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Sep 23, 2005

Keywords: USB, USB Charger, Li+ USB charger,
Lithium Ion USB charger, NiMH USB charger, USB battery
charger, charging batteries from USB, charging from USB,
charger, battery charger
Related
parts

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APPLICATION NOTE 3607

Charging batteries from USB

Abstract: USB
offers great opportunities as a power source for all types of
low power electronics, many of which are battery operated. The
widespread availability of USB presents unique opportunities
as well as challenges for battery-charging designs. This
article describes how to interface a simple battery charger to
a USB power source. This review of USB power bus
characteristics include an overview of NiMH and Li+ battery
technologies, charging methods, and charge termination
techniques, and a complete circuit example for smart-charging
NiMH cells from a USB port.

Introduction
The Universal
Serial Bus (USB) port is a bidirectional
data port with power and ground. Peripherals of all types can
be connected to the USB, including external drives, memory
devices, keyboards, mice, wireless
interfaces, video and still cameras, MP3 players, and
countless other electronics. Many of these devices are battery
powered, some with internal batteries. The widespread
availability of USB presents unique opportunities, as well as
challenges, for battery-charging designs. This paper describes
how to interface a simple battery charger to a USB power source.
This review of USB power bus characteristics includes voltage,
current limits, inrush
current, connectors, and cabling. An overview of nickel
metal hydride (NiMH) and lithium battery technologies,
charging methods, and charge-termination techniques is given.
A complete example circuit for smart-charging NiMH cells from
a USB port is presented, along with charging data.

USB characteristics
The USB bus can provide power for
low-power electronics. It is isolated from power mains and is
relatively well regulated. However, there are limitations on
available current and potential interactions between the load
and the host or power source.

The USB port consists of
a 90Ω bidirectional differential
shielded twisted pair, VBUS (+5V power), and
ground. These four wires are shielded with an inner shield of
solid aluminum and a stranded outer shield. Copies of the USB
2.0 specification are available free of charge from the USB organization.
Full compliance with the specification requires bidirectional
communication between the device and the host through a
function controller. The specification defines a unit load as
100mA (max). The maximum current that any device is allowed to
draw is five unit loads.

USB ports are classified as
either low-power ports, which supply up to one unit load, or
high-power ports which supply up to five unit loads. When
devices are first connected to the USB port, an enumeration
process identifies the device to determine its load
requirements. During this time, the device is only allowed to
draw one unit load from the host. After the enumeration
process, higher powered devices are permitted to draw higher
current if the power-management software in the host allows
it.

Some host systems (including downstream USB hubs)
have current limiting either through fuses or active current
sensors. If a USB device presents a high current (over one
unit) load to the USB port without enumerating, it can cause a
detectable overcurrent condition that could shut
down one or more of the USB ports in use. Many
commercially available USB devices, including stand-alone
battery chargers, draw over 100mA without a function
controller to handle the enumeration process; they run the
risk of causing problems for the host under the wrong
circumstances. For instance, if a device drawing 500mA is
plugged into a bus-powered USB hub, it could overload both the
hub port and the host port if it is not properly enumerated.

Further complications arise when the host operating
system is using advanced
power management, especially for notebook computers, and
is expecting the port current to be extremely low. In some
power-saving modes, the computer issues suspend commands to
USB devices, which are then expected to go to a low-power
mode. It is always a good idea to include a function
controller to communicate with the host even with low-power
devices.

The USB 2.0 specification is quite thorough
and specifies power quality, connector construction, cable
materials, allowable voltage drops, and inrush current.
Low-current and high-current ports have different
power-quality specifications. These are determined primarily
by the voltage drop in the connectors and cabling between the
host and the load, including voltage drop across a USB-powered
hub. A host, such as a computer or self-powered USB hub, has
high-current ports capable of supporting up to 500mA. Lower
current ports are found on passive, bus-powered USB hubs.
Table 1 gives the allowable tolerances for the voltage
at the pins on the upstream (source) side of the USB port for
high- and low-current ports.

Table 1. USB 2.0
specification power-quality standards

Parameter

Requirement

DC voltage, high-power port*

4.75V to 5.25V

DC voltage, low-power port*

4.40V to 5.25V

Maximum quiescent
current (low power, suspend mode)

500µA

Maximum quiescent current (high power, suspend
mode)

2500µA

Maximum allowable Input capacitance (load side)

10µF

Minimum required output capacitance (host side)

120µF ±20%

Maximum allowable inrush charge Into load

50µC

*These specifications apply
to the pins of the host or hub port connector on the upstream
side. Additional I x R drops due to cables and connectors must
be counted separately.

In hosts that are compatible
with the USB 2.0 specification, the upstream side of a
high-power port is provided with 120µF of low-ESR capacitance.
The input capacitance of attached USB devices is limited to
10µF, and the total allowable charge drawn from the host (or
powered hub) during an initial load connection is 50µC. Thus,
when a new device is connected to a USB port, the transient
voltage drop at the upstream port is less than half a volt. If
more capacitance is required for correct operation of the
load, it must be provided with an inrush current limiter to
charge the larger capacitance at no greater than 100mA.

The allowable DC voltage drops for a USB port having a
bus-powered USB hub with low-powered functions attached are
shown in Figure 1. A high-power load connected to an
unpowered hub has larger voltage drops than shown in Figure 1
and can overload the bus.

Figure
1. Drops larger than these allowable DC voltage drops from
host to low-power load can overload the bus.

Battery-charging requirements

Single-cell lithium
ion and lithium polymer
Present-day lithium
chemistries are typically 4.1V to 4.2V when the cells are
charged to their maximum-rated capacity. Newer, higher
capacity cells are being marketed with voltages in the 4.3V to
4.4V range. Typical prismatic lithium ion (Li+) and lithium
polymer (Li-Poly) have capacities of 600mAh to 1400mAh.

The preferred charge profile for both Li+ and Li-Poly
cells is to start the charge with a constant charge current
until the cell voltage reaches the rated voltage. When this
occurs, the charger then regulates the voltage across the
cell. These two regulation states are called constant-current
(CC) and constant-voltage (CV) charging; therefore, this type
of charger is usually referred to as a CCCV charger. When the
CCCV charger is in CV mode, the current into the cell begins
to drop. For typical charge rates, between 0.5C and 1.5C, the
transition between CC and CV mode occurs when the cell has
accepted about 80% to 90% of its full-charge capacity. Once
the charger is in CV charging mode, it then monitors the cell
current; when it reaches a low threshold (milliamps or tens of
milliamps), the charger terminates charge. A typical charge
profile for a lithium chemistry battery is shown in Figure
2.

Figure
2. Typical results are shown for a Li+ battery charged using a
CCCV charger.

The USB voltage drops shown in Figure
1 show that a low-powered port, on the downstream side of a
port-powered hub, would barely have enough headroom to charge
a cell to 4.2V. Small amounts of additional resistance
in the charge path could prevent proper charge.

Li+
and Li-Poly cells should be charged at moderate temperatures.
Manufacturers' recommended maximum charging temperatures are
typically in the +45°C to +55°C range, with allowable
discharge temperatures about 10°C higher. The materials used
in these cells are highly reactive and can ignite if the cell
temperature exceeds +70°C. Chargers for lithium chemistry
cells should be designed with a thermal cutoff circuit that
monitors cell temperature and terminates charging if the cell
temperature exceeds manufacturer-recommended maximum charging
temperatures.

Nickel metal hydride cells
NiMH cells are heavier and
have less energy density than lithium-based cells.
Historically, they have been less expensive than lithium, but
the price gap has recently been shrinking. NiMH cells are
available in standard sizes and are a direct replacement for
alkaline cells in most applications. Nominally, each cell is
1.2V, and when fully charged they are as high as
1.5V.

NiMH batteries are usually charged with a
constant-current source. As they reach the fully charged
state, an exothermic chemical reaction occurs that causes the
battery temperature to increase and the terminal voltage to
decrease. Either the rate of rise of the battery temperature
or a negative voltage change can be detected and used to
terminate the charge. These termination methods are known as
dT/dt and -ΔV, respectively. At very low charge rates, the
dT/dt and -ΔV effects become less pronounced and can be
difficult to detect accurately. The dT/dt and -ΔV responses
begin as the cell starts to become overcharged. Continued
charging beyond this point can damage the
cell.

Termination detection at charge rates above C/3
is much easier than at low charge rates. The temperature rise
is about 1°C/min, and the -ΔV response is more pronounced than
at lower rates. After fast-charge terminates, an additional
charging period at a reduced current is recommended to top off
the cell (top-off charge). When the top-off charge cycle is
done, a trickle-charge current of C/20 or C/30 counters the
effects of self-discharge and maintains the battery in a fully
charged state. A graph showing the cell voltage during a
charging cycle of a partially charged NiMH cell using a DS2712
NiMH charger is shown in Figure 3. In this graph, the
upper curve is data taken while current is being driven into
the battery; the lower curve shows data taken with the charge
current source turned off. In the DS2712, this difference in
voltage is used to distinguish between NiMH cells and alkaline
cells. If an alkaline cell is detected, the DS2712 will not
charge it.

Figure
3. A DS2712 charge controller is used to charge a NiMH
cell.

Switching vs. linear
The USB 2.0 specification allows
up to 100mA from a low-power port and 500mA from a high-power
port. If a linear pass element is used to regulate charge
current to the battery, these are the maximum allowable charge
currents. The power dissipated in the linear pass element
(Figure 4) is P = VQ x IBATT.
This causes power dissipation in the pass transistor and can
require the use of a heat
sink to prevent overheating.

Figure
4. Power dissipation is the battery current times the voltage
across the pass transistor.

For a nominal input
voltage of 5V, the pass element dissipates a varying amount of
power based on the type and number of cells, as well as the
battery voltage.

Figure
5. Power is dissipated in a linear pass element, with NiMH
cells being dissipated from a USB port at 5.0V input
voltage.

Figure 5 shows the calculated power
dissipation for a linear USB charger with NiMH cells at a
nominal input voltage of 5.0V. With single-cell charging, a
linear charger is only about 30% efficient; a two-cell charger
is 60% efficient. Charging a single cell at 500mA results in
up to 2W of power dissipation. This amount of power typically
requires a heatsink.
At 2W of dissipation, a +20°C/W heatsink heats up to about
+65°C from an ambient
temperature of +25°C, and it needs exposure to free air
for full performance. Inside an enclosure with still air, it
will get much hotter.

Several problems are solved by
using a switching-regulator-based charger. First, the cells
can be charged faster and at a higher current than with a
linear charger (Figure 6). Because less power is lost
as heat, the thermal
management problem is reduced. Also, the charger is more
reliable, as it runs cooler.

Figure
6. Charge times differ for a linear charger vs. a switching
charger when charging a single-cell NiMH
battery.

The calculated values in Figure 6 are
based on charging from a high-power USB port at about 90% of
the 500mA maximum allowable current. The switching
regulator in this example is assumed to be a
nonsynchronous buck converter with 77% efficiency.

Circuit example
The circuit shown in Figure 7
is a switch-mode buck
regulator for charging a single NiMH cell. It uses a
DS2712 charge controller to regulate charging current and
terminate charge. The charge controller monitors temperature,
battery voltage, and battery current. If the temperature is
above +45°C or below 0°C, the controller does not begin
charging the battery.

Figure
7. Schematic shows a single-cell NiMH charging from a USB
port.

In Figure 7, Q1 is the switching power
transistor for the step-down charger; L1 is the smoothing
inductor; and D1 is the freewheeling, or catch, diode.
Input C1 is a 10µF, very low-ESR ceramic filter capacitor.
Substituting tantalum or other electrolytic capacitors for C1
can adversely influence charger performance. R7 is the
current-sense resistor for the sense amplifier
of the current regulator. The reference voltage for the DS2712
is 0.125V with an hysteretic component of 24mV. Closed-loop,
switch-mode current control is provided through CSOUT. Gate
drive to Q1 is enabled when the gate of Q2 is pulled low by
the charge control pin, CC1. Both Q1 and Q2 are low-Vt
(gate-source threshold voltage) pMOSFETs. When CC1 and CSOUT
are both low, the drain to source voltage of Q2 is slightly
over one Vt. This voltage, plus the forward voltage drop of
CSOUT, establishes the available switching-gate drive voltage
for Q1. When CC1 is low, it enables closed-loop control of the
current. Startup switching waveforms are shown in Figure
8. The top waveform is the voltage across the 0.125Ω
current-sense resistor, and the bottom waveform is the Q1
drain to GND voltage. Initially, current ramps up in the
inductor while Q1 is ON (CC1 and CSOUT both low). When the
current reaches 0.125V, CSOUT goes high. Also, current ramps
down until the voltage on the current-sense resistor reaches
about 0.1V, whereupon CSOUT goes low again. This process
continues as long as CC1 is low.

Figure
8. Startup waveforms are shown for a USB NiMH
charger.

The DS2712's internal state machine
controls the gating action of CC1. At the beginning of charge,
the DS2712 performs cell qualification tests to make sure that
the cell voltage is between 1.0V to 1.65V, and to verify that
the temperature is between 0°C and +45°C. If the voltage is
below 1.0V, the DS2712 gates CC1 low at a duty factor of
0.125, which slow-charges the cell to prevent damage. Once the
cell voltage exceeds 1.0V, the state machine transitions into
fast-charge. The fast-charge duty factor is 31/32, or about
97%. The "skipped" pulse is used to perform impedance
tests on the cell to make sure that a high-impedance cell,
such as an alkaline cell, has not been installed in the
charger. Fast-charge continues until a ΔV of -2mV is detected.
If no -ΔV is detected, fast-charge continues until the
fast-charge timer expires or until an overtemperature or
overvoltage fault condition (including impedance failure) is
detected. When the fast-charge is done (either due to -ΔV or
expiration of the fast-charge timer) the DS2712 enters its
timed topoff mode, with a duty factor of 12.5% and a duration
of half of the programmed fast-charge timeout. After topoff is
complete, the charger goes into maintenance mode with a duty
factor of 1/64, and remains in maintenance until the cell is
removed or power is cycled.

The charger shown in
Figure 7 fast-charges a 2100mAh NiMH cell from a high-power
USB port in a little over two hours, with a full top-off
charge achieved in about three hours. Current drawn from the
port is 420mA. If enumeration with the host and high-current
enable is required, an open-drain
NMOSFET can be inserted in series between R9 and ground. If
the MOSFET
is off, TMR floats and the DS2712 is in its suspend
state.

Summary
The USB port is an economical and practical
power source for charging batteries for small consumer
electronics. To be fully compliant with the USB 2.0
specification, loads connected to the USB port must be capable
of bidirectional communication with the host. Loads must also
comply with power-management requirements, including low-power
modes and a means of allowing the host to determine when high
power is drawn from a port. While partially compliant systems
may operate compatibly with most USB hosts, they occasionally
give unexpected results. A good understanding of USB
requirements and expectations of the load are needed to make
the right tradeoff between full compliance and load
complexity.

A similar version of this article appeared
in the January, 2006 issue of Battery Power Products
Technology magazine.

Related parts

DS2711

Loose Cell NiMH
Chargers

Free
samples

DS2712

Loose Cell NiMH
Chargers

Free
samples

DS2715

NiMH Battery Pack
Charge Controller

Free
samples

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Download,
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APP 3607: Sep 23, 2005

AN3607, AN 3607, APP3607,
Appnote3607, Appnote 3607

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