Texas Instruments | How the SmartAmp Speaker Protection Algorithm Compensates the Effects of Series | Application notes | Texas Instruments How the SmartAmp Speaker Protection Algorithm Compensates the Effects of Series Application notes

Texas Instruments How the SmartAmp Speaker Protection Algorithm Compensates the Effects of Series Application notes
Application Report
SLAA858 – August 2018
How the SmartAmp Speaker Protection Algorithm
Compensates the Effects of Series Resistance
Supriyo Palit ........................................................................................................... Software Engineer
ABSTRACT
This document describes how the SmartAmp speaker protection algorithm can compensate the effect of
adding a series resistance with a loudspeaker when used with devices with IV sense capabilities, such as
TAS2560, TAS2557, TAS2559 and TAS2770.
1
2
3
4
5
Contents
Introduction ...................................................................................................................
Circuit Diagram ...............................................................................................................
SmartAmp Algorithm Implementation .....................................................................................
Summary ......................................................................................................................
References ...................................................................................................................
1
1
2
5
5
List of Figures
.............................................................................
1
Sense Voltage Including Series Resistance
2
Sense Voltage Without Series Resistance ............................................................................... 2
1
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The Texas Instruments SmartAmp speaker protection algorithm monitors the temperature and excursion
of the loudspeaker and allows to improve the loudspeaker performance. The voltage across the
loudspeaker and the current flowing through the loudspeaker is fed back to the SmartAmp algorithm by
the IV sense capabilities integrated in the SmartAmp devices. In some applications the loudspeaker is
connected to the SmartAmp amplifier output via a long trace on the PCB, flex cable or long wire, which
adds additional resistance in series with the loudspeaker. In some cases a series resistance is
intentionally added in the circuit to reduce idle channel noise.
2
Circuit Diagram
2.1
Application Case 1: Sense Voltage Including Series Resistance
SmartAmp
SPK_P
Rs
SPK_N
Rs
SNS_P
SNS_N
Figure 1. Sense Voltage Including Series Resistance
SLAA858 – August 2018
Submit Documentation Feedback
How the SmartAmp Speaker Protection Algorithm Compensates the Effects
of Series Resistance
Copyright © 2018, Texas Instruments Incorporated
1
Circuit Diagram
www.ti.com
In this application, the sense pins are connected right at the amplifier terminals, and the sense voltage
includes the series resistance.
The series resistance acts as a voltage divider and reduces the idle channel noise across the loudspeaker
terminals. This topology also helps in keeping the sense wire traces short.
2.2
Application Case 2: Sense Voltage Without Series Resistance
SmartAmp
SPK_P
Rs
SPK_N
Rs
SNS_P
SNS_N
Figure 2. Sense Voltage Without Series Resistance
In this application, the sense pins are connected right at the speaker terminals, and the sense voltage
does not include the series resistance.
Although the series resistance acts as a voltage divider, TAS2560, TAS2557 and TAS2559 devices have
post-filter feedback circuit configuration, which means that the series resistance is included within the
amplifier feedback circuit, hence the idle channel noise of the amplifier will be induced across the
terminals of the loudspeaker. Thus, this topology cannot be used to reduce idle channel noise.
3
SmartAmp Algorithm Implementation
One key assumption in all the following analysis is that the series resistor (RS) does not change value with
power or temperature.
Also, it’s important to note that any series resistance will result in power loss, for example, for a given
input power, the output power available in the speaker will be less, or for a given output power across the
speaker, the input power requirement will be higher. This means that the efficiency will reduce.
The expression for power across the speaker is given by Equation 1:
Pspk
Pin
R dc _ t
2 u RS
R dc_t
where
•
•
•
•
•
•
Pin = power delivered by the amplifier
Pspk = power across the speaker
2 × RS = series resistance
Rdc_t = DC resistance of the voice coil of the speaker at any given time (t)
Rdc_t is a time varying quantity because it changes with change in temperature of the voice coil of the
speaker
RS is assumed to be constant
(1)
Thus, the power loss across the series resistance is:
Ploss
2
Pin
Pspk
Pin
2 u RS
2 u RS
Rdc _ t
How the SmartAmp Speaker Protection Algorithm Compensates the Effects
of Series Resistance
Copyright © 2018, Texas Instruments Incorporated
(2)
SLAA858 – August 2018
Submit Documentation Feedback
SmartAmp Algorithm Implementation
www.ti.com
3.1
3.1.1
Application Case 1: Sense Voltage Including Series Resistance
Temperature
In this case, since, the sensed voltage includes the series resistance, hence, the SmartAmp algorithm will
measure the circuit resistance as:
R msr_t
2 u RS
R dc _ t
where
•
Rmsr_t = measured resistance at any given time (t)
(3)
Assume the measured resistance at factory calibration time is:
R msr
2 u RS
R dc
where
•
•
•
Rmsr = measured resistance
2 × RS = series resistance
Rdc = DC resistance of the voice coil of the speaker at factory calibration time
(4)
Assume, temperature of the speaker voice coil at time (t) is equal to T and at factory calibration time is
equal to T0
Then,
R dc ª¬1
R dc _ t
T 0 º¼
D T
where
•
α = temperature coefficient of resistance of the voice coil material
(5)
Hence,
'T
T T0
R dc _ t R dc
DR dc
(6)
Now,
R msr _ t
R msr
DR msr
R dc _ t
R dc
DR msr
R dc _ t
R dc
§ DR msr
¨¨
© R dc
·
¸¸ R dc
¹
(7)
Define,
D'
DR dc
R msr
D R msr
2uR S
R msr
(8)
Hence,
R msr _ t
R msr
D 'R msr
R dc _ t
R dc
DR dc
'T
(9)
Thus, from RS and Rmsr, the SmartAmp algorithm is able to calculate a modified temperature coefficient of
resistance (α') to compensate for the series resistance. With this compensation, the SmartAmp algorithm
can measure the voice coil temperature accurately even with the series resistance.
3.1.2
Excursion
The addition of series resistance is going to reduce the current flowing through the circuit and hence, for a
given input voltage, the excursion (and also the SPL (1)) is going to be less. However, it is not going to
affect the SmartAmp speaker protection algorithm reliability.
(1)
SPL = Sound Pressure Level is a measure of loudness of sound produced by a loudspeaker.
SLAA858 – August 2018
Submit Documentation Feedback
How the SmartAmp Speaker Protection Algorithm Compensates the Effects
of Series Resistance
Copyright © 2018, Texas Instruments Incorporated
3
SmartAmp Algorithm Implementation
3.2
3.2.1
www.ti.com
Application Case 2: Sense Voltage Without Series Resistance
Temperature
Since the sense voltage does not include the series resistor, hence, the temperature is going to be
measured accurately using the original temperature coefficient α,
'T
R dc _ t R dc
T T0
DR dc
(10)
So, there is no compensation required.
3.2.2
Excursion
Since TAS2560, TAS2557 and TAS2559 devices have post-filter feedback circuit configuration hence the
voltage across the sense terminals (SNS_P and SNS_N) will follow the voltage with which the SmartAmp
algorithm drives the amplifier. Hence the excursion will be estimated accurately. For devices without postfilter feedback, (for example, TAS2770) the SmartAmp algorithm will estimate the excursion accurately by
compensating for the series resistance.
3.2.3
Channel Gain (Only for Devices With Post-Filter Feedback)
With post-filter feedback, the voltage across the SPK_P and SPK_N terminals will be higher than the
output of the SmartAmp algorithm (which is equal to the voltage across the SNS_P and SNS_N
terminals), for example.
V a lgo
V SNS
V SPK
V AMP
Rdc _ t
Rdc _ t
2 u RS
where
•
•
•
•
Valgo = SmartAmp output equivalent voltage (including gain of the amplifier)
VSNS = Sense voltage across SNS_P and SNS_N
VSPK = Voltage across the speaker
VAMP = Voltage at the amplifier output across SNS_P and SNS_N
Without RS( which means, RS = 0 in Equation 11), V a lgo
chosen such that
V SNS
(11)
V AMP and the gain of the amplifier is
Valgo_max(maximum value of Valgo) = VSNS_max(maximum value of VSNS) = VAMP_max(maximum value of VAMP) = VMAX
where
•
VMAX is the rated maximum voltage of the amplifier such that there is no clipping.
(12)
From Equation 11, when, Valgo = VSNS = VSPK = VMAX,
V AMP _ max
V MAX
Rdc _ t
2 u RS
Rdc _ t
! V MAX
(13)
Thus, the amplifier will clip if the gain is kept at the same level as before.
To prevent clipping of the amplifier, the SmartAmp algorithm reduces Valgo using a post gain compensation
block such that,
V AMP _ max
V MAX and V a lgo
V SNS
V SPK
V MAX
Rdc _ t
Rdc _ t
2 u RS
(14)
Thus, the post gain compensation block is a gain multiplier whose value is
Rdc _ t
Rdc _ t
4
2 u RS
(15)
How the SmartAmp Speaker Protection Algorithm Compensates the Effects
of Series Resistance
Copyright © 2018, Texas Instruments Incorporated
SLAA858 – August 2018
Submit Documentation Feedback
Summary
www.ti.com
As an example, assume, RS = 1 Ω and minimum value of Rdc_t = 8 Ω (minimum value will cause maximum
post gain compensation).
Post Gain Compensation will be:
8
0.8 linear scale
1.94 db
8 2
(16)
Thus, the SmartAmp speaker protection algorithm will attenuate the output by –1.94 dB to ensure that
there is no clipping at the amplifier output terminals SKP_P and SPK_N.
4
Summary
The document explains how the SmartAmp algorithm is able to compensate the effect of series resistance
in the two most commonly used application cases. In application case 1, the compensation is done in the
temperature coefficient of resistance (for accurate temperature control) while in application case 2 the
compensation is done in the post gain block (to prevent amplifier clipping)
5
References
•
Texas Instruments, SmartPA Speaker Protection Algorithm application report
SLAA858 – August 2018
Submit Documentation Feedback
How the SmartAmp Speaker Protection Algorithm Compensates the Effects
of Series Resistance
Copyright © 2018, Texas Instruments Incorporated
5
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising