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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
Fourth Quarter, 2007
© Copyright 2007 Texas Instruments
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Texas Instruments Incorporated
IMPORTANT NOTICE
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accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
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Analog Applications Journal
High-Performance Analog Products
4Q 2007
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Data Acquisition
Using a touch-screen controller’s auxiliary inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Most touch-screen controllers (TSCs) have one or more auxiliary analog inputs that can be used to
monitor battery voltage or other voltage signals in addition to the TSC signals. This article describes
typical applications for these auxiliary inputs. Also discussed are design considerations for ESD
protection, circuit isolation after power down, and input-voltage ranges accepted by several devices.
Power Management
Driving a WLED does not always require 4 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
This article investigates the myth that a boost converter is always required to drive white-light-emitting
diodes (WLEDs). Several typical WLED datasheets are examined, revealing that many applications
require far less than the maximum-rated LED voltage. This article also shows that many applications
can be driven by an LDO-based LED driver, such as the Texas Instruments TPS75105, for an extremely
small and efficient solution.
Host-side gas-gauge-system design considerations for single-cell
handheld applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A host-side gas gauge offers economic advantages in systems with interchangeable battery packs since
it eliminates most redundant pack components. This article covers the problems of existing gas gauges
and the benefits of Impedance Track™ technology. Also included are design considerations for a host-
side gas-gauge and battery-charging system.
Using a buck converter in an inverting buck-boost topology. . . . . . . . . . . . . . . . . . . . . 16
The most common power-supply source for electronic devices has a positive output relative to ground.
Some applications like audio amplifiers, line drivers/receivers, and instrumentation amplifiers may
require a negative supply relative to ground. This article describes how to use a buck converter to
obtain a negative supply output. Included are discussions of basic buck topology, inverting buck-boost
topology, and design considerations required to obtain a stable negative power source.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
To view past issues of the
Analog Applications Journal, visit the Web site
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Analog Applications Journal
4Q 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:
Data Acquisition
Power Management
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
4Q 2007
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Texas Instruments Incorporated
Data Acquisition
Using a touch-screen controller’s
auxiliary inputs
By Wendy Fang, Precision Analog Applications, High-Performance Analog,
and Tony Chang, Precision Analog Nyquist, High-Performance Analog
Introduction
Texas Instruments (TI) touch-screen controllers (TSCs),
including the ADS7843/45/46, TSC2046, and TSC2003/4/5/6/7,
have touch-screen input pins and one or more non-touch-
screen or auxiliary analog input pins, such as the battery-
voltage-monitoring pin (V BAT ) of the TSC2046 or the AUX
pin of the TSC2007. These auxiliary inputs make it possible
to monitor the system’s battery level or other voltage signals
by sharing time with touch-screen inputs or using time
periods when the touch screen is not touched.
The auxiliary analog inputs of different TI TSCs may have
different input ranges and different levels of electrostatic-
discharge (ESD) protection, so certain requirements or
limitations should be considered when they are used. This
article discusses the general and specific features and limita-
tions of the TSC auxiliary (including battery-voltage) inputs.
Auxiliary analog inputs
Table 1 lists auxiliary analog input pins of TI’s current
TSCs. Inside a TSC, a MUX selects and connects one of
the analog inputs to the ADC via commands sent through
the SPI or I 2 C ports. Figure 1 shows an example.
Auxiliary input mode
TI’s TSCs can be operated in either differential or single-
ended (SE) input mode, but auxiliary analog inputs can be
measured and converted only in SE mode (see Figure 2).
Table 1. Auxiliary analog input(s) of TI TSC devices
AUXILIARY ANALOG INPUTS
TSC
V REF
MAIN FEATURES 1
NUMBER OF INPUTS
PIN NAMES
ADS7843
2
IN3, IN4
External
4-wire, SPI, command-based
ADS7845
1
AUXIN
External
5-wire, SPI, command-based
ADS7846
2
V BAT , AUX
Internal
4-wire, SPI, command-based
TSC2046
2
V BAT , AUX
Internal
4-wire, SPI, command-based
V BAT1 , IN1,
TSC2003
4
Internal
4-wire, I 2 C, command-based
V BAT2 , IN2
4-wire, I 2 C, register-based
TSC2004
1
AUX
External
TSC2005
1
AUX
External
4-wire, SPI, register-based
TSC2006
1
AUX
External
4-wire, SPI, register-based
TSC2007
1
AUX
Shared with V DD
4-wire, I 2 C, command-based
Figure 2. SE mode of a TSC
Figure 1. Block diagram of TSC’s internal
input circuit
V REF
TSC
TSC
V REF
X+
X–
Y+
Y–
Touch-
Screen
Driver’s
Interface
+REF
Analog
Input
+IN
SAR
ADC
SAR ADC
MUX
– IN
Temp
– REF
AUX
GND
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Analog Applications Journal
4Q 2007
High-Performance Analog Products
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