Manage Batteries Better
in Bluetooth Headsets
By Shadi Hawawini, Applications Engineer,
and George Paparrizos, Director of Marketing,
Summit Microelectronics, Sunnyvale, Calif.
Understanding safety, size and charging requirements, as well as innovative charging
methods, helps designers optimize battery
performance in Bluetooth applications.
T
he Bluetooth wireless
most of the main functions, such as
standard has experienced a
central processing, the transceiver
Bluetooth headset system
Li-polymer
battery
tremendous adoption rate
subsystem and several I/O interfaces.
Battery
2.8 V to 4.2 V
charger
over the last five years. IniThe audio codec and the speaker
80 mAh to
150 mAh
tial obstacles that hindered
driver comprise the second-mostBluetooth
Stepdown
first-generation devices have been
important system component. The
SoC
regulator
overcome and new applications have
codec, the speaker driver and the
Memory
Linear
been discovered. While the handset
corresponding algorithms determine
regulator
Audio codec
is the largest application for this
audio quality and signal integrity. The
and speaker
technology, the headset is the secondrequired memory for the system can
largest segment and a fast-growing
be integrated on the SoC or supplied
one. Improvements in industrial Fig. 1. In a typical Bluetooth headset system, by a dedicated memory chip.
design (size and fit), falling prices and the SoC integrates processing, transceiver
These various blocks in the design
new laws requiring hands-free calling and I/O functions. Memory, audio codec
usually require two to three differindicate a bright future for Bluetooth and the speaker, and power circuitry are
ent voltage rails for operation: one
success in the marketplace.
the other main circuit
elements.
External
for powering the digital core, one
Figure 1
With this rapid growth have come battery-charging circuitry is preferred.
for powering the I/Os and one for
battery-management challenges for
the audio subsystem. These power
Bluetooth headset designs, with the focus on safety, size
requirements are addressed by a combination of stepdown
and charging time. Although industrial design often takes
regulators and low-dropout linear regulators, all of which
precedence over design considerations of the battery and
are powered directly from the battery. The main areas of
battery-charger ICs, new technologies and battery-charging
focus on the power-system implementation are low noise
solutions are allowing these two previously contradicting
(because of Bluetooth’s RF sensitivity), high efficiency (for
goals to exist together. Modern Bluetooth headsets may
offering longer talk time and reducing thermal dissipanow be designed for a small solution size and attractive
tion) and minimum board space (for accommodating the
industrial design, while offering extended talk times and
required industrial design).
fast and safe battery charging.
Every headset also requires battery-charging circuitry for
To better understand how these solutions are poscharging the embedded battery and steering power to the
sible, let’s take a closer look at typical Bluetooth system
system. Some Bluetooth SoCs incorporate battery-charging
implementations. This will be followed by showing how
functionality. However, external battery-charging solutions
an innovative method for charging Bluetooth headsets via
are increasingly used to meet new system requirements, like
another portable device can be useful.
faster charging, reduced thermal dissipation on the SoC and
stricter safety features.
A Typical Headset
Modern headsets use battery packs with Li-polymer
A system diagram of a typical Bluetooth headset is shown
cells, which allow more flexible and thinner form factors
in Fig. 1. The Bluetooth system-on-chip (SoC) integrates
than Li-ion cells. Using polymer electrolytes, this battery
30
Power Electronics Technology October 2008
www.powerelectronics.com
technology allows the use of a thin foil casing, since it does
not require external pressure between the electrodes and
the separator.
This material advancement has led to products with
industrial designs that are both more usable and more
attractive for the consumer, because the battery can be
manufactured to fit most form factors. Typical battery capacities for mono headsets are 80 mAh to 150 mAh, while
stereo headsets use battery packs with capacities up to 500
mAh. Most modern system designs can be charged both
via an external wall adapter (a USB
or proprietary physical interface) or
a USB port on a notebook PC.[1]
Fig. 2. Chip-scale
packaging has
enabled the
miniaturization
of linear batterycharger circuits.
Size and Weight Challenges
Two of the main differentiating
features and selling points among
Bluetooth headset products are size
and weight. How small the product
dimensions and how low the weight
can be are highly dependent on the
battery pack used and its capacity,
as well as on the system design. Using battery-charging solutions that
are offered in chip-scale packaging
(CSP)[2], and which integrate many
of the required system functions
such as secondary system and battery protection, allows for minimum
board space.
CSP in its simplest form requires
three basic steps: the addition of a
passivation layer on top of the silicon
die, the deposit of an under bump
metallization (UBM) stack for forming the UBM pad, and the attachment
and reflow of the solder ball. This
technology allows the device size
to be identical to the die size, which
results in a very small chip size compared with DFN or QFN packaging
equivalents. In addition to the obvious area savings, CSP is generally
electrically superior to wire-bonded
packaging, offers an extremely low
profile (as low as 0.6 mm) and can
have a lower junction-to-ambient
resistance, thereby providing better
power dissipation.
The benefits of space savings and
cost outweigh the challenges associated with layout, given the small pitch
(0.4 mm or 0.5 mm) of such solutions.
However, most IC manufacturers
design the ball array configuration
such that inner balls handle digital,
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Power Electronics Technology October 2008
battery management
electrostatic discharge protection diode or
another IC that provides such functionality
Input
Battery
overvoltage
(and protection) between the power source
overvoltage
1-A fuse
protection
protection
Wall adapter,
and the system.
BATT
IN
car adapter
Battery
Li-ion
or
Even more significant is the protection
4.7
µF
battery + pack
USB port
against a faulty, stressed or unauthorized
Transient_ with
protection
voltage
THERM
battery pack. Each one of these cases can
IC
suppressor
SDA
DC = 6.2 V
result in a battery overvoltage or overtemBattery
SCL
Transient = 10 V
overtemperature
perature runaway situation with a high
USB500/100
protection
probability of battery explosion. In addiGND
tion to the battery protection that is located
inside the battery pack (for overvoltage,
undervoltage and overcurrent protection),
Fig. 3. Integrating battery-protection devices can eliminate the need for discrete com- it is recommended that additional safety
ponents or additional ICs (dotted box) and result in significant space and cost savings. measures be taken. Many systems monitor
charging time and battery-pack temperature to ensure stable environmental conditions during the
4.400
charging process. Furthermore, secondary-battery overvoltage protection for guarding against a protection IC failure
4.200
is also offered in modern battery-charging solutions.
Figure
3
4.000
Some of these protection features, commonly offered
3.800
by discrete components or additional ICs (see the dotted
3.600
box in Fig. 3) can be eliminated if integrated in the battery0.5C charging
charging IC. Such integration can result in significant space
3.400
1C charging
and cost savings, which also can be seen in Fig. 3.
2C charging
Battery pack voltage (V)
Power source
Power
source
output
voltage
Intelligent
battery charger
3.200
3C charging
3.000
2.800
0.00 0.20
0.40
0.60
0.80 1.00
Charge time (hours)
1.20
1.40 1.60
Fig. 4. Using higher charging rates such as charging currents can
save battery-charging time.
low-current signals that require only narrow traces while
the power traces are on the outside, which greatly simplifies
Battery pack charge time comparison (130 mAh, 25°C)
the layout of such
devices. The high and fast-growing adoption rate of this packaging
type in high-volume portable
Figure 4
applications demonstrates the maturity and manufacturability of this technology (Fig. 2).
System Safety
The inherent sensitivity of Li-based rechargeable batteries has introduced strict safety requirements for new
systems. Many new designs incorporate secondary protection features, thereby adding a new layer of protection
outside the battery pack. Many of the new safety requirements are proactively addressing extreme consumer and
system behavior by taking into account potential failures
in the complete system chain: power source, battery pack
and device.
One of the most common failure cases is the use of a
faulty, noncompliant or poorly regulated power source
(wall adapter, car adapter, etc.). This can lead to overvoltage
conditions at the input of the battery-charger IC, and consequently to device failure. A common protection against such
failures is the addition of a transient-voltage suppressor, an
32
Power Electronics Technology October 2008
Battery-Charging Challenges
Battery technology has played a major role in the adoption of handheld equipment in today’s society. Nowadays,
advancements in the Li-based battery technology allow
a 6% capacity increase, on average, per year for a given
battery size. This, combined with the fact that newer Bluetooth SoCs reduce necessary power consumption, allows
Bluetooth headsets to use smaller batteries.
Despite this trend, the demand for faster charging is
another big differentiating factor for many headsets. Some
headset designers are requiring charging rates of 2C or even
3C, a requirement that SoCs with integrated battery chargers cannot accommodate, thereby creating the need for an
external battery-charging IC. The C rating for charging is a
normalized charging specification based on the fast charge
current and battery capacity. Therefore, 2C for a 130-mAh
battery translates to a fast charging at 260 mA, and at 3C the
fast charge current is 390 mA. These charging rates need to
be confirmed with the corresponding battery manufacturer
to ensure that they will be safe for the battery and not cause
any failures. Fig. 4 demonstrates the charge-time savings by
using higher charging rates (i.e., charging currents).
Another consideration for the battery charger is the
power dissipation (PDISS) in the pass element for a linear
battery charger such as Summit’s SMB139. The linear battery charger regulates the output voltage/battery voltage
(VBATTERY) and battery-charge current (ICHARGE CURRENT) by
dissipating the excess power from the input (VIN) as heat.
This describes the power loss of the linear battery charger:
PDISS = (VIN – VBATTERY) ICHARGE CURRENT . (Eq. 1)
www.powerelectronics.com
battery management
USB
Battery
current
Batterycharger
IC
Battery
current
Connection
via micro A/B
USB cable
D+ VBUS
USB On-the-Go
5 V at 250 mA
ID GND D-
Li-ion
battery Batterycharger
+
IC
_
D+ VBUS
VBUS
current
Cellular phone
ID GND D-
System
System
current
Charge
current
Bluetooth headset
Li-ion
battery
+
_
System
USB
Fig. 5. Using the USB On-the-Go standard allows the charging of a Bluetooth headset by a cellular handset.
Eq. 1 should be used to ensure that the battery-charger
its battery to be charged by using power from the cellular
IC does not become too hot to cause the battery-charger IC
phone’s battery. Hence, battery charging is not limited by
to enter into thermal foldback. This is a condition in which
the absence of a USB port (notebook) and/or a wall power
the actual charge current is reduced from the expected valueFigure 5source. This implementation addresses a very realistic
to ensure the IC is not damaged, or that the battery-charger
consumer behavior scenario, given that a high number of
IC does not get hotter than what is desired for comfortable
cellular phone users also own a Bluetooth headset. PETech
consumer use during charging.
Knowing the package junction-to-ambient resistance
References
(θJA), the maximum junction temperature (TJ) and the
1. USB-IF, USB2.0 specification, www.usb.org.
ambient temperature (TA), the maximum power dissipation
2. FlipChip, www.flipchip.com/services/wafer_level/.
can be calculated as:
T - TA
PDISS = J
.
(Eq. 2)
q JA
Digital control of the battery-charger device (via an I2C
or a serial-parallel interface) provides more flexibility in
designs that take advantage of higher charge-current rates.
The real-time control allows charging to be enabled and
Model numbers
available with tabs:
well controlled based on certain system and environmental
BR1225A
BR2450A
parameters (battery-pack temperature, battery voltage, etc.),
BR1632A
BR2477A
making faster battery charging safe and effective.
BR2330A
BR2777A
Lithium Coin Cell Batteries
Portability
While usable battery life for many of the modern electronic equipment has reached acceptable levels, the goal of
true portability can only be achieved via innovative system
designs that allow a wide range of battery-charging alternatives for the user. An innovative and versatile charging
method is enabled by the increasing adoption of the USB
On-the-Go standard in new portable consumer devices.
While the aim of the On-the-Go standard is to address the
need for user friendliness and compatibility by allowing
portable devices to be connected to each other without
the need for a USB host (most commonly a PC), its power
attributes can be used for portable-to-portable device
charging.
Such an implementation is shown in Fig. 5. In this case,
the cellular phone is the power-providing device and delivers the required 5 V (±5%) and a predetermined current to
the connected Bluetooth headset. This power can be used
by the headset as the input to its battery charger, allowing
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3V, BR“A” series poly-carbonmonofluoride lithium
batteries operate at extended temperature range.
Features:
• Extended operating temperature range of -40°C to 125°C
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For More Information:
Toll Free: 1-877-726-2228 (1-877-PANABAT)
E-Mail: oembatteries@us.panasonic.com
Online: www.panasonic.com/batteries
33
Power Electronics Technology October 2008