June 2015
6
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NTERNET
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HINGS
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PPLICATIONS
Extending the battery life
of wearable devices
By Landa Culbertson,
Mouser Electronics
This article gives just a small look at
low power management technology
and how products can work together
to create the ultra-low power systems
to drive the emerging wearables
market and other applications where
conserving battery life and/or conser-
ving harvested energy is critical.
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24/04/15 12:18
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To gain market acceptance and meet the
projected 380 million units in use worldwide
by 2018, extending battery life in wearable
devices is critical. Energy harvesting, wireless
charging, battery management, power man-
agement and low power solutions are all con-
siderations. Energy harvested from the body
(heat or movement), or from the environment
(ambient light), yields insu cient power for
something like a smartwatch. Texas Instru-
ments Ultra Low Power Harvester Power Man-
agement IC with Boost Charger bq25570, can
take 300 to 400 millivolts collected by energy
harvesters and boost it to 3 to 5 volts - enough
to charge a battery. Although insu cient to
power a smartwatch, it could extend its run-
time and also includes a highly e cient nano-
power buck converter to provide the option of
a second power rail to the system.
e new TI bq25100 single-cell Li-ion charger
provides a tiny solution for traditional battery
charging via a USB port or adapter. Half the
size of existing charger solutions, it supports
low cost unregulated adapters which may be
used in the cost-sensitive wearables market.
Maxim’s highly integrated MAX14676/76A
Wearable Charge Management Solution ICs
provide another option for battery charge
management in wearable applications. ey
include not only a linear battery charger, but
also numerous low-power powermanagement
peripherals to save system board space while
extending battery life. Wireless charging via
a charging base is popular and convenient for
consumers. Qi is an international standard for
interoperability of wirelessly charged device
- any Qi certi ed wireless power receiving
device, such as the Moto 360 smartwatch,
can work with any Qi certi ed charging base.
Wireless charging solution reference design
from TI, TIDA-00318, suits 135mA charge
current applications and is just 5x15mm2.
Any wearable implementing the TIDA-00318
design should be able to gain Qi certi cation
and work with any Qi charging base. For an
even smaller wireless charging receiver solu-
tion capable of delivering up to 2W of power,
TIDA-00329 reference design is only 5.23mm
x 5.48mm. On the wireless power transmit-
ter or charging-base side, TIDA-00334 ref-
erence design is a low power design laid out
in a 30mm area that matches the diameter of
the round Wurth coil 760308101103 - slightly
larger than a 2 Euro coin.
Ultra-low power conversion is critical to
achieving optimal battery life in wearable
devices. Some of the latest low power prod-
ucts or highly e cient dc-dc converter prod-
ucts currently available include: TI TPS727xx
series of 250mA LDOs with ultra-low Iq of just
7.9µA, very low dropout, high PSRR of 70dB
at 1kHz for quiet performance in RF appli-
cations, and TI TPS82740A and TPS82740B
200mA step-down converter modules with
95% conversion e ciency, 360nA Iq during
active operation and 70nA during standby.
Step-up or boost conversion is generally not
as e cient as step-down conversion, so step-
ping up from the battery voltage is o en nec-
essary to power various circuits in the system,
especially displays. New Maxim 1A step-up
converter, MAX8627, generates a boosted out-
put voltage from 3V to 5V from a single-cell
Li-ion battery, with up to 95% e ciency and
consumes just 20µA Iq. Silicon Labs TS33x
step-up converters have Iq as low as 150nA
and step up input voltage from 0.9V to 3.6V to
eight selectable output voltages ranging from
1.8V to 5V.
Higher power consuming functions such as
processing and displaying can be o oaded to
a smartphone, tablet, or PC. Bluetooth Smart
or Bluetooth Low Energy now automatically
built into most new smartphones is the de facto
wireless communication standard for wear-
ables. Optimized for low power operation, TI
Bluetooth Wearable Watch Development Sys-
tem TI Meta Watch enables rapid development
of connected watch applications and includes a
watch with a display, a 3 ATM water resistant
stainless steel case, leather strap, a mineral glass
crystal, vibrating motor, three-axis accelerome-
ter and an ambient light sensor.
