regulation can be set by selecting
a programming resistor (RPROG) from PROG to VSS.
The charge current can be calculated using the
following equation:
EQUATION 1:
PROGRAM FAST CHARGE
CURRENT
1000VI REG = ---------------R PROG
Where:
RPROG
=
kilo-ohms
IREG
=
milliamperes
MCP73831 Flowchart.
© 2007 Microchip Technology Inc.
DS01088A-page 5
AN1088
Charge Current (mA)
CHARGE STATUS INDICATOR
The charge status output of the MCP73831 has three
different states: High (H), Low (L), and High-Impedance (Hi-Z). The charge status output can be used to
illuminate 1, 2, or tri-color LEDs. Optionally, the charge
status output can be used as an interface to a host
microcontroller.
550
500
450
400
350
300
250
200
150
100
50
0
Table 4 summarize the state of the status output during
a charge cycle.
2
4
6
8
10
12
14
16
18
20
TABLE 4:
Charge Cycle State
Programming Resistor (kΩ)
FIGURE 4:
IOUT vs. RPROG.
Figure 4 shows the relationship between fast charge
current and programming resistor.
The preconditioning trickle charge current and the
charge termination current are ratio metric to the fast
charge current based on the selected device option.
CONSTANT-VOLTAGE MODE
When the voltage at the VBAT pin reaches the
regulation voltage, VREG, constant voltage regulation
begins. The regulation voltage is factory set to 4.2V,
4.35V, 4.40V, or 4.50V with a tolerance of ±0.75%.
STATUS OUTPUT
Shutdown
Hi-Z
No Battery Present
Hi-Z
Constant-Current Fast Charge
L
Preconditioning
L
Constant Voltage
L
Charge complete - Standby
H
TYPICAL APPLICATION
500 mA Li-Ion Battery Charger
VIN
4.7 µF
VBAT 3
4.7 µF
4 V
DD
CHARGE TERMINATION
The charge cycle is terminated when, during ConstantVoltage mode, the average charge current diminishes
below a percentage of the programmed charge current
(established with the value of the resistor connected to
the PROG pin). A 1 ms filter time on the termination
comparator ensures that transient load conditions do
not result in premature charge cycle termination. The
percentage or ratio of the current is factory set. The
charge current is latched off and the MCP73831 enters
a Charge Complete mode.
AUTOMATIC RECHARGE
The MCP73831 continuously monitors the voltage at
the VBAT pin in the Charge Complete mode. If the
voltage drops below the recharge threshold, another
charge cycle begins and current is once again supplied
to the battery or load.
THERMAL REGULATION AND THERMAL
SHUTDOWN
MCP73831
PROG
470Ω
1
STAT
+ Single
Li-Ion
- Cell
5
VSS 2
2 kΩ
MCP73831
FIGURE 5:
MCP73831 Typical
Application Circuit.
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 battery charger
and the ambient cooling air. The worst-case situation is
when the device has transitioned from the Preconditioning mode to the Constant-Current mode.
In this situation, the battery charger has to dissipate the
maximum power. A trade-off must be made between
the charge current, cost and thermal requirements of
the charger.
The MCP73831 limits the charge current based on the
die temperature. The thermal regulation optimizes the
charge cycle time while maintaining device reliability.
The MCP73831 suspends charge if the die temperature exceeds 150°C. Charging will resume when the
die temperature has cooled by approximately 10°C.
DS01088A-page 6
© 2007 Microchip Technology Inc.
AN1088
=
the maximum input voltage
IREGMAX
=
the maximum fast charge current
VPTHMIN
=
the minimum transition threshold
voltage
4.0
400
3.0
300
2.0
200
MCP73831-2AC/IOT
VDD = 5.2V
RPROG = 2 kΩ
1.0
100
0.0
0
240
VDDMAX
500
210
Where:
5.0
180
REGMAX
150
)×I
120
PTHMIN
90
–V
60
DDMAX
600
0
PowerDissipation = ( V
6.0
Charge Current (mA)
POWER DISSIPATION
Battery Voltage (V)
EQUATION 2:
TYPICAL CHARGE PROFILE
30
The power dissipation has to be considered in the
worst-case.
Time (minutes)
EXAMPLE 1:
POWER DISSIPATION
EXAMPLE
FIGURE 7:
MCP73831 Typical Charge
Profile in Thermal Regulation
(1000 mAh Battery).
Assume:
VIN
=
5V ±10%
IREGMAX
=
550 mA
VPTHMIN
=
2.7V
Power
Dissipation
=
(5.5V - 2.7V) x 550 mA = 1.54W
DEVICE OVERVIEW
EXTERNAL COMPONENTS
The MCP73831 is stable with or without a battery load.
A minimum capacitance of 4.7 µF is recommended to
bypass the VBAT pin to VSS and VIN pin to VSS to
maintain good AC stability in the constant-voltage
mode. A single resistor between PROG pin and VSS is
required to control fast charge current. Equation 1 and
Figure 4 can be applied to find RPROG value. LED and
RLED are required for status indicator.
525
RPROG = 2 kΩ
450
The MCP73812 Simple, Miniature Single-Cell Fully
Integrated Li-Ion/Li-Polymer Charge Management
Controller is designed for use in space limited and cost
sensitive applications. The MCP73812 provides
specific charge algorithms for single cell Li-Ion or LiPolymer battery to achieve optimal capacity in the
shortest charging time possible. Along with its small
physical size and the low number of external
components required make the MCP73812 ideally
suited for portable applications.
The MCP73812 employs a constant current/constant
voltage charge algorithm like MCP73831. The constant
voltage regulation is fixed at 4.20V, with a tight
regulation tolerance of 1%. The constant current value
is set with one external resistor. The MCP73812 limits
the charge current based on die temperature during
high power or high ambient conditions. This thermal
regulation optimizes the charge cycle time while
maintaining device reliability.
THERMAL REGULATION
Charge Current (mA)
Example 2: Design Ultra Low-Cost Li-ion
/ Li-Polymer Battery Charge Management
With MCP73812 [9]
375
300
225
150
The MCP73812 is fully specified over the ambient
temperature range of -40°C to +85°C. The MCP73812
is available in a 5-Lead, SOT-23 package.
75
155
145
135
125
115
95
105
85
75
65
55
45
35
25
0
Junction Temperature (°C)
FIGURE 6:
Thermal Regulation.
© 2007 Microchip Technology Inc.
DS01088A-page 7
AN1088
FAST CHARGE: CONSTANT-CURRENT MODE
During the constant current mode, the programmed
charge current is supplied to the battery or load. For the
MCP73812, the charge current is established using a
single resistor from PROG to VSS. The MCP73812
shares the same program method with MCP73831.
The program resistor and the charge current are calculated using the Equation 1. Refer to Figure 4 for the
Charge Current and Programming Resistor.
SHUTDOWN MODE*
VDD < VPD
STANDBY MODE*
CE = Low
CONSTANT-VOLTAGE MODE
CONSTANT CURRENT
MODE
Charge Current = IREG
When the voltage at the VBAT pin reaches the regulation voltage, VREG, constant voltage regulation begins.
The regulation voltage is factory set to 4.2V with a
tolerance of ±1.0%.
CHARGE TERMINATION
VBAT < VREG
VBAT = VREG
CONSTANT VOLTAGE
MODE
Charge Voltage = VREG
The charge cycle is terminated by removing the battery
from the charger, removing input power, or driving the
charge enable input (CE) to a logic low. An automatic
charge termination method is not implemented.
AUTOMATIC RECHARGE
* Continuously
Monitored
FIGURE 8:
MCP73812 Flowchart.
CHARGE QUALIFICATION AND
PRECONDITIONING TRICKLE CHARGE
The MCP73812 does not employ under voltage lockout
(UVLO). When the input power is applied, the input
supply must rise 150 mV above the battery voltage
before the MCP73812 becomes operational.
The automatic power down circuit places the device in
a shutdown mode if the input supply falls to within
+50 mV of the battery voltage. The automatic circuit is
always active. Whenever the input supply is within
+50 mV of the voltage at the VBAT pin, the MCP73812
is placed in a shutdown mode. During power down
condition, the battery reverse discharge current is less
than 2 µA.
For a charge cycle to begin, the automatic power down
conditions must be met and the charge enable input
must be above the input high threshold.
The MCP73812 does not support preconditioning of
deeply depleted cells, and it begins with fast charge
once charging conditions satisfy.
The MCP73812 does not support automatic recharge
cycles since automatic charge termination has not
been implemented. In essence, the MCP73812 is
always in a charge cycle whenever the qualification
parameters have been met.
THERMAL REGULATION AND THERMAL
SHUTDOWN
The MCP73812 limits the charge current based on the
die temperature. The thermal regulation optimizes the
charge cycle time while maintaining device reliability.
The MCP73812 suspends charge if the die temperature exceeds 150°C. Charging will resume when the
die temperature has cooled by approximately 10°C.
The thermal shutdown is a secondary safety feature in
the event that there is a failure within the thermal
regulation circuitry.
TYPICAL APPLICATION
500 mA Li-Ion Battery Charger
VIN
1 µF
4 V
DD
VBAT
3
+ Single
Li-Ion
- Cell
1 µF
1 CE
PROG
5
VSS
2
2 kW
MCP73812
FIGURE 9:
tion Circuit.
DS01088A-page 8
MCP73812 Typical Applica-
© 2007 Microchip Technology Inc.
AN1088
The MCP73812 shares similar application with
MCP73831, but Charge Enable (CE) is designed to
replace charge status pin. A logic high enables battery
charging while a logic low disables battery charging.
The charge enable input is compatible with 1.8V logic.
TYPICAL CHARGE PROFILE
The power dissipation has to be considered in the
worst case. The power dissipation for the MCP73812 is
same as the MCP73831. Therefore, equation 2 will be
applied for the MCP73812 power dissipation calculation.
MCP73831 VS. MCP73812
EXAMPLE 2:
MCP73812 shares same charge profile with
MCP73831, but no available preconditioning and automatically charge termination.
TABLE 5:
MCP73831 VS. MCP73812
MCP73831 MCP73812
Cost
Power Dissipation Example
Assume:
Low
Ultra Low
Applications
Simple
Simple
Space Requirement
Small
Small
VIN
=
5V ±10%
Voltage Reg. Accuracy
±0.75%
±1.0%
IREGMAX
=
500 mA
Yes
Yes
VPTHMIN
=
2.7V
Programmable Current
Note 1
Power
Dissipation
=
(5.5V - 2.7V) x 500 mA = 1.4W
UVLO
Yes
No
Preconditioning
Yes
No
End-of-Charge Control
Yes
No
EXTERNAL COMPONENTS
Charge Status
Yes
No
The MCP73812 is stable with or without a battery load.
A minimum capacitance of 1 µF is recommended to
bypass the VBAT pin to VSS and VIN pin to VSS to
maintain good AC stability in the constant-voltage
mode. A single resistor between PROG pin and VSS is
required to control fast charge current. Equation 1 and
Figure 4 can be applied to find RPROG value. LED and
RLED are required for status indicator.
Charge Enable PIN
No
Yes
Automatic Recharge
Yes
No
THERMAL REGULATION
Note 1:
Charge Current (mA)
525
Automatic Power-Down
Yes
No
Thermal Regulation
Yes
Yes
Fully Integrated
Yes
Yes
Voltage Reg. Options
Note 2
Yes
No
RPROG = 2 kΩ
450
375
2:
300
225
150
75
155
145
135
125
115
95
105
85
75
65
55
45
35
25
0
MCP73812 family is also available in
selectable Charge Current: 85 mA or
450 mA for applications charging from
USB port with device number MCP73811. Refer to MCP73811/2 Data
Sheet (DS22036) for detail information.
MCP73831 voltage regulation is fixed with
four available options: 4.20V, 4.35V,
4.40V or 4.50V. MCP73812 comes with a
standard 4.20V constant voltage
regulation.
Junction Temperature (°C)
FIGURE 10:
Thermal Regulation.
© 2007 Microchip Technology Inc.
DS01088A-page 9
AN1088
CONCLUSION
REFERENCES
Li-Ion batteries are not only good NiMH and NiCd
batteries substitutes for advanced portable electric
devices, but also for cost-sensitive designs. Although,
high capacity, compact size, light weight and maximum
charge/discharge cycles do not exist in the same
package; there is always a trade-off when engineers/
designers select the key factors for the design. Due to
the phase out rate of today’s portable electric products,
charge/discharge cycles is always the first to be eliminated. The aging issue of Li-Ion batteries are often
ignored and rarely recommended to customers for the
same reason.
[1]
“Lithium Batteries”, Gholam-Abbas Nazri and
Gianfranco Pistoia Eds.; Kluwer Academic
Publishers, 2004.
[2]
“Handbook of Batteries, Third Edition”, David
Linden, Thomas B. Reddy; McGraw Hill Inc,
2002.
[3]
”Batteries in a Portable World Second Edition”,
Isidor Buchmann; Cadex Electronics Inc., 2000.
[4]
“Portable Electronics Product Design and
Development”, Bert Haskell; McGraw Hill, 2004.
[5]
“Brief of Li-Polymer Battery’s Research and
Development”, W.T. Wen; Taiwan National
Science Cuncil Monthly No.7, 2001.
[6]
AN960, “New Components and Design Methods
Bring Intelligence to Battery Charger Applications”, Terry Cleveland and Catherine Vannicola; Microchip Technology Inc., DS00960,
2004.
[7]
AN947, “Power Management in Portable Applications: Charging Lithium-Ion/Lithium-Polymer
Batteries”, Scott Dearborn; Microchip Technology Inc., DS00947, 2004.
[8]
Microchip RTC Training Class: “Portable Power
Management”, Microchip Technology Inc., 2006.
[9]
MCP73811/2 Data Sheet, “Simple, Miniature
Single-Cell, Fully Integrated Li-Ion/Li-Polymer
Charge Management Controllers”, Microchip
Technology Inc., DS22036, 2007.
[10]
MCP73831/2 Data Sheet, “Miniature SingleCell, Fully Integrated Li-Ion/Li-Polymer Charge
Management Controllers”, Microchip Technology Inc., DS21984, 2006.
Selecting the right charge management controller can
improve the product performance, reduce design time,
simplify design cycle and optimize cost performance.
The MCP73831 is a good solution to meet all of the
above needs. For systems that do not require many
features and are designed on a tight budget, the
MCP73812 is the right candidate to perform well in
battery charging applications.
DS01088A-page 10
© 2007 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.
•
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© 2007, Microchip Technology Incorporated, Printed in the
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© 2007 Microchip Technology Inc.
DS01088A-page 11
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DS01088A-page 12
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