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Texas Instruments Relay Replacement for Brushed DC Motor Drive in Automotive Applications Application notes
Application Report
SLVA837 – November 2016
Relay Replacement for Brushed DC Motor Drive in
Automotive Applications
Ishtiaque Amin, Philip Beard
ABSTRACT
Many modern automotive applications use relays for driving different loads for power distribution. Such
applications include power outlets, AC clutch, seat heaters, sunroofs, rear windshield defrost, and HVAC
blowers. Some of these applications use brushed DC (BDC) motors to drive a load. This application report
describes how Texas Instrument’s automotive gate driver devices, in addition to MOSFETs, can be used
to replace the mechanical relays in applications with a BDC motor. This document also highlights some of
the benefits of using solid-state devices (SSDs).
When used as intended, SSDs have a high lifetime of survival, usually outlasting the equipment in which
they are installed. These devices operate silently because they have no mechanical moving parts, which
helps reduce electrical interference. SSDs can typically function over a wide range of input voltages, can
drive a wide range of motors, and consume little power even at a high supply voltage. These device do
not produce any arc which makes them suitable for use in extreme environments. SSDs have no moving
parts, so physical shock, vibration, and other environmental conditions have a limited effect on the device
integrity. SSDs are often used in applications requiring high frequency switching, where mechanical relays
fail to perform, to improve efficiency.
Contents
Motor Control and Applications ............................................................................................ 3
1.1
Typical Relay Applications ......................................................................................... 3
1.2
Solid-State Design Considerations ............................................................................... 4
2
Design Constraints .......................................................................................................... 5
2.1
Coil Suppression ................................................................................................... 5
2.2
Switching Time ...................................................................................................... 5
2.3
Bounce Factor ....................................................................................................... 7
2.4
Integrated Protection .............................................................................................. 7
2.5
Interfacing With the Microcontroller .............................................................................. 8
3
Environmental Constraints.................................................................................................. 8
3.1
Shock and Vibration Limitations .................................................................................. 8
3.2
Audible Noise........................................................................................................ 8
3.3
Reliability ............................................................................................................. 8
4
Solution-Size Comparison .................................................................................................. 9
4.1
Typical Relay-Solution Size ....................................................................................... 9
4.2
Typical Motor Driver Solution Size .............................................................................. 10
4.3
Comparison ........................................................................................................ 11
5
Motor Drivers by Texas Instruments ..................................................................................... 11
5.1
Integrated Diagnostics ............................................................................................ 11
5.2
Slew-Rate Control ................................................................................................. 12
5.3
Current Regulation With Integrated Current-Sense Amplifier ............................................... 12
Appendix A
Glossary ............................................................................................................ 13
1
List of Figures
1
BDC Motor Applications in a Vehicle...................................................................................... 3
2
SPDT Configuration ......................................................................................................... 3
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3
H-Bridge Control of BDC Motor Using Relays ........................................................................... 4
4
H-Bridge Control ............................................................................................................. 4
5
Suppression Circuit for High-Voltage Transients ........................................................................ 5
6
Relay Switching Time and High-Voltage Breakdown ................................................................... 6
7
MOSFET Switching Time ................................................................................................... 7
8
Relay Interface Circuit With MCU
9
10
11
12
13
14
......................................................................................... 8
Switching Cycle Comparison ............................................................................................... 9
Typical Relay Dimensions .................................................................................................. 9
Typical MOSFET Dimensions ............................................................................................ 10
DRV8702-Q1EVM With MOSFETs and Motor Driver IC.............................................................. 10
OCP Implementation in DRV870x-Q1 ................................................................................... 11
Current Regulation With DRV870x-Q1 .................................................................................. 12
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All trademarks are the property of their respective owners.
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Motor Control and Applications
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1
Motor Control and Applications
Today's automotive industry is experiencing an increasing number of motors used in a car because of
automation, enhanced safety, and luxury benefits. Motors are found in applications such as electric power
steering, brakes, engine and transmission, body, and trunk. Figure 1 shows an overview of the areas
where BDC motors can be used in a car.
Figure 1. BDC Motor Applications in a Vehicle
A BDC motor can be driven in one of two ways. One way to drive a BDC motor is with mechanical relays
and the other is with solid-state electronic devices such as MOSFETs and a gate driver integrated circuit
(IC). If the motor size is small, the FETs can be integrated with the motor driver IC in one device. The
following sections explore both these design methods.
1.1
Typical Relay Applications
A relay is an electromagnetic switch that turns on or off based on an external electrical signal, and is used
to drive a high current load. Relays isolate low power circuits (for example, the microcontroller) from high
power circuits (for example, the BDC motor). Relays are activated by energizing a coil wound in a soft iron
core. In automotive applications, the most common type of relay used is the single-pole double-throw
(SPDT) as shown in Figure 2. Other relay configurations include single-pole single-throw, double-pole
single-throw, and double-pole single-through.
A
B
C
Figure 2. SPDT Configuration
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Motor Control and Applications
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The working principle of such a relay is simple: when the coil is not energized, the points B and C are
connected. When electricity passes through the coil, the points A and C are connected thereby driving the
load the relay is connected with.
A common automotive application for mechanical relays is to drive a BDC motor in an H-bridge topology.
This type of configuration allows for bidirectional motor rotation by changing the direction of current
through the motor. Figure 3 shows an example of an H-bridge configuration using two SPDT relays.
VBAT
M
Relay
Relay
Figure 3. H-Bridge Control of BDC Motor Using Relays
By using two relays the motor can go into brake (decay) mode by shorting the motor terminals together.
When both terminals are shorted the energy stored in the motor is quickly dissipated and the motor will
come to a complete stop. Each relay essentially acts as a switch by connecting each of the motor phase
leads to either power supply (usually vehicle battery) or ground (GND). While this topology is simple, the
amount of board space required by relays can be significantly high compared to the SSD solution. With an
increasing need for smaller boards by automotive customers, SSDs are the more attractive option.
1.2
Solid-State Design Considerations
By using an IC as the motor driver and discrete MOSFETs for the four switching positions in an H-Bridge,
relays can be replaced in most automotive applications. Figure 4 shows how such electronic components
can be used to drive a BDC motor with four external power MOSFETs in an H-bridge configuration.
VCC
HS1
HS2
M
LS1
LS2
Motor Driver Device
Figure 4. H-Bridge Control
In an H-bridge configuration, the following occurs:
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•
•
•
•
When HS1 and LS2 are on at the same time, the motor rotates in one direction, or forward drive.
When HS2 and LS1 are on at the same time, the motor rotates in the opposite direction, or reserve
drive.
When LS1 and LS2 are on simultaneously, the phase terminal of the motor is shorted to (slowly) decay
the energy stored in the windings. A similar way of power dissipation can be achieved by switching on
both HS1 and HS2 simultaneously and the energy stored in the motor will decay faster.
A faster decay mode can be achieved by flip stating the drive configuration. For example, if the motor
is in forward drive, the faster decay mode can be obtained by switching on LS1 and HS2
simultaneously to dissipate energy stored in the motor faster.
The rate of switching of the MOSFETs is determined by the host microcontroller (MCU) which commands
the motor driver to perform pulse-width modulation (PWM) at a given frequency. MOSFETs switch on or
off at a much faster rate than mechanical relays which helps with electromagnetic interference (EMI)
reduction and improves on switching losses.
2
Design Constraints
While choosing between a relay and SSD solution, a designer must consider multiple design factors,
some of which are highlighted in this section. Both solutions have their advantages in a given system,
however, the SSD solution tends to provide more flexibility and reliability.
2.1
Coil Suppression
Rapidly de-energizing relays forces the collapsing magnetic field to produce a significant voltage spike
because it must dissipate all the stored energy caused by the rapid change of the current flow. These
large voltage transients create noise in the system and produce EMI. Therefore, an external circuitry is
required in a relay system to suppress high voltage transients.
Power
Supply
Induced
Load
Figure 5. Suppression Circuit for High-Voltage Transients
Such relay coil suppression can be implemented by using external components which are a reversed
biased rectifier in series with a Zener diode, both in parallel with the relay coil as shown in Figure 5.
2.2
Switching Time
Because of the pitting that results from high-voltage switching, the on/off times of the device are
mechanically limited to the voltage limit created by the coil suppressor. This may not be an ideal solution
in systems that require faster switching of the output. The high voltage breakdown created by switching
between relay contacts results in high temperatures that pool metal and damage relay contacts over time.
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Design Constraints
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Close
Open
Current
(flow)
Time
Close
Voltage
(pressure)
Open
Figure 6. Relay Switching Time and High-Voltage Breakdown
One of the limitations for using relays is that they have much higher switching time. When compared to
MOSFETs relays are slow devices typically having switching and settling time in the range of 5 to 15 ms.
Such a range may be too slow for many automotive applications as it contributes to additional power
losses during the switching intervals. Looking at MOSFETs with similar drive capability (CSD18540Q5B)
we find the following information regarding switching times.
Table 1. MOSFET Switching Time
Parameter
Description
td(on)
Turnon delay time
Turnon delay time is the time taken to charge the
input capacitance of the MOSFET before the drain
current conduction can start.
tr
Rise time
During the rise time (tr) the MOSFET gate voltage
rises to a sufficient level to drive the MOSFET, and
the drain current rises from zero to full-on current.
td(off)
Turnoff delay time
Turnoff delay (td(off)) is the time taken to discharge the
gate capacitance after the MOSFET has been
switched off.
tf
Fall time
The fall time (tf) is the time required for the drain
current fall to zero.
Test Conditions
VDS = 30 V, VGS = 10 V,
IDS = 28 A, RG = 0 Ω
Typical
Value
Unit
6
ns
9
ns
20
ns
3
ns
Use Equation 1 to calculate the MOSFET turnon time (ton). Use Equation 2 to calculate the MOSFET
turnoff time (toff)
ton = td(on) + tr = 6 ns + 9 ns = 15 ns
toff = td(off) + tf = 20 ns + 3 ns = 23 ns
(1)
(2)
Figure 7 shows the timing diagram for a MOSFET switching on and off.
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Design Constraints
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V
100%
10%
td(off)
tr
VDS
VGS
10%
10%
10%
10%
10%
tf
td(on)
(1)
VGS is the gate-to-source voltage.
(2)
VDS is the drain-to-source voltage.
t
Figure 7. MOSFET Switching Time
The comparison table (Table 2) shows the difference is switching times between relays and MOSFETs.
For detailed information on how a MOSFET operates, refer to Understanding IDRIVE and TDRIVE in TI
Motor Gate Drivers (SLVA714).
Table 2. Switching Time Comparison
Relay
5 to 15 ms
MOSFET
Turnon Time
Turnoff Time
15 ns
23 ns
The faster switching time for a MOSFET provides designers with the advantage of reducing switching
losses in the system. This is particularly beneficial when doing variable speed control of a motor by pulse
width modulation (PWM). Changing the duty cycle of the PWM signal will vary the motor speed; the higher
the duty cycle, the faster the motor with rotate in a given direction. For a relay to perform a similar
operation it must engage and disengage the metal contacts which raise reliability concerns of the part (as
described in Section 3.3). Also, each engaging or disengaging cycle of the contacts results in 5 to 15 ms
of switching time loss making the systems less thermally efficient. As a result, the SSD solution provides a
major advantage for speed control of a motor by varying the duty cycle of the PWM pulse to drive the
external MOSFETs in the H-bridge.
2.3
Bounce Factor
When an electromagnetic relay switches on, a bounce time is present. The bounce is an internal,
undesired event where the contacts open and close intermittently for a period of time. As with any
mechanical component, constant bouncing produces contact wear, such as metal degradation between
contacts, or even contact welding or arching during these making-and-breaking events. This wear impacts
the overall integrity and can reduce longevity of the device.
2.4
Integrated Protection
Relay based systems require protection features to be implemented with discrete circuitry. This additional
circuitry requires additional board space because of the external components being added to the solution.
The addition of components adds more failure points which can have adverse effect on reliability and
safety of the application. Also, a higher component count increases the cost of the overall system and
typically requires a larger controller board size. With the SSD approach, numerous protection features can
be integrated within the motor driver IC that puts fewer burdens on the host microcontroller for fault
detection. Such integrated protection features can include overcurrent, overvoltage, and overtemperature
protection. For more details on some of these protection features, see Section 5.1.
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Design Constraints
2.5
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Interfacing With the Microcontroller
In the SSD solution, the motor driver IC can be directly connected to the MCU with a digital interface. The
motor driver IC amplifies the signal from the MCU to drive external MOSFETs to energize a motor.
If a relay is used, it should not be directly connected to a MCU because the MCU cannot able to source
enough current to energize a relay. Also, a relay is activated by energizing the coils during which the MCU
can receive negative voltage transients caused by the back-EMF of the relay. These negative will make
the MCU stop working and, in some cases, may damage the MCU.
As a result, relays are typically interfaced to the MCU by a circuit similar to the one shown in Figure 8.
VCC
A
B
C
MCU
Signal
R
Load
Figure 8. Relay Interface Circuit With MCU
The circuit requires two more components, at a minimum, which translates to more board space and
additional failure points.
3
Environmental Constraints
In addition to design constrains, special attention must be given to environmental factors when selecting a
relay-based solution. While some of these constraints can be aesthetic in nature, others can have an
impact on the device performance and safety.
3.1
Shock and Vibration Limitations
In automotive body applications relays are susceptible to physical vibration and shock (both electrical and
mechanical) and therefore designers must carefully consider the placement, packaging, and orientation of
relays on a controller board. If abnormal vibration or shock is received, it will cause the relay to
malfunction and can result in damage or component deformation.
3.2
Audible Noise
When a relay switches on and off, the contacts are engaged and disengaged rapidly which causes a
clicking sound. Depending on the application, this clicking sound could be undesired, especially in highend vehicles. An SSD solution provides a noise-free and fast switching operation because no mechanical
parts move inside. As a result, hardware designers may prefer to go with an SSD approach to eliminate
the clicking noise made by relays.
3.3
Reliability
Most mechanical relays are typically rated for 100,000 cycles for electrical endurance. Beyond these rated
number of cycles, damage on the contacts can occur which increases the risk of failure. When switching
off inductive loads, electrical arc discharge occurs between the relay contacts resulting in wear and fatigue
over time.
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1 200 000
1 000 000
Cycles
800 000
600 000
400 000
200 000
0
Relays
MOSFETs
Figure 9. Switching Cycle Comparison
The graph in Figure 9 provides a rough comparison of switching cycles between the solutions. MOSFETs
can typically have 10 times more switching cycles over lifetime. The effectively lifetime of an SSD solution
can be infinite in comparison to the lifetime of the mechanical parts in a car. Silicon is very robust and has
a much longer life cycle compared relays. The ICs are subjected to harsh test conditions for AEC-Q100
qualification requirements that ensure high reliability and longer life cycle. Also, an SSD can be used in
high-frequency switching circuits because they typically have much higher switching cycles over lifetime
compared to mechanical relays (see Table 2 for comparison).
4
Solution-Size Comparison
4.1
Typical Relay-Solution Size
The solution size can vary significantly depending on the type of application. For the purpose of this
application report, the solution size for Figure 3 will be explored in more details. For a given application,
set the parameters as follows:
• Supply voltage = 13.5 V
• Motor current = 15 A (rms)
• Temperature = 25°C
With these settings, a suitable automotive-SPDT relay can be one with 15-A continuous current rating.
Figure 10 shows the mechanical dimensions of such a device.
15±0.5
R0.3 (All round)
15±0.5
16.4±0.3
11±0.5
2.8±0.1
0.8 +0.05
±0.03
Figure 10. Typical Relay Dimensions
To drive a BDC motor, two of these relays is required. Each relay requires approximately 232.5 mm2 of
the area of the controller board area. Therefore, the total board area for just two relays is 2 × 232.5 mm2 =
465 mm2.
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Solution-Size Comparison
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Additional components for high-voltage transient suppression and protection circuits for system
diagnostics will be required. These components have not been considered in the board area calculation.
Some relays can be larger (or smaller) in size than the one discussed with a similar continuous-current
rating.
The height of the relays could be of another disadvantage because mechanical CAD designers must
consider at least a 16.4-mm vertical keep-out distance when designing board covers and enclosures. Most
electronic components will be less than 1.5 mm in height providing the benefit of better packaging.
4.2
Typical Motor Driver Solution Size
Keeping the same parameters listed in Section 4.1, this example uses the DRV8702-Q1EVM components
for comparison.
The DRV8702-Q1EVM has one DRV8702-Q1 motor driver device and four CSD18540Q5B FETs for
driving a BDC motor. The EVM configuration is similar to the one shown in Figure 4. Figure 11 shows the
mechanical dimensions of the FETs. The DRV8702-Q1 driver comes in a 5-mm × 5-mm QFN package.
2
7
1
3
6
5 mm
4
5
1 mm
8
5.5 mm
Figure 11. Typical MOSFET Dimensions
Using the footprint area, the following areas can be calculated:
• Area of each MOSFET = 27.5 mm2
• Total area of all 4 MOSFETs = 27.5 mm2 × 4 = 110 mm2
• Area of one DRV8702-Q1 motor driver device = 25 mm2
Total controller board area required is therefore 25 mm2 plus 110 mm2, which equals 135 mm2 for the
solid-state solution.
Figure 12 shows the solution size of SSD with DRV8702-Q1 gate driver IC on a DRV8702-Q1EVM. The
white box marks the DRV8702-Q1 gate driver IC, and the yellow box outlines the MOSFETs.
Figure 12. DRV8702-Q1EVM With MOSFETs and Motor Driver IC
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4.3
Comparison
At a quick glance, the relay-based solution requires at least three times more board space compared to
the SSD solution without even considering the additional components required for diagnostic and
protection features. Also, the relay is 16 times taller than the CSD18540Q5B FET. The 135-mm2 area is
inclusive of numerous integrated protection features that can reduce external components to lower the
cost of the overall system and save board space.
5
Motor Drivers by Texas Instruments
Relay based direction control of a BDC motor can be replaced with TI's solid-state devices listed as
follows:
• DRV8702-Q1
• DRV8703-Q1
The DRV8702-Q1 and DRV8703-Q1 devices are H-bridge gate drivers (also called gate drivers or
controllers). The device integrates FET gate drivers to control four external N-channel MOSFETs in Hbridge configuration, as shown in Figure 4. The device is also capable of driving each half-bridge
independently which can be useful to drive two unidirectional motors. The device can be powered by a
wide supply voltage between 5.5 V and 45 V. The device significantly reduces external component count
of discrete motor driver systems by integrating the required MOSFET drive circuitry into a single device.
Also, the DRV870x-Q1 device adds protection features above traditional discrete implementations, such
as undervoltage lockout, overcurrent protection, gate-driver faults, and thermal shutdown. These
integrated features are always missing when using relays, and must be implemented externally by adding
components.
In addition, the DRV8703-Q1 driver incorporates a serial-peripheral interface (SPI) module that provides
added flexibility to the customer to program different parameter settings for optimal motor drive operation.
The SPI register also provides detailed reporting of fault conditions. For detailed information about these
devices, refer to DRV870x-Q1 Automotive H-Bridge Gate Driver (SLVSDR9).
5.1
Integrated Diagnostics
This section highlights some of the protection features available in the DRV870x-Q1 motor driver device.
5.1.1
Overcurrent protection (OCP)
The voltage across each external MOSFET is monitored by the device and compared with a threshold that
the customer can select to trigger an overcurrent condition. The DRV8702-Q1 device provides 5 settings
using hardware interface, and the DRV8703-Q1 device provides 8 settings through SPI for the overcurrent
threshold. These settings provide the customer flexibility to set different overcurrent trip points for different
BDC motor sizes by keeping the same gate driver IC and external MOSFETs.
Figure 13 shows how the OCP feature is implemented inside the device. When an overcurrent event is
triggered, the H-bridge is disabled to protect the motor, MOSFETs, and the gate driver device.
VVM
VDS
Monitor
Vref
Digital
Core
VDS
Monitor
Vref
Figure 13. OCP Implementation in DRV870x-Q1
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Motor Drivers by Texas Instruments
5.1.2
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Thermal Shutdown
A temperature sensor inside the device constantly monitors the die temperature to prevent the device from
overheating and causing an overtemperature event. When the die temperature exceeds the recommended
operation condition, the H-bridge is disabled to protect itself from permanent damage.
5.1.3
Undervoltage Lockout
At any time during operation if the supply voltage drops below the recommended operating voltage, the
device goes into a lockout state and disables the external H-bridge. This lockout occurs to prevent
overdriving the charge pump inside the device. The power supply is constantly monitored by the device to
ensure that it is not driving a load under extremely low voltages.
5.2
Slew-Rate Control
The DRV870x-Q1 device includes the IDRIVE feature that allows adjustable slewing of the external
MOSFETs at any moment without adding external components to or removing them from the system. This
feature allows the customer to fine tune the switching performance of the MOSFETs with regards to
radiated emissions, efficiency, and the MOSFETs body-diode recovery inductive spikes. Mechanical relays
will always switch at a given frequency, so if the customer wants improve system-level efficiency by
reducing switching losses, the relays must be replaced.
5.3
Current Regulation With Integrated Current-Sense Amplifier
The DRV870x-Q1 device features an integrated current-sense amplifier (CSA) for measuring low-side
current. This amplifier provides feedback to the MCU on how much current is being drawn by the load
during normal operation. With the CSA, the device can perform current regulation based on an analog
voltage reference which reduces dependency on the MCU to regulate the motor current. To use the CSA,
the source pins of both the low-side MOSFETs in the H-bridge must be connected to a power (shunt)
resistor as shown in Figure 14.
VBAT
HS1
DRV870x-Q1
HS2
From
DRV870x-Q1
LS2
From
DRV870x-Q1
M
LS1
Current-Sense Amplifier
SP
Digital
+
Power or
shunt resistor
AV
SN
Core
±
SO
VREF
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Figure 14. Current Regulation With DRV870x-Q1
When the voltage across the power resistor exceeds the reference voltage (Vref), the H-bridge enters
brake mode and energy stored in the motor is dissipated through the low-side MOSFETs. To implement
such a feature with relays, external CSA devices must be implemented to monitor motor current, which
adds to the cost of the system, introduces additional failure points, and requires more board space.
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Appendix A
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Glossary
A.1
Nomenclature Used in this Document
The following acronyms and initialisms are used in ths document:
BDC — Brushed direct current
CSA — Current-sense amplifier
IC — Integrated circuit
MCU — Microcontroller unit
MOSFET — Metal-oxide semiconductor field-effect transistor
OCP — Overcurrent protection
PWM — Pulse width modulation
SPDT — Single pole double throw
SSD — Solid state device
For a more comprehensive list of terms, acronyms, and definitions, refer to the TI Glossary (SLYZ022).
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requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
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Copyright © 2016, Texas Instruments Incorporated
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