AN971

AN971
USB Port-Powered Li-Ion/Li-Polymer Battery Charging
Author:
Scott Dearborn
Microchip Technology Inc.
INTRODUCTION
The Universal Serial Bus (USB) allows many computer
peripherals to be easily swapped without having to turn
off the computer. Today, a variety of handheld, batteryoperated peripherals provide USB ports to facilitate
data transfer to and from a host computer. With the
introduction of the new USB 2.0 specification, CD/DVD
players, MP3 players, cameras, personal data
assistants and even cell phones can transfer data at
rates up to 480 Mbps.
These peripherals are, in some instances, selfpowered. As a result, many of these peripherals do not
take full advantage of the USB port. Often overshadowed by the data interface is the power capability that
a USB port provides. Microchip's MCP73853/55 and
MCP73861 advanced, fully-integrated, single-cell LiIon/Li-Polymer charge-management devices allow
these peripherals to utilize the full “power” of the USB
port.
USB-POWERED, SINGLE-CELL,
LI-ION BATTERY CHARGER
Many of the self-powered peripherals use a separate
power supply for battery charging even while
connected to a USB port. With the MCP73853/55,
harnessing the power of the USB port for battery charging becomes extremely simple.
A USB port transfers signal and power over a fourconductor cable, depicted in Figure 1. Each USB port
provides a limited amount of power over the cable. The
amount of current provided from a USB port is defined
in terms of unit loads, where one unit load is defined as
100 mA. The number of unit loads that a port can
supply is an absolute maximum, not an average over
time. A port may be configured as either low-power (at
one unit load) or high-power (providing up to five unit
loads). All ports default to low-power. The transition to
high-power is under the software’s control. It is the
responsibility of the host software to ensure that
adequate power is available before allowing
peripherals to consume high power.
© 2005 Microchip Technology Inc.
VBUS
D+
DGND
FIGURE 1:
VBUS
D+
DGND
USB Cable.
The USB high-power port is specified for a maximum
current of five unit loads (or 500 mA), with a voltage of
5V, +5%. The USB specification allows for voltage
drops in the USB connectors and cables of up to
350 mV when delivering 500 mA. The maximum cable
length is specified as 16 feet. Shorter cables will
introduce less voltage drop. The minimum voltage seen
at the peripheral device will be 4.4V.
Limiting the current draw to 400 mA produces an
absolute minimum voltage of 4.47V, providing 0.27V of
headroom to fully charge a single-cell, Li-Ion battery.
The extremely low dropout voltage (200 mV at 400 mA)
of the MCP73853/55 makes them an attractive and
simple approach for a USB Li-Ion/Li-Polymer battery
charger.
Figure 2 and Figure 3 illustrate USB-powered, singlecell Li-Ion/Li-Polymer battery chargers. The USB controller communicates with the host to determine if the
peripheral is connected to either a low-power or highpower port. The controller then sets the appropriate
charge current setting. The MCP73853/55 employs
typical charge current settings of 85 mA and 400 mA
for low-power and high-power, respectively. These settings ensure that the absolute maximum rating of the
USB port is not exceeded. Optionally, the high-power
charge current setting can be adjusted lower by placing
a resistor between the MCP73853/55 and the USB
controller. The appropriate value resistor can be
determined from the formula:
13.2 – 33.3 × I REG
R PROG = --------------------------------------------14.1 × I REG – 1.2
Where:
IREG is the desired high-power charge current
setting in amps
RPROG is in kilo-ohms
DS00971A-page 1
AN971
VBUS
GND
STAT1 STAT 2 EN VSS2
16
VSET
15
14
13
1
12
2
11
SUSPEND
D+
D-
VDD1
USB CONTROL
+ Single
Lithium-Ion
Cell
VBAT2
MCP73853
VDD2
100 mA
500 mA
VBAT
VBAT3
VSS1
3
10
4
9
5
6
7
PROG THREF THE RM
VBAT1
VSS3
8
TIMER
CTIMER
RPROG
RT1
RT2
FIGURE 2:
Monitoring.
USB-powered, Single-cell, Li-Ion/Li-Polymer Battery Charger with Cell Temperature
VBUS
GND
STAT1
VSET
D+
D-
SUSPEND
USB CONTROL
VDD1
VSS1
PROG
100 mA
500 mA
FIGURE 3:
DS00971A-page 2
1
10
2
9
3
MCP73855
8
4
7
5
6
EN
VBAT2
VBAT
+ Single
Lithium-Ion
Cell
VBAT1
VSS2
TIMER
CTIMER
Small PCB Area USB-powered, Single-cell, Li-Ion/Li-Polymer Battery Charger
© 2005 Microchip Technology Inc.
AN971
ADDITIONAL USB SUPPORTED
FEATURES
FASTER CHARGE CYCLE TIMES
WITH WALL ADAPTER
In addition to the maximum permissible current draw on
the USB power port, additional considerations must be
adhered to as well. Any peripheral connected to a USB
port must support a Suspend state as defined by the
USB specification. A peripheral can enter the Suspend
state from either power state (low power or high
power). The allowed Suspend current is a function of
unit load allocation. Ports operating in the Low-power
mode are limited to 500 µA of Suspend current. Ports
operating in the High-power mode are limited to
2.5 mA.
The USB high-power port is specified for a maximum
current of five unit loads, or 500 mA. Utilizing
Microchip’s MCP73861, charge currents up to 1.2A can
be realized from an alternate power source (such as a
wall cube).
In the circuits of Figures 1 and 2, the peripheral
device’s system power, including the USB controller, is
derived directly from the battery. In this manner, the
MCP73853/55 controls the charging of the battery,
while maintaining the current draw from the USB port
below the specification limits. The Suspend state is
entered when the USB controller pulls the enable pin of
the MCP73853/55 low. Disabled, the MCP73853/55
consumes less than 1 µA of current. The peripheral
Suspend current can be computed as the current from
VBUS through the bus pull-up and pull-down
termination resistors plus 1 µA.
In-rush current must also be considered whenever a
peripheral is connected to a USB port. The VBUS power
lines at the port are bypassed with a minimum low-ESR
capacitance of 120 µF. The maximum load of the
peripheral device is limited to 44Ω, in parallel with
10 µF of capacitance. This ensures that the VBUS
voltage does not get pulled below its minimum
operating voltage when a peripheral is connected to the
port. The maximum droop allowed on VBUS is 330 mV.
The MCP73853/55 requires a minimum input capacitance of 1 µF, although 4.7 µF is recommended. This is
well below the maximum USB specification. In addition,
the MCP73853/55 controls the slew rate of the charge
current when transitioning from preconditioning to fast
charge, and when a new charge current setting is
requested (i.e., from low-power to high-power). These
unique features prevent large slugs of current from
flowing to the charger output capacitor and battery,
causing the VBUS voltage to droop excessively.
Another important specification to consider when connecting to the USB power bus is that no peripheral shall
supply (source) current on VBUS at any time. The
MCP73853/55 employs integrated reverse discharge
circuitry that prevents current to flow from the battery
onto VBUS.
Figure 4 depicts the MCP73861 in a system that
accepts input power from a USB port or alternate
power source. This solution requires only a handful of
external components, in addition to the MCP73861.
In this manner, charge currents up to 1.2A can be realized when the alternate power source is present. When
only USB power is present, the P-channel MOSFET is
supplying power to the charger from VBUS. In this case,
the programming resistor is not optional. A minimum
resistance of 2.2 kΩ must be placed between the USB
controller and the MCP73861 PROG input. The 2.2 kΩ
programming resistor sets the typical high-power
charge current to 420 mA. The high-power charge
current setting can be adjusted lower by utilizing a
higher value resistor. The appropriate value resistor
can be determined from the formula:
13.2 – 11 × I REG
R PROG = ---------------------------------------12 × I REG – 1.2
Where:
IREG is the desired high-power charge current
setting in amps
RPROG is in kilo-ohms
When an alternate power source is connected, the
P-channel MOSFET is turned off. Input power is
supplied to the charger from the alternate power
source through the Schottky diode. The P-channel
MOSFET prevents current from being supplied on
VBUS. The alternate power source must provide a
voltage greater than the maximum VBUS specification,
minus the difference between the forward voltage
drop of the MOSFET body diode and the forward
voltage drop of the Schottky diode. An absolute
minimum voltage of 5V is recommended for the
alternate power source. The MCP73861 is rated for a
maximum input voltage of 12V, so higher voltages can
be utilized.
The maximum charge current is increased to 1.2A by
pulling the MCP73861 PROG input to ground. The
additional N-channel MOSFET is turned on when the
alternate power source is present. Increasing the
charge current significantly decreases the charge cycle
time.
Figure 5 depicts complete charge cycles utilizing the
MCP73861 with a high-power USB port and an
alternate power source. The charge cycles depicted
were performed on a 1400 mAh Li-Ion battery pack.
Initial conditions were near full discharge.
© 2005 Microchip Technology Inc.
DS00971A-page 3
AN971
While charging from the USB port, it takes
approximately one hour longer until the end of charge
is reached. The MCP73861 scales the charge termination current proportionately with the fast charge current.
The result is an increase of 40% in charge time with the
benefit of a 2% gain in capacity and reduced power
dissipation. The change in termination current 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.
+6V
VBUS
GND
STAT1 STAT 2 EN VSS2
16
15
14
13
VSET
1
12
2
11
SUSPEND
D+
D-
VDD1
USB CONTROL
VDD2
VSS1
100 mA
500 mA
VBAT
VBAT3
+ Single
- Lithium-Ion
Cell
VBAT2
MCP73861
3
10
4
9
5
6
7
VBAT1
VSS3
8
PROG THR EF TH ERM TIMER
CTIMER
RPROG
RT1
RT2
FIGURE 4:
USB-powered or Externally-powered, Single-Cell, Li-Ion/Li-Polymer Battery Charger
with Cell Temperature Monitoring.
.
MCP73861 Charge Cycles, 1400 mAh Battery
VBAT @ Wall Adapter Input
1.40
Voltage (Volts)
4.00
1.20
VBAT @ USB Input
3.50
1.00
3.00
IBAT = 1.2A
2.50
0.80
2.00
0.60
1.50
0.40
1.00
IBAT = 420 mA
0.50
0.00
0.0
50.0
100.0
150.0
Current (Amps)
4.50
0.20
0.00
200.0
Time (minutes)
FIGURE 5:
DS00971A-page 4
MCP73861 Charge Cycle Waveforms.
© 2005 Microchip Technology Inc.
AN971
CONCLUSION
REFERENCES
When utilizing Microchip’s MCP73853/55 or
MCP73861, harnessing the power of the USB port for
battery charging becomes extremely simple. The
devices adhere to all the specifications governing the
USB power bus.
1.
2.
Three stand-alone linear charging solutions for Li-Ion/
Li-Polymer batteries were presented. The guidelines
and considerations presented in this application note
should be taken into account whenever the power of a
USB port is interfaced.
3.
4.
5.
6.
7.
8.
9.
© 2005 Microchip Technology Inc.
Universal Serial Bus Specification, Revision 2.0
MCP73853/55 Data Sheet, “USB Compatible
Li-Ion/Li-Polymer Charge Management Controllers”, DS21915, Microchip Technology Inc.,
2004.
MCP73861 Data Sheet, “Advanced Single or
Dual Cell, Fully Integrated Li-Ion/Li-Polymer
Charge Management Controllers”, DS21893,
Microchip Technology Inc., 2004.
David Linden, Thomas B. Reddy, Handbook of
Batteries, Third Edition (New York: McGraw-Hill,
Inc., 2002).
AN947: “Power Management in Portable Applications: Charging Lithium-Ion/Lithium-Polymer
Batteries”, Scott Dearborn, DS00947, Microchip
Technology Inc., 2004.
ADN008: “Charging Simplified for High Capacity
Batteries”, Bonnie Baker, DS21864, Microchip
Technology Inc., 2004.
Http://www.powercellkorea.com
Http://sanyo.com/batteries/lithium_ion.cfm
Http://lgchem.com/en_products/electromaterial/battery/battery.html
DS00971A-page 5
AN971
NOTES:
DS00971A-page 6
© 2005 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
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Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
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•
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Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
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DS00971A-page 7
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DS00971A-page 8
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