AN1276

AN1276
Design A Low-Cost Lithium Iron Phosphate (LiFePO4)
Battery Charger With MCP73123
Author:
Brian Chu
Microchip Technology Inc.
INTRODUCTION
Demand of fast-discharge rated energy storage
sources for Electrical Vehicle (EV), Hybrid Electrical
Vehicle HEV) or portable power tools have driven the
commercial development of Lithium Iron Phosphate
(LiFePO4) batteries. The traditional LiFePO4 battery
systems usually require high voltages or large
capacities. However, the nature of its characters, such
as longer cycle life than typical Li-Ion (Lithium Iron)
batteries, better resistance to thermal runaway and
higher output and peak current rating make them ideal
candidates to RC (remote control) toys and backup
power applications.
The typical capacity of LiFePO4 battery cells are
available in the ranged from 500 mAh to 2300 mAh.
They are usually rated at 3.2V. There are systems or
applications that do not require large capacity (multiple
cells in parallel) or high voltage (multiple cells in series)
battery packs. Figure 1 illustrates a charge cradle that
can range from one cell to ‘n’ cells batteries. Each
power path has one IC (Integrated Circuit) to manage
the charge profile and display the state of charge.
Status
Most LiFePO4 battery manufacturers have different
charge and discharge specifications for their batteries.
However, all LiFePO4 share Constant CurrentConstant Voltage (CC-CV) algorithm with Li-Ion batteries. The preferred charge voltage is typically 3.6V. The
termination current can be either fixed value or ratio of
fast charge current. Unlike Li-Ion chemistry, LiFePO4
can be charged with higher C rate.
Note:
Please consult the battery manufacturer
for the desired maximum charge rated.
Microchip’s MCP73123 family is developed to simplify
the design for mid to low range capacity LiFePO4
batteries or if the total charge time is not critical for
larger capacity applications.
This application note is intended to provide design
guidance for designers who are interested in taking
advantage of using Microchip’s MCP73123 to charge
LiFePO4 batteries to reduce the product development
cycle, cost and time to market.
Status
MC P73123
LiFeP O 4
FIGURE 1:
MCP 73123
LiFePO 4
LiFePO4 Charger Cradle Illustration of the MCP73123.
© 2009 Microchip Technology Inc.
DS01276A-page 1
AN1276
MCP73123 DEVICE DESCRIPTION
MCP73123 DEVICE FEATURES
The MCP73123 is a highly integrated Lithium Iron
Phosphate (LiFePO4) battery charge management
controller for use in space-limited and cost-sensitive
applications. The MCP73123 provides specific charge
algorithms for LiFePO4 batteries to achieve optimal
capacity and safety in the shortest charging time
possible. Along with its small physical size, the low
number of external components make the MCP73123
ideally suitable for various applications. The absolute
maximum voltage, up to 18V, allows the use of
MCP73123 in harsh environments, such as low cost
wall wart or voltage spikes from plug/unplug.
• Constant Current / Constant Voltage Operation
with Thermal Regulation
• 4.15V Undervoltage Lockout (UVLO)
• 18V Absolute Maximum Input with OVP:
- 6.5V - MCP73123
• High Accuracy Preset Voltage Regulation
Through Full Temperature Range (-5°C to +55°C):
- + 0.5%
• Battery Charge Voltage Options:
- 3.6V - MCP73123
• Resistor Programmable Fast Charge Current:
- 130 mA - 1100 mA
• Preconditioning of Deeply Depleted Cells:
- Available Options: 10% or Disable
• Integrated Precondition Timer:
- 32 Minutes or Disable
• Automatic End-of-Charge Control:
- Selectable Minimum Current Ratio:
5%, 7.5%, 10% or 20%
- Elapse Safety Timer: 4 HR, 6 HR, 8 HR or
Disable
• Automatic Recharge:
- Available Options: 95% or Disable
• Two Charge Status Output Available - On or Flash
• Soft Start
• Temperature Range: -40°C to +85°C
• Packaging:
- DFN-10 (3 mm x 3 mm)
The MCP73123 employs a constant current-constant
voltage charge algorithm. The 3.6V per cell factory
preset reference voltage simplifies design with
2V preconditioning threshold. The fast charge,
constant current value is set with one external resistor
from 130 mA to 1100 mA. The MCP73123 also 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.
The PROG pin of the MCP73123 also serves as enable
pin. When a high impedance is applied, the MCP73123
will be in standby mode.
The MCP73123 is fully specified over the ambient
temperature range of -40°C to +85°C. The MCP73123
is available in a 10 lead, DFN package.
This Applications Note shows how to design a simple
Lithium Iron Phosphate battery charge management
system with Microchip’s MCP73123 for cost-sensitive
applications.
References to documents that treat these subjects in
more depth and breadth have been included in the
“References” section.
Note:
MCP73223 is also available for dual cell
charger to charge two LiFePO4 in series.
DS01276A-page 2
© 2009 Microchip Technology Inc.
AN1276
TABLE 1:
AVAILABLE FACTORY PRESET OPTIONS
Precondition
Timer
Elapse
Timer
End-ofCharge
Control
Automatic
Recharge
Output
Status
2V
Disable /
32 Minimum
Disable / 4 HR /
6 HR / 8 HR
5% / 7.5% /
10% / 20%
No /
Yes
Type 1 /
Type 2
4V
Disable /
32 Minimum
Disable / 4 HR /
6 HR / 8 HR
5% / 7.5% /
10% / 20%
No /
Yes
Type 1 /
Type 2
Charge
Voltage
OVP
Preconditioning
Charge Current
Preconditioning
Threshold
3.6V
6.5V
Disable / 10%
7.2V
13V
Disable / 10%
TABLE 2:
STANDARD SAMPLE OPTIONS
Part
Number
VREG
OVP
IPREG/IREG
Pre-charge
Timer
Elapse
Timer
MCP73123-22SI/MF
3.6V
6.5V
10%
32 Min.
6 HR
10%
95%
71.5%
Type 1
MCP73223-C2SI/MF
7.2V
6.5V
10%
32 Min.
6 HR
10%
95%
71.5%
Type 1
Note 1:
2:
3:
4:
5:
6:
7:
8:
9:
Note:
ITERM/IREG VRTH/VREG VPTH/VREG
Output
Status
IREG: Regulated fast charge current.
VREG: Regulated charge voltage.
IPREG/IREG: Preconditioning charge current; ratio of regulated fast charge current.
ITERM/IREG: End-of-Charge control; ratio of regulated fast charge current.
VRTH/VREG: Recharge threshold; ratio of regulated battery voltage.
VPTH/VREG: Preconditioning threshold voltage.
Type 1 Output Status - Open-drain.
Type 2 Output Status - Open-drain with 50% duty cycle on/off.
Customers should contact their distributor, representatives or field application engineer (FAE) for support and sample.
Local sales offices are also available to help customers. A listing of sales offices and locations is included in the back of
this document. Technical support is available through the web site at: http//support.microchip.com.
Above information is available in the MCP73123/223 data sheet (DS22191).
MCP73123 Typical Application
1
Ac-dc-Adapter
VDD
VBAT
2 VDD
VBAT
3
4
4.7 µF
+
4.7 µF
7
STAT
PROG
1-Cell
LiFePO4
Battery
10
1 kΩ
5 NC
6 NC
FIGURE 2:
VSS
VSS
9
1.15 kΩ
-
8
Typical MCP73123 Applications.
© 2009 Microchip Technology Inc.
DS01276A-page 3
AN1276
LIFEPO4 CHARGER DESIGN GUIDE
EXTERNAL CAPACITORS
Figure 2 depicts the typical application circuit.
Designing with the MCP73123 is easy with minimum
four external components. The output status pin
connects to either MCU or LED for different display
methods. Table 1 provides the available options of the
MCP73123. The options in Table 2 are standard
samples and can be obtained quickly. The MCP73123
is available in the 3 mm x 3 mm DFN package,
as shown in Figure 3.
The MCP73123 is stable with or without a battery load.
In order to maintain good AC stability in the Constantvoltage mode, a minimum capacitance of 1 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. A minimum of 16V
rated 1 µF, is recommended to apply for output
capacitor and a minimum of 25V rated 1 µF, is
recommended to apply for input capacitor for typical
applications.
MCP73123/223
3x3 DFN *
VDD 1
VDD 2
VBAT 3
VBAT 4
NC 5
10 PROG
EP
11
9 VSS
8 VSS
7 STAT
6 NC
* Includes Exposed Thermal Pad (EP); see DS22191.
FIGURE 3:
TABLE 3:
MLCC CAPACITOR EXAMPLE
MLCC
Capacitors
Temperature
Range
Tolerance
X7R
-55°C to +125°C
±15%
X5R
-55°C to +85°C
±15%
MCP73123/223 Package
For non-standard combinations of options, contact your
local Microchip representatives or distributors. This
section will offer detailed design guide to develop a
LiFePO4 battery charger system.
Power Supply Input (VDD)
The MCP73123 operates from 4.15V to 5.8V or 6.5V,
However, the MCP73123 can protect up to 18V absolute maximum voltage when the power supply is instable or when the end user accidently plug in the wrong
ac-dc adapter. The selected input capacitor needs to
meet the desired design specifications.
Battery Charger Output (VBAT)
The MCP73123 regulates VBAT pin to 3.6V when
charge begins. When 3.6V is detected, the algorithm
moves to constant voltage range until minimum current
is satisfied or elapse timer is up for automatic
termination. The output capacitor will ensure the loop
stability when the battery is disconnected.
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 output load current. A 1 µF ceramic,
tantalum or aluminum electrolytic capacitor at the
output is usually sufficient to ensure stability.
Fast Charge Current Set (PROG)
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. The program resistor and
the charge current are calculated using the following
equation:
EQUATION 1:
CHARGE CURRENT
I REG = 1104 × R
– 0.93
Where:
RPROG
=
kilo-ohms (kΩ)
IREG
=
milliampere (mA)
EQUATION 2:
SELECT RESISTOR
R PROG = 10
I REG ⎞ ⎞
⎛ log ⎛ ----------- ⁄ ( – 0.93 )
⎝ ⎝ 1104⎠ ⎠
Where:
DS01276A-page 4
RPROG
=
kilo-ohms (kΩ)
IREG
=
milliampere (mA)
© 2009 Microchip Technology Inc.
AN1276
Table 4 provides commonly seen E96 (1%) and E24
(5%) resistors for various charge current to reduce
design time.
TABLE 4:
RESISTOR LOOKUP TABLE
Charge
Recommended Recommended
Current (mA) E96 Resistor (Ω) E24 Resistor (Ω)
Battery Charge Status Outputs (STAT)
The charge status outputs are open-drain outputs with
two different states: Low (L), and High Impedance
(Hi-Z). The charge status outputs can be used to
illuminate LEDs. Optionally, the charge status outputs
can be used as an interface to a host microcontroller.
Table 5 summarize the state of the status outputs
during a charge cycle.
130
10k
10k
150
8.45k
8.20k
200
6.20k
6.20k
250
4.99k
5.10k
300
4.02k
3.90k
Shutdown
Hi-Z
350
3.40k
3.30k
Standby
Hi-Z
L
L
TABLE 5:
STATUS OUTPUTS
CHARGE CYCLE STATE
400
3.00k
3.00k
Preconditioning
450
2.61k
2.70k
500
2.32k
2.37k
Constant Current Fast
Charge
550
2.10k
2.20k
Constant Voltage
STAT
L
600
1.91k
2.00k
Charge Complete - Standby
Hi-Z
650
1.78k
1.80k
Temperature Fault
700
1.62k
1.60k
750
1.50k
1.50k
1.6 second 50% D.C.
Flashing (Type 2)
Hi-Z (Type 1)
800
1.40k
1.50k
Timer Fault
850
1.33k
1.30k
900
1.24k
1.20k
1.6 second 50% D.C.
Flashing (Type 2)
Hi-Z (Type 1)
950
1.18k
1.20k
Preconditioning Timer Fault
1000
1.10k
1.10k
1.6 second 50% D.C.
Flashing (Type 2)
Hi-Z (Type 1)
1100
1.00k
1.00k
Constant current mode is maintained until the voltage
at the VBAT pin reaches the regulation voltage, VREG.
When constant current mode is invoked, the internal
timer is reset.
PROG pin also serves as charge control enable. When
a typical 200 kΩ impedance is applied to PROG pin,
the MCP73123 is disabled until the high impedance is
removed.
© 2009 Microchip Technology Inc.
DS01276A-page 5
AN1276
SUMMARY
The MCP73123 helps designers to reduce design
complexities and minimize external components for
LiFePO4 charger cradles or chargers. Integrated input
overvoltage protection and battery short protection
allow seamless switching between different input/
output voltage conditions. The MCP73123 also offers
built-in preconditioning timer and overall elapse timer to
prevent overcharge of a bad battery.
Due to the power dissipations in the linear charger
design, the thermal foldback provides better heat
management that prevents the system temperature
from increasing and prolong the life of the products.
Figure 4 depicts the complete charge cycle of a
1100 mAh rated LiFePO4 battery. The charge current is
set at 1A. At the beginning of charge cycle, the battery
voltage is 2V when input voltage is 5V. The 3 watts
power dissipation triggers the thermal foldback to
begin. Unlike Li-Ion batteries, LiFePO4 batteries can
restore energy back faster if battery capacity and fast
charge current speed are equal. A typical Li-Ion battery
may require 2-3 hours when charge with 1C rate.
“C” Rate Definition: 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 or ampere-hours.
7.0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Battery Voltage (V)
Thermal Regulation
6.0
5.0
4.0
3.0
2.0
VDD = 5V
RPROG = 1 kΩ
1.0
0.0
0
10
20
30
40
50
Time (Minutes)
60
Supply Current (A)
Note:
Figure 5 shows half of top layer of the
MCP73X23EV-LFP evaluation board. There are two
independent circuits on the MCP73X23EV-LFP for
single-cell and dual-cell applications. The user’s guide
and Gerber file for the MCP73X23EV-LFP are available
on Microchip’s website.
MCP73X23EV-LFP
FIGURE 5:
Board.
MCP73X23 Evaluation
REFERENCES
[1]
MCP73123/223 Data Sheet, “Lithium Iron
Phosphate
(LiFePO4)
Battery
Charge
Management Controller with Input Overvoltage
Protection”,
Microchip
Technology
Inc.,
DS22191, ©2009.
[2]
“Lithium Batteries”, Gholam-Abbas Nazri and
Gianfranco Pistoia Eds.; Kluwer Academic
Publishers, ©2004.
70
FIGURE 4:
Typical MCP73123 Charge
Profile (1100 mAh LiFePO4 Battery Cell).
DS01276A-page 6
© 2009 Microchip Technology Inc.
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DS01276A-page 8
© 2009 Microchip Technology Inc.