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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
First Quarter, 2007
© Copyright 2007 Texas Instruments
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Texas Instruments Incorporated
IMPORTANT NOTICE
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enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
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TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI's standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for
their products and applications using TI components. To minimize the risks associated with customer products
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Analog Applications Journal
High-Performance Analog Products
1Q 2007
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Power Management
Selecting the correct IC for power-supply applications . . . . . . . . . . . . . . . . . . . . . . . . . . 5
To select the right power ICs for multiple voltage rails, factors like cost, solution size, power source,
duty cycle, and output power must be considered. This article shows how to determine the best solution
for a typical power-supply application. Included are criteria for choosing the best topology and ICs;
interface considerations; and a method for evaluating system efficiency.
LDO white-LED driver TPS7510x provides incredibly small solution size . . . . . . . . . . . . 9
White-LED backlighting for LCD screens is growing in popularity. Compared to historical solutions using
charge pumps or inductor boost converters, the LDO topology of the TPS7510x white-LED driver
eliminates EMI, requires no external components, and is available in chip-scale and QFN packages.
Power management for processor core voltage requirements . . . . . . . . . . . . . . . . . . . 11
High-performance processors put high demands on power supplies in terms of voltage tolerance, noise
suppression, and fast transient response. This article covers how TI’s new PTH power modules can
provide up to an 8× reduction in power-supply output capacitance, a faster response to load transients,
and enhanced stability when ultra-low ESR capacitors are used.
Amplifiers: Op Amps
Accurately measuring ADC driving-circuit settling time . . . . . . . . . . . . . . . . . . . . . . . . . 14
Measuring ADC driving-circuit settling time with an oscilloscope can be misleading due to parasitic
capacitance and inductance introduced by the probe. This article describes a practical method to
accurately measure driving-circuit settling time without additional components that may affect the
measurement. This method can easily be implemented as a built-in self-test function.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
To view past issues of the
Analog Applications Journal, visit the Web site
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Analog Applications Journal
1Q 2007
High-Performance Analog Products
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Introduction
Texas Instruments Incorporated
Introduction
Analog Applications Journal is a collection of analog application articles
designed to give readers a basic understanding of TI products and to provide
simple but practical examples for typical applications. Written not only for
design engineers but also for engineering managers, technicians, system
designers and marketing and sales personnel, the book emphasizes general
application concepts over lengthy mathematical analyses.
These applications are not intended as “how-to” instructions for specific
circuits but as examples of how devices could be used to solve specific design
requirements. Readers will find tutorial information as well as practical
engineering solutions on components from the following categories:
• Power Management
• Amplifiers: Op Amps
Where applicable, readers will also find software routines and program
structures. Finally, Analog Applications Journal includes helpful hints and
rules of thumb to guide readers in preparing for their design.
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Analog Applications Journal
High-Performance Analog Products
1Q 2007
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Texas Instruments Incorporated
Power Management
Selecting the correct IC for power-
supply applications
By William Hadden (Email: willhadden@ti.com)
Applications Engineer, High Performance Analog, Low Power dc/dc
Selecting the proper integrated circuit (IC)
for a power-supply application may seem like
an easy task. However, as newer consumer
electronics come out that require multiple
voltage rails, the task becomes more com-
plex. To select the correct IC(s) for the job,
many factors such as cost, solution size,
power source, duty cycle, and required output
power must be weighed. These factors must
be ranked by importance and the power
supplies selected accordingly. In this article,
we will determine the best solution for the
application shown in Figure 1.
Our example application is portable, requires
the lowest possible battery consumption and
a small form factor, and operates from a single-
cell Li-ion battery that is charged whenever the
12-V supply is available. We want to keep the
cost to a minimum; but this can be sacrificed
for space considerations, which are the most
important requirements, followed by the high-
est efficiency possible to extend battery life.
Selecting the best topology
First, let’s examine the power requirements
of each rail to determine what kind of dc/dc
converter should be used (i.e., inductive
switcher, linear regulator, or charge pump).
Inductive switchers are usually the best
choice for highest efficiency. The inductive
switcher circuits require a switching element,
rectifier, inductor, and input and output capac-
itors. For many applications, the solution size
can be reduced by choosing a device where
the IC switching element and rectifier are
internal. These circuits have typical efficien-
cies ranging from 80 to 96%, depending on
the load. Switching converters usually
require more space due to the size of the inductor and are
generally more expensive. The switching converter also
causes electromagnetic interference (EMI) radiation from
the inductor and noise on the output due to the switching.
Low-dropout linear regulators (LDOs) step down dc
voltages by dropping the input voltage across a pass element.
The benefit of this topology is that it requires only three
parts (pass element and input/output capacitors). LDOs
are usually a cheaper solution and are much less noisy
Figure 1. Power-supply application
12-V
Supply
Li+ Battery
Charger
5-V, 2-A DC/DC
Supply
3.5- to 4.2-V
Li+ Battery
AC-Adapter/Battery
Switchover Circuit
1.50-V, 100-mA
DC/DC Supply
2.5-V, 50-mA
DC/DC Supply
ASIC
1.25
-V,
DC/DC Supply
300-mA
FPGA
1.65
-V,
DC/DC Supply
112-mA
Microprocessor
-V,
DC/DC Supply
3.3
420-mA
than inductive switchers. The device input current is
equal to the load current, so the efficiency of the solution
is equal to the output-to-input voltage ratio. The drawback
of this solution is the low efficiency for high input-to-output
voltage ratios. All of the power is dissipated by the pass
element, which means that an LDO is not an ideal solution
for high-current applications where the input-to-output
difference is large. These high-power applications require
heat sinking, which increases the solution size.
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Analog Applications Journal
1Q 2007
High-Performance Analog Products
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