Battery Power - March/April 2013 - (Page 10)
Feature
Simplify Peripheral Connectivity and Extend Battery Life
In Mobile Applications
Timothy Hegarty, Principal Applications Engineer,
Isolated and Protection Solutions
Texas Instruments
With the advent of and voracious demand for new handset,
tablet and notebook devices with lower profile and smaller form
factors, engineers called upon to design solutions that fit into the
required footprint dimensions face a particularly arduous task.
Furthermore, consumers, while seeking increased functionality
and features with their portable devices, place a substantial premium on battery capacity by seeking longer run time between
charge cycles. The upshot is that the battery ends up vying for
valuable device real estate with the system board, and the PCB
area available to portable device designers is even more restricted than at first glance.
The system power demands of these devices are inherently
more demanding with new audio, video and processing specifications. For example, smartphone handsets now must seamlessly accommodate multi-mode 3G and 4G wireless communication standards, latest generation wireless local area networks
(WLAN), Bluetooth and near field communication (NFC)
protocols, touch screen control, and a multi-core, multi-GHz
applications processor platform1. Power management of the
subsystem blocks becomes a key concern, both in active mode
and sleep mode operating regimes. To minimize battery drain,
it becomes strategically advantageous to entirely disconnect a
particular sub-system block when it is not required and reconnect when needed.
Also, a must have for these devices are connection ports such
as USB, stereo headphone mini-jack or proprietary connection
that enable a user to hookup various accessories to their device.
However, power transfer from a portable device to a downstream accessory needs to be carefully managed to avoid inrush
current and mitigate voltage transient events upon connection
and disconnection. The accessory typically sends a handshake
signal to indicate that it is ready to accept downstream power
transfer. Only then should the device’s CPU or system controller authorize current flow. Well-defined overload protection is
also mandated to smoothly charge large capacitive loads upon
peripheral connection and to reliably manage fault modes that
may occur at the load.
Peripheral Connectivity Power Management
There are a number of ways to solve and overcome the
peripheral connectivity application challenges referenced above,
the most convenient being a load switch IC implementation.
This is a series power switch that provides a smart and cost-effective connect/disconnect power distribution function. Specifically, the power switch is implemented using a MOSFET in the
power path with its body diode directed such that it cannot pass
the load current when the MOSFET is off.
10
Battery Power • March/April 2013
Of course, it is critical to minimize power dissipation to
reduce die operating temperature, particularly with small footprint ICs. MOSFET switch on-state resistance has a positive
temperature coefficient, which means an increase of junction
temperature compounds upon itself to increase even further until
a steady state thermal equilibrium is obtained. What is more,
the local ambient and PCB temperatures in high-density PCB
layouts can become elevated quickly due to the mutual heating
effect inherent with high-power dissipation from immediately
adjacent power components, for example: RF PA modules, DC/
DC switching regulators, low noise LDO linear regulators,
class-D audio amplifiers, display drivers, battery charger ICs,
and the like1. These devices can dissipate significant heat, at
least on a transient basis. The power and ground planes common
to and layered beneath the devices act as heat spreaders throughout the PCB. The thermal properties of the IC package are key
to achieving the desired thermal performance. Specifically,
low-thermal impedance from junction to soldered pin serves to
conduct a large portion of the heat flux through the device pins
and utilizes the PCB substrate as a heatsinking element.
Incorporated in the solution with the power MOSFET should
be an accurate current sense functional block, a series current
sense resistor for example, to protect the input supply in case of
large bulk capacitance, over-current or short circuit at the load.
Optimal solutions can then avail of a brick-wall current limit
characteristic. This leverages an internal current-mode loop that
regulates the load current to an essentially constant setpoint as
the output voltage falls from its nominal level during an overload event. Moreover, it is important to facilitate smooth entry
into and recovery from current limit as substantial inductive
circuit parasitics can occur with long connection lines.
Circuit Implementation
A newly designed smart power switch IC solution in ultraminiature package combining low on-state resistance and
accurate current limit protection2 is now available to address
the peripheral connectivity requirements described above. The
LM34904 from TI comes in a 1.2-mm by 0.8-mm chip scale
package and is rated for 0.5 A continuous current. The schematic
in Figure 1 demonstrates how this power switch connects to a
device CPU (or microcontroller) and an accessory load.
The power switch IC implementation saves board space,
extends battery life and monolithically integrates several safety
and protection features. For example, the CPU can check the
status of an accessory detect control input and selectively turn
on (or off) the MOSFET switch using an enable input. This provides power to the accessory or dynamically conserving power
when needed. An open-drain output flag provides switch status
to the CPU. Power transfer can also be initiated based on this
signal transition.
www.BatteryPowerOnline.com
http://www.BatteryPowerOnline.com
Table of Contents for the Digital Edition of Battery Power - March/April 2013
Battery Power - March/April 2013
Contents
Editor's Choice
GTS and RGIS Launch Battery Test & Replace Service for Mobile Devices
Leyden Energy Raises $10 Million for Commercialization of Silicon Anode Mobile Pouch Cells and Automotive Start-Stop Batteries
Features
Circuit Protection Approach for High-Rate Discharge Li-Ion Battery Application
Battery Power 2013 Conference Preview
Simplify Peripheral Connectivity and Extend Battery Life in Mobile Applications
Managing Lithium-Chemistry Batteries: It’s Mostly
About Their Temperature
Lead Acid Batteries: A Proven Technology Marches
Forward
New Products
Batteries
Components
Charging & Testing
Power Supplies & Conversion
ICs & Semiconductors
Departments
Industry New
Marketplace
Battery Power - March/April 2013
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2017spring
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2016winter
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2016fall
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2016summer
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2016spring
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2015winter
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2015fall
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2015summer
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2015spring
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2014fall
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2014summer
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2014spring
https://www.nxtbook.com/nxtbooks/webcom/batterypower_2014winter
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20131112
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20130910
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20130708
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20130506
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20130304
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20130102
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20121112
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20120910
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20120506
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20120304
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20120102
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20111112
https://www.nxtbook.com/nxtbooks/webcom/batterypower_20110910
https://www.nxtbookmedia.com