User’s Guide
TPS40055-Based Design Converts
12-V Bus to 1.8 V at 15 A (HPA070)
User’s Guide
1
DYNAMIC WARNINGS AND RESTRICTIONS
It is important to operate this EVM within the input voltage range of 0 V to 14 V
.
DC
DC
Exceeding the specified input range may cause unexpected operation and/or irreversible damage to the EVM.
If there are questions concerning the input range, please contact a TI field representative prior to connecting
the input power.
Applying loads outside of the specified output range may result in unintended operation and/or possible
permanent damage to the EVM. Please consult the EVM User’s Guide prior to connecting any load to the EVM
output. If there is uncertainty as to the load specification, please contact a TI field representative.
During normal operation, some circuit components may have case temperatures greater than 50°C. The EVM
is designed to operate properly with certain components above 50°C as long as the input and output ranges are
maintained. These components include but are not limited to linear regulators, switching transistors, pass
transistors, and current sense resistors. These types of devices can be identified using the EVM schematic
located in the EVM User’s Guide. When placing measurement probes near these devices during operation,
please be aware that these devices may be very warm to the touch.
Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright 2004, Texas Instruments Incorporated
3
SLUU186 − March 2004
TPS40055-Based Design Converts 12-V Bus to
1.8 V at 15 A (HPA070)
Mark Dennis
System Power
Contents
1
2
3
4
5
6
7
8
9
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Component Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Test Results and Performance Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
EVM Assembly Drawing and PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
List of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1
Introduction
In many modern electronic applications there is a growing demand for circuits to convert a12-V bus to digital
voltages as low as, but not limited to 1.8 V. The current requirements can range from below 1 A to over 15 A.
For high-efficiency and small circuit size the TPS40055 wide-input synchronous buck controller can be used
to provide the necessary control and drive functions to implement these converters. The TPS40055EVM−001
operates at 300 kHz and delivers 1.8 V at 15 A with efficiency over 90% for much of the load range, and a full
load efficiency of 88%.
The TPS40055 synchronous buck controller offers a variety of user programmable functions such as operating
frequency, soft-start time, voltage feed-forward, high-side current limit, and external loop compensation. This
controller also provides a regulated 10-V bias voltage which supplies onboard drivers for the N-channel switch
and synchronous rectifier MOSFETs, utilizing adaptive gate drive logic to prevent cross conduction of the power
[1]
MOSFETs.
2
Features
The specification of this design is as follows:
D
D
D
D
D
D
D
D
92% peak efficiency at 6 A
88% peak efficiency at 15 A
1.8V output at 15 A
V
range from 10 V
to 14 V
DC DC
IN
Small circuit size 1.4” x 2.5” SMT design, components on single side
Line/load regulation < 0.5%
High-frequency 300-kHz operation
Transient deviation 60 mV with 10-A load step
4
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
3
Schematic
Figure 1. TPS40055EVM−001 (HPA070) Schematic
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
5
SLUU186 − March 2004
4
Component Selection
4.1 TPS40055 Device Selection
The TPS4005x family of parts offers a range of output current configurations including source only (TPS40054),
source/sink (TPS40055), or source/sink with V
prebias (TPS40057). In this converter the TPS40055 with
OUT
source/sink capability is selected. This serves to maintain continuous inductor ripple current all the way to zero
load to improve the small signal loop response by preventing the inductor current from transitioning to the
discontinuous current mode.
The TPS4005x family is packaged in TI’s PWP PowerPAD thermally enhanced package which should be
soldered to the PCB using standard solder flow techniques. The PowerPAD technology uses a thermally
conductive epoxy to attach the integrated circuit die to the leadframe die pad, which is exposed on the bottom
of the completed package. The PWP PowerPAD package has a θ = 2°C/W which helps keep the junction
JC
temperature rise relatively low even with the power dissipation inherent in the onboard MOSFET drivers. This
power loss is proportional to switching frequency, drive voltage, and the gate charge needed to enhance the
N-channel MOSFETs. Effective heat removal allows the use of ultra-small packaging while maintaining high
component reliability.
[2]
The technical brief, PowerPAD Thermally Enhanced Package contains more information on the PowerPAD
package.
4.2 Frequency of Operation
The clock oscillator frequency for the TPS40055 is programmed with a single resistor from RT (pin 2) to signal
ground. The following equation (1) from the datasheet allows selection of RT in kΩ for a given switching
frequency in kHz.
1
(1)
R + R2 +
* 23 (kW)
T
*6
f
17.82 10
SW
For 300-kHz operation, R2 is selected to be 165 kΩ.
For a particular operating frequency, the PWM ramp time must be programmed via the resistor R
connected
KFF
to V . Also, the selection of R
programs the V voltage at which the circuit starts operation. This prevents
IN
KFF
IN
the circuit from starting at low voltages, which can lead to current flow larger than desired. R
using equation (2).
is programmed
(2)
KFF
* 3.5 ǒ58.14 R ) 1340Ǔ (kW
+ R6 + ǒV
Ǔ
)
R
KFF
IN(min)
T
Where V
is the minimum startup input voltage, and R is in kΩ. Note that internal tolerances have been
IN(min)
T
incorporated into this equation, so the actual V
frequency of 300-kHz, the R
of the input voltage should be used. For an oscillator
IN(min)
value of 71.5 kΩ is selected.
KFF
6
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
4.3 UVLO Circuitry
The user programmable UVLO built into the TPS4005x provides hysteresis for transients shorter than a total
count of seven cycles. If the input voltage to the converter can be slowly rising around the minimum V range,
IN
external hysteresis can be incorporated to prevent multiple on/off cycles during startup or shutdown. These
on/off cycles are a result of line impedance external to the EVM causing V to the module to drop when under
IN
load, which causes the programmable UVLO threshold to be crossed repetitively.
In this converter, C1 and D1 are added to form a peak detector from the lower gate drive which is only active
when the converter is operating. This provides a bias source to deliver hysteresis current from the peak detector
voltage to the lower KFF voltage of 3.5 V, enabling the designer to alter the programmable UVLO shutdown
point. The bias is not present during startup, so the circuit starts as expected from the R
calculation.
KFF
In this application, R4 is selected to provide a hysteresis current of 20% I
(3).
. R4 can be calculated from equation
KFF
ǒV * 3.5Ǔ
R
KFF
PD
(3)
R
+ R4 +
HYS
0.2 ǒV
* 3.5Ǔ
IN(min)
where
D
D
V
is the voltage on the peak detector
PD
V
is the desired start voltage used in the determination of R
IN(min)
KFF
In a typical case, V
= 8V, and R4 is found to be 247 kΩ, and a standard value of 243kΩ is selected. Testing
PD
shows the startup voltage to be 9.2 V, and the shutdown voltage to be 8.5 V.
4.4 Inductance Value
The output inductor L1 value used in the circuit of Figure 2 was selected from equation (4).
V
V
OUT
OUT
(4)
L +
1 *
f I
V
RIPPLE
IN(min)
in which I
is usually chosen to be in the range between 10% and 40% of I
there is a ripple current of 3 A, and the inductance value is 1.7 µH.
. With I = 20% of
RIPPLE
RIPPLE
OUT
I
OUT(max)
4.5 Input capacitor selection
Bulk input capacitor selection is based on allowable input voltage ripple and required RMS current carrying
capability. In typical buck converter applications, the converter is fed from an upstream power converter with
its own output capacitance. In this converter, ceramic capacitors capable of meeting circuit requirements are
provided onboard. For this power level, input voltage ripple of approximately 250 mV is reasonable, and the
minimum capacitance is calculated in (5).
I V
I D t
D V
O
15 A 1.8 V
0.25 V 10 V 300 kHz
(5)
C
+
+
+
+ 36 mF
IN
D V V f
IN
S
Also consider the RMS current rating required for the input capacitors (6).
V
OUT
1.8
10
Ǹ
(6)
Ǹ
i ^ I
D + I
Ǹ
+ 15
+ 6.4 A
OUT
OUT
V
IN
To meet this requirement with the smallest cost and size two 22 µF, 16 V, X5R ceramic capacitors (C12, C14)
are installed on the board. In the 1812 case, the parts are able to carry approximately 4 A each. These
RMS
capacitors function as power bypass components and should be located close to the MOSFET packages to
keep the high-frequency current flow in a small, tight loop.
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
7
SLUU186 − March 2004
4.6 Output Capacitor Selection
Selection of the output capacitor is based on many application variables, including function, cost, size, and
availability. The minimum allowable output capacitance is determined by the amount of inductor ripple current
and the allowable output ripple, as given in equation (7).
I
3 A
RIPPLE
(7)
C
+
+
+ 83 mF
OUT(min)
8 f V
8 300 kHz 15 mV
RIPPLE
In this design, C
is 83 µF with V
=15 mV to allow for some margin. However, this only affects the
OUT(min)
RIPPLE
capacitive component of the ripple voltage, and the final value of capacitance is generally influenced by ESR
and transient considerations. The voltage component due to the capacitor ESR.
V
15 mA
3 A
RIPPLE
RIPPLE
(8)
C
v
+
+ 5 mW
ESR
I
An additional consideration in the selection of the output inductor and capacitance value can be derived from
examining the transient voltage overshoot which can be initiated with a load step from full load to no load. By
equating the inductive energy with the capacitive energy the equation (9) can be derived:
ǒ Ǔ2 ǒ Ǔ2
ǒ I
Ǔ
L
* I
OL
OH
(
)
(9)
2
1.7 mH 15 A
L I
V
C v
+
+ 1034 mF
+ ǒ(
) Ǔ
O
2
ǒV Ǔ2
* ǒV Ǔ2
2
2
)
(
1.9 V * 1.8 V
f
i
where
• I = full load current
OH
• I = no load current
OL
• V = allowed transient voltage rise
f
• V = initial voltage
i
For compactness while maintaining transient response capability, two 470-µF POSCAP capacitors (C16, C17)
are fitted in parallel. The total ESR of these capacitors is approximately 5 mΩ. An additional 47-µF, 6.3-V ceramic
capacitor C15 is placed in parallel with the POSCAPs to help suppress high frequency noise generated by the
fast current transitions as the current switches between the input and output circuits during each switching cycle.
4.7 MOSFET selection
Proper MOSFET selection is essential to optimize circuit efficiency. To operate with high current it is important
to choose a package which allows the generated heat to be removed from the package as easily as possible.
Various MOSFETs with a package similar to the SO−8 footprint are considered for this application, and devices
with reduced junction-case thermal impedance are selected.
For the upper switch Q1, a Hitachi HAT2168H MOSFET with low gate charge (typically 27 nC at 10 V) and with
an R
of 6 mΩ is selected to keep the switching losses to a minimum. The low-side rectifier switch Q2 was
DS(on)
chosen as a Hitachi HAT2167H, which has slightly more gate charge (43 nC at 10 V) but lower R
= 4.2
DS(on)
mΩ to minimize conduction losses. A schottky diode, D2, is placed across Q2 in this high current design to carry
some of the high circulating current during short circuit conditions.
8
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
4.8 Short Circuit Protection
The TPS40055 implements short circuit protection by comparing the voltage across the topside MOSFET while
it is ON to a voltage developed across R due to an internal current source of 10 µA inside pin 16. Both of these
LIM
voltages are negative with respect to V . From the datasheet equation, R
is defined as:
IN
R
DS(on) (max)
LIM
I
V
OC
OS
(10)
R
+ R9 +
+
+ (W)
LIM
1.12 I
I
SINK
SINK
where
• I is the overcurrent set point equal to the DC output current plus one-half the inductor ripple current
OC
• V is the overcurrent comparator offset, and Isink is the current into ILIM (pin 16).
OS
Using worst case tolerances the value of R
rated current under all conditions. In a worst case condition, R =R9 and
should be maximized to ensure that the converter can deliver full
LIM
LIM
(
)
(
)
15 A ) 1.5 A 7.9 mW 1.45
* 30 mV
8.65 mA
(11)
R
+
)
+ 16.0 kW
LIM
1.12 8.65 mA
The standard value of 16.2 kΩ was selected. This ensures that we can deliver a minimum of 15 A before current
limit is activated. There is also a small capacitor, C7, placed in parallel with R9 to filter the signal.
4.9 Snubber Component Selection
Initially, the junction of Q1, Q2, and L1 was ringing at a frequency near 100 MHz with a peak voltage near 30
V. This was due to the extremely fast switching speed of the MOSFETs and the lack of any cross−conduction.
C13 was added to shunt the high-frequency ringing to ground and the peak voltage is now below 25 V.
4.10 Compensation Components
The TPS40055 uses voltage mode control with feed-forward in conjunction with a high-frequency error amplifier
to implement closed loop control. The power circuit L-C double pole corner frequency f occurs at 3.8 kHz, and
C
the output capacitor ESR zero is located at approximately 38 kHz. The feedback compensation network is
implemented to provide two zeroes and three poles. The first pole is placed at the origin to improve DC
regulation.
The first zero is placed at 2.8 kHz, just below the L-C corner frequency.
1
(12)
(13)
f
+
Z1
2p R5 C5
The second zero is selected to be coincident with the L-C corner frequency of 3.8 kHz,
1
f
+
Z2
(
)
2p R7 ) R8 C6
The second pole is placed near the ESR zero frequency at 37 kHz.
1
f
+
(14)
(15)
P1
C4 C5
C4)C5Ǔ
ǒ
2p R5
and the third pole is placed at 150 kHz, which is one-half the switching frequency.
1
f
+
P2
2p R8 C6
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
9
SLUU186 − March 2004
5
Test Setup
Figure 2 illustrates the basic test setup needed to evaluate the TPS40055EVM−001.
5.1 DC Input Source
The input voltage source should be capable of supplying between 10 V
A of current. For best results the input leads should be made with a wire of 18AWG or larger.
and 14 V
and rated for at least 4
DC
DC
5.2 Output Load
The output load can be either an electronic load or a resistive load configured to draw between 0 A and 15 A.
The output leads should be made with a wire of 16AWG or larger diameter wire. Monitor the output voltage on
the PCB by connecting a voltmeter to TP9 and TP10 to prevent voltage drops through PCB traces and the output
terminal block which can lead to substantial measurement errors.
5.3 Oscilloscope Probe Test Jacks
An oscilloscope probe test jack (TP8) has been included to allow monitoring the ourput voltage ripple.
5.4
Fan
There is no cover to prevent the user from probing the internal circuit nodes. There are components that can
get hot to the touch (above 60°C) in normal operation. A small fan delivering more than 15 cfm should be used
when operating at and near full load.
V
Test Points
V
IN
TP1 = V (+)
IN
10 V to 14 V
IN
TP2 = V (−)
IN
+
−
Fan
V
Test Points
OUT
TP9 = V
(+)
OUT
(−)
− +
TP10 = V
OUT
V
LOAD
1.8 V / 15 A
Figure 2. Test Setup
10
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
6
Test Results / Performance Data
6.1 Efficiency and Power Loss
Figure 3 shows the efficiency as the load is varied from 1 A to over 15 A. The typical efficiency remains over
90% as the load ranges from 3 A to 12 A.
POWER DISSIPATION
EFFICIENCY
vs
LOAD
vs
LOAD
7
94
92
90
88
86
84
82
80
78
6
5
4
3
2
1
0
0
2
4
6
8
10
12
14
16
0
2
4
6
8
10
12
14
16
I
− Output Current − A
OUT
I
− Output Current − A
OUT
Figure 3
Figure 4
6.2 Closed Loop Performance
OVERALL GAIN AND PHASE
vs
FREQUENCY
160
140
120
100
80
50
40
30
20
10
0
Phase
Gain
60
40
−10
−20
20
0
−30
100
1 k
f − Frequency − kHz
10 k
100 k
Figure 5.
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
11
SLUU186 − March 2004
6.3 Output Ripple and Transient Response
Figure 6 shows the typical output voltage ripple with I
=15 A to be less than 20 mVpp.
OUT
The transient response is shown in Figure 7 as the load is stepped from 5 A to 15 A. The voltage deviation is
less than 60 mV.
OUTPUT VOLTAGE RIPPLE
TRANSIENT RESPONSE
I
= 15 A
OUT
V
RIPPLE
(10 mV/div)
I
5 A/div
OUT
t − Time − 1 µs/div
t − Time − 200 µs/div
Figure 6
Figure 7
7
EVM Assembly Drawing and PCB Layout
Figure 8. Top Side Component Assembly
12
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
Figure 9. Top Side Copper
Figure 10. Internal Layer 1 Copper
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
13
SLUU186 − March 2004
Figure 11. Internal Layer 2 Copper
Figure 12. Bottom Layer Copper
14
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
SLUU186 − March 2004
8
List of Materials
Table 1 lists the parts values of the evaluation board. These values can be modified to meet the application
requirements.
Table 1. TPS40055EVM−001 (HPA070) List of Materials
REFERENCE
DESIGNATOR
QTY
DESCRIPTION
SIZE
MFR
Vishay
PART NUMBER
C1, C4
2
2
1
2
Capacitor, ceramic, 470 pF, 50 V, X7R, 10%
Capacitor, ceramic, 22 µF, 16 V, X5R, 20%
Capacitor, ceramic, 47 µF, 6.3 V, X5R, 20%
Capacitor, POSCAP, 470 µF, 4 V, 10 mΩ, 20%
805
1812
VJ0805Y471KXAAT
C4532X5R1C226MT
C4532X5R0J476MT
4TPD470M
C12, C14
C15
TDK
1812
TDK
(1)
C16, C17
7343 (D)
Sanyo
C2, C8, C10,
C11, C18
5
Capacitor, ceramic, 0.1 µF, 25 V, X7R, 10%
805
Vishay
VJ0805Y104KXXAT
C3
1
1
2
1
1
1
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Capacitor, ceramic, 2.2 nF, 50 V, X7R, 10%
Capacitor, ceramic, 5.6 nF, 50 V, X7R, 10%
Capacitor, ceramic, 4.7 nF, 50 V, X7R, 10%
Capacitor, ceramic, 100 pF, 50 V, NPO, 10%
Capacitor, ceramic, 1 µF, 16−V, X5R, 10%
Diode, switching, 10 mA, 85 V, 350 mW
Diode, schottky, 3 A, 40 V
805
805
Vishay
Vishay
Vishay
Vishay
TDK
VJ0805Y222KXAAT
VJ0805Y562KXAAT
VJ0805Y472KXAAT
VJ0805A101KXAAT
C2012X5R1C105KT
C5
C6,C13
C7
805
805
C9
805
D1
SOT23
SMC
Vishay−Liteon BAS16
D2
IR
30BQ040
J1, J2
Terminal block, 4-pin, 15 A, 5.1 mm
0.80 x 0.35
OST
ED2227
HC1−1R7
HAT2168H
HAT2167H
Std
(1)
L1
Inductor, SMT, 1.7−µH, 22.3 A, 1.8 mΩ
0.512 x 0.512 Coiltronics
(1)
Q1
MOSFET, N−channel, V
MOSFET, N−channel, V
30 V, R
6 mΩ, I 30 A
LFPAK
LFPAK
805
Hitachi
Hitachi
Std
DS
DS
D
(1)
Q2
30 V, R
4.2 mΩ, I 40 A
DS
DS
D
R1
Resistor, chip, 1 kΩ, 1/10−W, 1%
Resistor, chip, 0 Ω, 1/10−W, 5%
Resistor, chip, 0 Ω, 1/10−W, yy%
Resistor, chip, 20 Ω, 1/10−W, 5%
R10
R11
R12
R2
805
Std
Std
805
Std
Std
805
Std
Std
Resistor, chip, 165 kΩ, 1/10−W, 1%
Resistor, chip, 5.49 kΩ, 1/10−W, 1%
Resistor, chip, 243 kΩ, 1/10−W, 1%
Resistor, chip, 10 kΩ, 1/10−W, 1%
Resistor, chip, 71.5 kΩ, 1/10−W, 1%
Resistor, chip, 8.66 kΩ, 1/10−W, 1%
Resistor, chip, 226 Ω, 1/10−W, 1%
Resistor, chip, 16.2 kΩ, 1/10−W, 1%
805
Std
Std
R3
805
Std
Std
R4
805
Std
Std
R5
805
Std
Std
R6
805
Std
Std
R7
805
Std
Std
R8
805
Std
Std
R9
805
Std
Std
TP1, TP4,
TP5, TP7, TP9
5
4
1
JACK, test point, red
Farnell
240−345
240−333
TP2, TP3,
TP6, TP10
JACK, test point, black
Adaptor, 3.5 mm probe clip
Farnell
131−4244−00 or
131−5031−00
TP8
0.2
Tektronix
U1
−−
1
1
Wide input synchronous buck controller
PCB, 2.85” x 2” x .062 In
PWP16
TI
TPS40055PWP
HPA070
Std
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
15
SLUU186 − March 2004
9
References
1. Data Sheet, TPS40055 Wide-Input Synchronous Buck Controller (SLUS593).
2. Technical Brief, PowerPAD Thermally Enhanced Package (SLMA002).
16
TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)
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