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Texas Instruments Load Sharing Concepts: Implementation for Large-Signal Applications Application notes
Application Report
SBOA127 – October 2011
Load Sharing Concepts: Implementation for Large-Signal
Applications
Kristoffer Flores, Xavier Ramus
............................................................................. High-Speed Products
ABSTRACT
Selecting a suitable large-signal swing operational amplifier driver for instrumentation or test and
measurement applications can be challenging. In addition to developing the necessary theoretical
background for load sharing, this report provides bench results to demonstrate the implementation and
benefits of load sharing and output paralleling.
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2
3
4
5
Contents
What Constitutes a Large Signal? ........................................................................................ 2
Load Sharing Amplifiers .................................................................................................... 3
Load Sharing Amplifiers and Distortion Performance .................................................................. 7
THS3091 Test Circuits and Load Sharing Performance ............................................................... 9
Conclusion .................................................................................................................. 12
List of Figures
1
1-kHz Harmonic Distortion vs. Output Voltage (THS6182) ............................................................ 2
2
Load Sharing Conceptual Block Diagram
3
4
5
6
7
8
...............................................................................
Two THS3091 Amplifiers in Load Sharing Configuration ..............................................................
Simplified Load Sharing Circuit ...........................................................................................
Example of Mismatched Amplifier Output Voltages Producing Unbalanced Amplifier Currents .................
Difference in Amplifier Currents for Two Load Sharing THS3091 Amplifiers vs Series Output Resistor ........
Harmonic Distortion vs. Load Resistance Graph from OPA695 Data Sheet ........................................
Reference THS3091 and THS3091 Load Sharing Test Configurations .............................................
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4
5
6
7
8
9
9
32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Single
THS3091 Circuit Configuration .......................................................................................... 10
10
32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two THS3091
Amplifiers in Load Sharing Configuration .............................................................................. 10
11
64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Single
THS3091 Circuit Configuration .......................................................................................... 11
12
64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two THS3091
Amplifiers in Load Sharing Configuration .............................................................................. 11
13
Harmonic Distortion vs Frequency, Single THS3091 Circuit Configuration........................................ 12
14
Harmonic Distortion vs Frequency, Two THS3091 Amplifiers in Load Sharing Configuration .................. 12
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1
What Constitutes a Large Signal?
1
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What Constitutes a Large Signal?
Most operational amplifier data sheets define large signal as any output signal that swings greater than
2 VPP for ±5-V amplifiers and 5 VPP for ±15-V amplifiers. However, the performance that is specified as
large signal depends on the intended application as well as the internal architecture of the specific device.
For example, a low-voltage, +5-V, fully-differential, voltage-feedback architecture analog-to-digital
converter (ADC) driver amplifier such as the THS4521 specifies a 2-VPP large-signal bandwidth because 2
VPP is a common ADC analog input range. On the other hand, a high-voltage ,+28-V, current-feedback
architecture, high output power line driver device such as the THS6204 specifies large-signal performance
for 4-VPP to 20-VPP output.
In fact, the output voltage swing of an amplifier can affect the bandwidth by more than 50% at 20-VPP
output compared to a small-signal (200 mVPP) bandwidth as a direct result of slew rate limitation or
loading. For a detailed discussion on the parameters affected by the output voltage swing, refer to the
application note Large-Signal Specifications for High-Voltage Line Drivers (SBOA126), available for
download from the TI website. As a first-order rule, distortion performance is also degraded by 6 dB for
second-harmonic distortion and 12 dB for third-harmonic distortion every time the signal doubles. This
behavior is shown in Figure 1 for a 1-kHz signal using the THS6182.
THS6182: G = -1 V/V, 1 kHz
HARMONIC DISTORTION vs OUTPUT VOLTAGE
-100
Harmonic Distortion (dBc)
-105
Second Harmonic (dBc)
-110
-115
-120
-125
-130
-135
Third Harmonic (dBc)
-140
-145
VS = ±15 V
-150
1
10
Output Voltage (VPP)
Figure 1. 1-kHz Harmonic Distortion vs. Output Voltage (THS6182)
Slew rate, output voltage swing, and output current are key operational amplifier parameters to consider
for large signals applications, in particular when targeting low distortion as key design parameter. As a
reminder, here is a brief summary of these three specific parameters and the respective relationships to
bandwidth, distortion, and amplifier architecture.
1. Slew rate is directly related to the large-signal bandwidth of an operational amplifier. Slew rate
limitation may also be a factor in the distortion of the amplifier. As a rule of thumb, to be able to
support 80 dBc for a given frequency, the amplifier achievable slew rate must be 20x the slew rate
requirement to support the signal at that frequency.
2. The amplifier output current sourcing and sinking ability into the load determine the output voltage
swing range of the device. As the load current increases, distortion suffers. Additionally, high-speed
large signal swings into a capacitive load require the amplifier to quickly charge and discharge the
load. Depending on the load capacitance, the limited amplifier current sourcing and sinking ability may
lead to a lower effective slew rate into the capacitive load.
3. The amplifier output is distorted as it approaches the output voltage swing range as a result of
compression because the transistor swing is getting too close to the voltage rail.
In high voltage swing applications, where an operational amplifier is pushed to drive close to its supply rail,
it is possible to drive several identical operational amplifiers in parallel and combine the outputs to achieve
higher bandwidth and lower distortion. The remainder of this application report develops this concept of
load sharing, and demonstrates how this technique can be used to reduce current sourcing and sinking
requirements in the amplifier.
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2
Load Sharing Amplifiers
2.1
Concepts
The fundamental concept of load sharing is to drive a load using two or more of the same operational
amplifiers. Each amplifier is driven by the same source. Figure 2 shows three THS3091 amplifiers sharing
the same load.
THS3091
THS3091
VOUT
VIN
THS3091
Figure 2. Load Sharing Conceptual Block Diagram
This concept effectively reduces the current load of each amplifier by 1/N, where N is the number of
amplifiers.
The balance of this report focuses on selection of component values and demonstrated performance
improvements.
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Figure 3 shows two THS3091 devices configured in a typical load sharing configuration.
RG1
250 W
RF1
1 kW
V+
V1
15 V
V-
RSOURCE
50 W
VIN
V+
RG2
250 W
RF2
1 kW
V2
-15 V
TL1
Characteristic
Impedance
50 W
VOUT
RLOAD
50 W
V-
RT2
100 W
+
RS1
100 W
THS3091
U1
RT1
100 W
V-
+
RS2
100 W
THS3091
U2
V+
Figure 3. Two THS3091 Amplifiers in Load Sharing Configuration
In this example, each THS3091 is configured in a noninverting gain of +5 V/V with the two noninverting
inputs tied to a common input signal, VIN. Several items are worth noting:
• Each output has 100-Ω resistors in series. This configuration provides matching to the 50-Ω load.
Looking from the load, then, both resistors appear to be in parallel, presenting a 50-Ω matched
impedance to the 50-Ω load through the transmission line.
• The matched load 50-Ω impedance minimizes reflections at the load end of the transmission line. This
reduced reflection, however, comes at the expense of 6-dB attenuation of the signal that reaches the
load.
• The matched input impedance is realized with two 100-Ω resistance that appear to be in parallel with
the 50-Ω source.
For applications where back or double termination is not required or desired, a non-zero resistor on each
amplifier output is highly recommended; the resistors help to balance the current load so that each
amplifier provides the same current. Because both amplifiers are low-impedance, this configuration also
minimizes the tendency of one amplifier driving the other amplifier, as could be the case if the output
offset voltages were different.
The next section focuses on the dc offset contribution that results in unequal load current distribution.
Differences in gain from one amplifier to the next because of mismatched resistors also leads to
imbalanced output voltages and load currents.
Note that the topic of gain mismatch is not developed in this report, but may need to be considered,
depending on the final application.
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2.2
Output DC Offset
One guideline for selecting the value of the series output resistors can be determined by analyzing the
circuit of Figure 3. The goal of load sharing amplifiers is for the current load of each amplifier to be
reduced by sharing the load current requirement. This method requires that neither amplifier supplies the
majority of the load current and that neither amplifier sources current into or sinks current from the output
of the other amplifier. If these conditions were to occur, the circuit would not work at all, and the purpose
of the load sharing approach is defeated entirely.
Note that on one hand, the current requirement for a given amplifier has been reduced, but the amplifier
must support the full voltage swing. This approach only relaxes the driving capability requirement of the
driver and increases the pool of amplifiers to select from. This increased selection pool further allows the
design to achieve higher bandwidth than would otherwise have been possible with monolithic amplifiers.
Unequal load currents can result when load sharing amplifier outputs are mismatched in voltage. One
source of mismatch is output-referred offset voltage. The worst-case, output-referred offset voltage can be
calculated using Equation 1.
R
R
VOS_RTO = |VOS| · (1 + F ) + |IB+| · RNE · (1 + F ) + |IB-| · RF
RG
RG
(1)
Where VOS is the input offset voltage, IB+ is the noninverting input bias current, IB– is the inverting bias
current, and RNE is the equivalent resistance looking out of the noninverting terminal.
For the THS3091, the maximum input offset voltage is 4 mV; the maximum noninverting and inverting
input bias currents are both 20 μA. Looking out of the noninverting terminal of either amplifier, the
equivalent resistance is RT1 || RT2 || RSOURCE = 25 Ω. Using these values in Equation 1, the output-referred
offset voltage is calculated as Equation 2 shows.
1 kW
1 kW
VOS_RTO = 4 mV · (1 +
) + 20 μA · 25 W · (1 +
) + 20 μA · 1000 W = 42.5 mV
250 W
250 W
(2)
For a worst-case analysis, it is typical to presume that the offset can either be positive or negative;
consequently, the final output-referred offset voltage is ±42.5 mV. For two amplifiers in a load sharing
configuration, an offset of ±42.5 mV for each amplifier indicates that a voltage as high as 85 mV can be
developed between the two outputs at dc, assuming perfectly-matched gain-setting resistors.
Although dual amplifiers should have better matching than single operation amplifier, a dual operational
amplifier in a single package approach is not recommended because of limited power-supply rejection
(PSR) isolation in the standard dual footprint package. In a dual standard footprint package, the
power-supply pins are shared; this configuration could lead to positive feedback on the other amplifier,
and result in oscillations. The other issue to consider if looking into a dual operational amplifier in a single
package is thermal power dissipation, especially where heavy loads are concerned.
The circuit shown in Figure 3 can be simplified for further analysis as shown in Figure 4. Consider a case
where the input to the load sharing amplifiers is at 0 V. Because of the non-ideal offset and bias currents
of the amplifiers, the outputs are not 0 V. In the worst-case situation, the output of Amplifier U1 is the
positive worst-case, output-referred offset voltage and the output of Amplifier U2 is the negative
worst-case offset.
RS
U1
VAMP1 = +VOS_RTO
0V
VOUT
RLOAD
RS
U2
VAMP2 = -VOS_RTO
Figure 4. Simplified Load Sharing Circuit
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With matched series output resistors (RS), the voltage at the load (VOUT) is 0 V. The current from Amplifier
1 is simply +VOS_RTO/RS and the current from Amplifier 2 is –VOS_RTO/RS.
Amplifier U1 is sourcing current into the output of Amplifier U2 without generating any signal on the load.
This event is not a desired behavior; at best, this configuration would dissipate power, and at worst, it
could damage the amplifiers. RS is therefore necessary in order to limit the current that one amplifier
attempts to drive into the other in this worst-case condition.
The difference in the two currents is given by Equation 3.
V
V
V
IDIFF = OS_RTO - - OS_RTO = 2 · OS_RTO
RS
RS
RS
(
(
(3)
The effect of this difference in current is best illustrated with an example. If RS is chosen to be 5 Ω for a
configuration with two THS3091 amplifiers in a load sharing configuration, the worst-case difference in the
two amplifier output currents is 17 mA (= 2 ● 42.5 mV / 5 Ω). During normal operation, if the nominal
output of each amplifier in Figure 5 is 5 V, the worst case is that the output of Amplifier U1 is 5.0425 V
and the output of Amplifier 2 is 4.9575 V (or the other way around) because of the mismatched offset
voltages. Figure 5 shows the resulting currents. As expected, Amplifier U1 supplies 17 mA more current to
the load than does Amplifier U2. The total current to the load is the same, but one amplifier always
appears to be hotter than the other, and will be more susceptible to failure. In this example, Amplifier U1
also shows harmonic distortion that is worse than expected.
RS = 5 W 56.11905 mA
U1
VAMP1 = 5.0425
95.2381 mA
VOUT = 4.7619 V
RLOAD = 50 W
R1 = 5 W 39.11905 mA
U2
VAMP2 = 4.9575
Figure 5. Example of Mismatched Amplifier Output Voltages Producing Unbalanced Amplifier Currents
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The difference in amplifier currents can be plotted against the series resistor, RS. Figure 6 shows the
difference in amplifier currents for two load sharing THS3091 amplifiers versus the series resistor, RS.
With an RS of 10 Ω, the difference in amplifier currents is reduced to 4.25 mA.
DIFFERENCE IN AMPLIFIER CURRENTS
vs SERIES OUTPUT RESISTOR
Difference in Amplifier Currents (mA)
50
45
40
35
30
25
20
15
10
5
0
0
10
20
30
40
50
Series Output Resistor (W)
Figure 6. Difference in Amplifier Currents for Two Load Sharing THS3091 Amplifiers vs Series Output
Resistor
3
Load Sharing Amplifiers and Distortion Performance
In addition to providing higher output current drive to the load, the load sharing configuration can also
provide improved distortion performance. In many cases, an operational amplifier shows better distortion
performance as the load current decreases (that is, for higher resistive loads) until the feedback resistor
starts to dominate the current load. In a load sharing configuration of N amplifiers in parallel, the
equivalent current load that each amplifier drives is 1/N times the total load current. For example, in a
two-amplifier load sharing configuration with matching resistance (refer to Figure 3) driving a resistive
load, RL, each series resistance is 2 ● RL and each amplifier drives 2 ● RL.
A convenient indicator of whether an op amp will function well in a load sharing configuration is the
characteristic performance graph of harmonic distortion versus load resistance. Such graphs can be found
in most of TI’s high-speed amplifier data sheets. These graphs can be used to obtain a general sense of
whether or not an amplifier will show improved distortion performance in load sharing configurations.
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Load Sharing Amplifiers and Distortion Performance
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For example, Figure 7 (from the OPA695 product data sheet) provides a figure of 10-MHz harmonic
distortion versus load resistance. This graph shows 9-dB improvement in second-order harmonic
performance with a load of 200 Ω compared to a 100-Ω load. Third-order harmonic performance also
shows an improvement of about 6 dB. Consequently, the OPA695 may be a good candidate for a load
sharing configuration of two OPA695 amplifiers driving a 100-Ω load. The apparent load to each OPA695
will be double the shared resistor load, or 200 Ω.
10-MHz HARMONIC DISTORTION
vs LOAD RESISTANCE
Harmonic Distortion (dBc)
-50
VO = 2 VPP
G = 8 V/V
-60
Second Harmonic
-70
-80
Third Harmonic
-90
-100
50
100
500
Load Resistance (W)
Figure 7. Harmonic Distortion vs. Load Resistance Graph from OPA695 Data Sheet
It is important to note, however, that harmonic distortion does not always improve monotonically as the
load resistance increases. Figure 7 shows that for the OPA695, for example, third-harmonic distortion
reaches a minimum at a 200-Ω load. With a 500-Ω load, third-harmonic distortion is virtually the same as
that observed with a 100-Ω load, while second-harmonic distortion continues to improve. Distortion
performance also varies with the output signal swing, typically degrading as the output swing increases,
though not always monotonically.
Product data sheets that do not include harmonic distortion versus load resistance graphs usually include
distortion versus frequency graphs for different load resistances. The typical distortion performance at a
specific frequency for two different loads may also be included in the Electrical Specifications table. For
example, the THS3091 data sheet includes typical distortion performance graphs for 100-Ω and 1-kΩ
loads. The distortion performance at 10 MHz for a 2-V/V gain configuration and 2-VPP output are also
included in the Electrical Characteristics table for this device, and are given in Table 1. The data show that
second-harmonic distortion performance is better with a 1-kΩ load than with a 100-Ω load; however,
third-harmonic distortion is better with a 100-Ω load than with a 1-kΩ load.
Suppose the THS3091 is chosen to drive a 20-VPP signal into a 100-Ω load (in other words, a
double-terminated, 50-Ω cable) and load sharing is being considered. No definitive conclusion can be
reached from the available 100-Ω and 1-kΩ performance data and additional characterization would be
required.
Table 1. Harmonic Distortion for 100-Ω and 1-kΩ Loads (from THS3091 Product Data Sheet)
THS3091
TYP
PARAMETER
Second Harmonic Distortion
Third Harmonic Distortion
8
CONDITIONS
+25°C
MIN/MAX OVER
TEMPERATURE
+25°C
0°C to
70°C
–40°C to
+85°C
UNIT
G = 2, RF = 1.21 kΩ,
VO = 2 VPP, f = 10 MHz
RL = 100 Ω
66
dBc
RL = 1 kΩ
77
dBc
G = 2, RF = 1.21 kΩ,
VO = 2 VPP, f = 10 MHz
RL = 100 Ω
74
dBc
RL = 1 kΩ
69
dBc
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4
THS3091 Test Circuits and Load Sharing Performance
As is the case in every design, lab evaluation is the best gauge of performance. Two test circuits are
shown in Figure 8, one for a single THS3091 amplifier driving a double-terminated, 50-Ω cable and one
with two THS3091 amplifiers in a load sharing configuration. In the load sharing configuration, the two
100-Ω series output resistors act in parallel to provide 50-Ω back-matching to the 50-Ω cable.
RG
250 W
RF
1 kW
V-
RS
50 W
RSOURCE
50 W
VOUT
THS3091
U3
RT
50 W
VIN
TL2
Characteristic
Impedance
50 W
RG1
250 W
RF1
1 kW
V+
V1
15 V
V-
RSOURCE
50 W
VIN
RLOAD
50 W
V+
V+
RG2
250 W
RF2
1 kW
V2
-15 V
TL1
Characteristic
Impedance
50 W
VOUT
RLOAD
50 W
V-
RT2
100 W
+
RS1
100 W
THS3091
U1
RT1
100 W
V-
+
RS2
100 W
THS3091
U2
V+
Figure 8. Reference THS3091 and THS3091 Load Sharing Test Configurations
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Figure 9 and Figure 10 show the 32-MHz, 18-VPP sine wave output amplitudes for the single THS3091
configuration and the load sharing configuration, respectively, measured using an oscilloscope. An ideal
sine wave is also included as a visual reference (the dashed red line).
Figure 9 shows visible distortion in the single THS3091 output. In the load sharing configuration of
Figure 10, however, no obvious degradation is visible.
32-MHz SINE WAVE OUTPUT
SINGLE THS3091
15
Output Voltage (V)
10
5
0
-5
-10
-15
0
10
20
30
40
50
Time (ns)
Figure 9. 32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Single
THS3091 Circuit Configuration
32-MHz SINE WAVE OUTPUT
LOAD-SHARING (DUAL) THS3091
15
Output Voltage (V)
10
5
0
-5
-10
-15
0
10
20
30
40
50
Time (ns)
Figure 10. 32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two
THS3091 Amplifiers in Load Sharing Configuration
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Figure 11 and Figure 12 show the 64-MHz sine wave outputs of the two configurations from Figure 8.
While the single THS3091 output is clearly distorted in Figure 11, the output of the load sharing
configuration in Figure 12 shows only minor deviations from the ideal sine wave.
64-MHz SINE WAVE OUTPUT
SINGLE THS3091
15
Output Voltage (V)
10
5
0
-5
-10
-15
0
5
10
15
20
25
Time (ns)
Figure 11. 64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output),
Single THS3091 Circuit Configuration
64-MHz SINE WAVE OUTPUT
LOAD-SHARING (DUAL) THS3091
15
Output Voltage (V)
10
5
0
-5
-10
-15
0
5
10
15
20
25
Time (ns)
Figure 12. 64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two
THS3091 Amplifiers in Load Sharing Configuration
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Conclusion
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The improved output waveform as a result of load sharing is quantified in the harmonic distortion versus
frequency graphs shown in Figure 13 and Figure 14 for the single amplifier and load sharing
configurations, respectively. While second-harmonic distortion remains largely the same between the
single and load sharing cases, third-harmonic distortion is improved by approximately 8 dB in the
frequency range between 20 MHz to 64 MHz.
HARMONIC DISTORTION vs FREQUENCY
SINGLE THS3091
-10
VO = 20 VPP (at amplifier output)
VO = 10 VPP (at load)
RS = 50 W
RL = 50 W
Harmonic Distortion (dBc)
-20
-30
-40
-50
-60
-70
-80
Second Harmonic
Third Harmonic
-90
1
10
100
Frequency (MHz)
Figure 13. Harmonic Distortion vs Frequency, Single THS3091 Circuit Configuration
HARMONIC DISTORTION vs FREQUENCY
LOAD SHARING (DUAL) THS3091
-10
VO = 20 VPP (at amplifier output)
VO = 10 VPP (at load)
RS (Each Amplifier) = 100 W
RL (Shared) = 50 W
Harmonic Distortion (dBc)
-20
-30
-40
-50
-60
-70
-80
Second Harmonic
Third Harmonic
-90
1
10
100
Frequency (MHz)
Figure 14. Harmonic Distortion vs Frequency, Two THS3091 Amplifiers in Load Sharing Configuration
5
Conclusion
The operational amplifier limitations imposed by slew rate and output current must be considered when an
application requires large output signal swings. Output current is an especially important factor. This
application note reviewed and explained a load sharing method to increase the output current ability of an
amplifier and potentially improve distortion performance.
12
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the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Applications
Audio
www.ti.com/audio
Communications and Telecom www.ti.com/communications
Amplifiers
amplifier.ti.com
Computers and Peripherals
www.ti.com/computers
Data Converters
dataconverter.ti.com
Consumer Electronics
www.ti.com/consumer-apps
DLP® Products
www.dlp.com
Energy and Lighting
www.ti.com/energy
DSP
dsp.ti.com
Industrial
www.ti.com/industrial
Clocks and Timers
www.ti.com/clocks
Medical
www.ti.com/medical
Interface
interface.ti.com
Security
www.ti.com/security
Logic
logic.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Power Mgmt
power.ti.com
Transportation and Automotive www.ti.com/automotive
Microcontrollers
microcontroller.ti.com
Video and Imaging
RFID
www.ti-rfid.com
OMAP Mobile Processors
www.ti.com/omap
Wireless Connectivity
www.ti.com/wirelessconnectivity
TI E2E Community Home Page
www.ti.com/video
e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2011, Texas Instruments Incorporated
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