Texas Instruments | Diagnostics and Protections in Automotive Audio Systems (Rev. A) | Application notes | Texas Instruments Diagnostics and Protections in Automotive Audio Systems (Rev. A) Application notes

Texas Instruments Diagnostics and Protections in Automotive Audio Systems (Rev. A) Application notes
Diagnostics and Protections in Automotive Audio
Systems
Jayson Johnston
In automotive audio systems, a reliable audio amplifier
has a large impact on system performance. That is
why TI’s Class-D automotive amplifiers have
integrated protections and diagnostics, which ensure
amplifier and system reliability. To differentiate the
two, protections typically refer to features which
actively step-in during fault conditions, while
diagnostics are used to search for fault conditions.
Protection
Traditional protection features almost always cause
the amplifier to shut down. TI has made several
advancements that allow the amplifier to remain active
during manageable fault conditions. One example of
enhanced protections is Cycle-by-Cycle Current
Control, or CBC. CBC is activated in overcurrent
situations when the current limit threshold is met. As
shown in Figure 1, the current output is actively limited
by reducing the duty cycle of the output PWM over
several switching cycles, which protects the amplifier
from overcurrent damage.
shorted load on a single output). Temperature warning
and protection measures for each channel operate in
the same manner as the global warning and protection
measures, but the primary difference is that a single
channel can be shut down and audio can continue
playback via the remaining channels.
New 2-MHz automotive Class-D audio amplifiers like
TI’s TPA6304-Q1 are able to protect themselves from
excessive temperatures by limiting the gain to reduce
output power. This is triggered by either the global or
per channel temperature threshold being exceeded.
The attack and release time, which corresponds to the
rate of change in the gain, are programmable. This
allows for the engineer to choose their level of
aggressiveness in limiting gain and thermal exposure.
The attack and release times are selectable from 100
ms to 1600 ms, and the maximum attenuation is 12
dB.
DC offset protection in TI’s Class-D automotive amps
is also implemented independently per channel.
Undervoltage and overvoltage protections are, of
course, implemented and improved. TI has
revolutionized this feature with industry-leading
operating range for Class-D amplifiers. TI's automotive
Class-D amplifiers like the new TPA6304-Q1 operate
down to 4.5 V, as opposed to competitors which
require 5 V to 6 V. Low operating voltage is very
important in automotive applications, as audio is
required in start/stop conditions for information, in the
form of chimes originating from the cluster.
Figure 1. CBC Timing Diagram for Automotive
Class-D Audio Amps
Overtemperature protection traditionally uses a single
“global” temperature sensor near the center of the
amplifier. When the global sensor measures the
temperature as exceeding a set warning threshold,
then a global temperature warning signal is activated.
If the temperature continues to rise until it reaches the
global overtemperature shutdown threshold, then the
amplifier will automatically shut down, and audio
playback for all four channels ceases.
Overtemperature warning and protection measures
can be further enhanced by allowing for thermal
monitoring on a “per channel” basis. Overtemperature
monitoring and protection on a per channel basis
features additional temperature sensors at the output
of each channel, where more localized temperature
events may occur (such as an event where there is a
SLOA282A – July 2019 – Revised September 2019
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Another protection important to the automotive industry
is load dump protection. TI’s automotive Class-D
amplifiers all feature load dump protection up to 40 V.
While clipping is sometimes desired when recording
music, audio engineers typically try to minimize
clipping in the audio amplifier. Warnings for clipping
and overtemperature are provided via the external
WARN pin, and through the I2C bus.
Diagnostics
Diagnostics can be split into two subsections: DC and
AC diagnostics. TI offers fully integrated diagnostic
abilities in automotive Class-D amplifiers.
DC diagnostics function by sending a low-current DC
signal to test for fault conditions. These conditions are:
• Short to supply
• Short to ground
• Short across load
Diagnostics and Protections in Automotive Audio Systems Jayson Johnston
Copyright © 2019, Texas Instruments Incorporated
1
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•
Open load
This prevents the amp from delivering full power to a
faulty load, which protects the amplifier and speaker
from damage. The load resistance can also be
measured in this manner.
Some speaker configurations use a passive crossover
network to separate the high- and low-frequency
components of a signal, allowing for a woofer and
tweeter to be connected on the same channel. This is
opposed to connecting the woofer and tweeter on two
separate channels. Figure 2 illustrates a simplified
passive crossover network. The coupling capacitor
between the tweeter and amplifier blocks DC signals,
so an AC signal must be used instead. This is called
AC diagnostics. Using AC diagnostics allows for
speaker detection in a passive crossover and
impedance measurement.
Figure 2. Simplified Passive Crossover Network
An internal signal generator can be found in devices
like TI’s new 2-MHz audio amplifier, TPA6304-Q1.
This internal signal generator provides the AC signal
for diagnostics. The signal can range from 1 kHz to 20
kHz, and is selectable via I2C.
Diagnostics are typically required during manufacturing
for test and validation purposes. Diagnostics results
are available via I2C for quick validation. TI also offers
a powerful audio design tool, PurePath™ Console,
which makes evaluation easy by providing a simple,
and easily understandable, GUI. Figure 3 shows the
interface and results of the AC diagnostics test used to
detect a properly connected load.
Figure 3. PurePath™Console Diagnostic Interface
As automobiles become more connected, more data is
being sent over the air (OTA) to manufacturers. Load
diagnostics, both AC and DC, let auto OEMs
understand what is happening to speakers on the road
through OTA data, and can notify the driver of issues
that require attention. For example, virtual engine
sound systems (VESS) and telematics applications are
improved when speaker connectivity is monitored, due
to the potentially lifesaving nature of the feature.
Diagnostics can be used to test for a connected load
by initiating the test via I2C when audio is not in use.
The impedance and phase measurement can then be
transmitted OTA for OEMs to characterize speakers
over a product lifetime. Results are also valuable to
alert drivers of any problems. This is just one way
diagnostics can be used to improve system reliability
and safety on the road.
Table 1 below lists the newest automotive audio
amplifiers which feature integrated diagnostic and
protection features outlined in this note.
Resources
• Texas Instruments, DC and AC Load Diagnostics
Using 75-W TAS6424-Q1 Class-D Audio Amplifier
• Texas Instruments, Instrument clusters: moving
beyond chimes and dings
• PurePath™Console graphical development suite
for audio system design and development
Table 1. Automotive Audio Amplifiers With Load Diagnostics and Protections
2
DEVICE
INPUT TYPE
CHANNELS
OPERATING RANGE (V)
CURRENT LIMIT (A)
TAS6421-Q1
Digital
1
4.5 V to 26.4 V
6.5 A
TAS6422-Q1
Digital
2
4.5 V to 26.4 V
6.5 A
TAS6424-Q1
Digital
4
4.5 V to 26.4 V
6.5 A
TAS6424M-Q1
Digital
4
4.5 V to 18 V
6.5 A
TAS6424L-Q1
Digital
4
4.5 V to 18 V
4.8 A
TPA6404-Q1
Analog
4
4.5 V to 18 V
6.5 A
TPA6304-Q1
Analog
4
4.5 v to 18 V
6.5 A
Diagnostics and Protections in Automotive Audio Systems Jayson Johnston
SLOA282A – July 2019 – Revised September 2019
Submit Documentation Feedback
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