Texas Instruments | Pop Reduction for the TPA1517 Audio Power Amplifier | Application notes | Texas Instruments Pop Reduction for the TPA1517 Audio Power Amplifier Application notes

Texas Instruments Pop Reduction for the TPA1517 Audio Power Amplifier Application notes
Application Report
SLOA108 – August 2004
Pop Reduction for the TPA1517 Audio Power Amplifier
Mark Ginsberg...................................................................................... HPL/Audio Power Amplifiers
ABSTRACT
The TPA1517 audio power amplifier is a powerful and versatile device that can deliver
greater than 6 W of stereo power into loads as low as 4 Ω.
One phenomenon of the TPA1517, however, is the undesirable pop heard when the
device comes out of standby mode. This application report is for circuit designers who
wish to reduce this pop to lower levels.
This document can help circuit designers to better understand what pop is, why it
occurs with this device, why the pop-reduction circuit works, and the tradeoffs that
occur when different components are used in the circuit.
1
Introduction
The pop heard when an audio amplifier comes in and out of shutdown or standby, or is simply powered on
and off, can directly affect the enjoyment the listener experiences when they turn on the television, stereo,
powered speakers, portable radio, or any other device that plays audio. Most of us know this phenomenon
as the thud or crack heard when turning on the receiver at home, or the powered speakers for the
computer. For most people, this is considered a minor nuisance at most, and eventually they ignore it.
This usually occurs if the annoying sound is moderate to begin with.
In certain circumstances, the pop can be so loud as to cause irritation, agitation, and perhaps even
physical discomfort to the ear.
Well-designed circuitry can either eliminate or greatly reduce the amount of pop that actually arrives at the
speaker, and subsequently to the listener’s ear. This application report provides that circuitry, as well as
explanations and alternatives. Scope captures show just what the pop really is, and how the circuitry
works to reduce or eliminate it. The impact of this particular pop solution on the total harmonic distortion
plus noise vs output power and frequency are minimal, and sweeps taken with an Audio Precision™
analyzer are provided to demonstrate this.
2
What Causes Pop in the TPA1517?
The pop addressed in this application report is the undesirable noise heard when the part is taken out of
standby mode, as well as the power-up and power-down sequences (addressed in Section 5).
The pop heard when the device is put into standby mode is minimal, but the pop heard when the
TPA1517 comes out of standby mode is significant. It is caused by two events happening simultaneously:
the inputs biasing up to the proper level, and a change in the output bias level.
Figure 1 and Figure 2 are scope captures of what the typical noise pop looks like. A Texas Instruments
evaluation module (EVM) was used with a 4-Ω load and a 12-V supply. Note the shape of the output
traces, both before and after the output decoupling capacitor. The sharp transients are the same. Also
note that the dc level of Trace 2 drops to 0 V for 40 ms to 50 ms before slowly rising back up to midrail.
Figure 2 shows more detail at the time the device is brought from the standby state to the active state, but
fails to demonstrate the long time necessary for the dc voltage to properly bias.
SLOA108 – August 2004
Pop Reduction for the TPA1517 Audio Power Amplifier
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What Causes Pop in the TPA1517?
Standby Pin
Voltage On
Output Pin
Voltage On Load After
Decoupling Capacitor
Figure 1. TPA1517 Pop With a 4-Ω Load and a 12-V Supply
Standby Pin
Voltage On
Output Pin
Voltage On Load After
Decoupling Capacitor
Figure 2. TPA1517 Pop With a 4-Ω Load and a 12-V Supply (Zoom In)
2.1
How the Input Bias Causes Pop
The dc bias at the input stage of the TPA1517 is nominally 2.1 V regardless of the supply voltage. When
the TPA1517 is placed into standby mode, the input bias voltage drops, often by several hundred millivolts
or more. When the device is returned to an active state, the input bias voltage quickly returns to its
nominal level of 2.1 V. The farther away from 2.1 V the input bias voltage rests in standby, the louder the
pop is when returning to an active state. Figure 3 graphically demonstrates what input noise pop looks like
at the load in a 12-V, 4-Ω system. Trace 1 is the voltage on the STANDBY pin. Trace 2 is the voltage on
the load side of the output decoupling capacitor, ac coupled.
2
Pop Reduction for the TPA1517 Audio Power Amplifier
SLOA108 – August 2004
www.ti.com
What Causes Pop in the TPA1517?
Standby Pin
Voltage On Load After
Decoupling Capacitor
Figure 3. TPA1517 Pop Caused by the Inputs
2.2
How the Output Bias Causes Pop
The dc bias at the output stage of the TPA1517 is nominally VCC/2. This is done so that the output signal
can have a high output swing in both the positive and negative directions without one side being clipped
before the other. Unlike many other TI Audio Power Amplifier devices, when the TPA1517 is placed into
standby mode, the outputs do not go to ground. Rather, the outputs remain at dc midrail. However, during
the transition from standby to active, the outputs can exhibit brief but sharp transient spikes in the dc
voltage. These spikes, which can be several volts in magnitude, propagate to the speaker and cause the
loud pop sound. This happens because the voltage variation is so quick that the dc-blocking capacitor fails
to see this as a change in dc, and therefore allows the signal to pass through
Figure 4 is a scope capture of what the pop, caused by the output bias voltage, looks like at the load in a
12-V, 4-Ω system. Note the large voltage spike of approximately 5 V on both Traces 2 and 3.
Standby Pin
Voltage On
Output Pin
Voltage On Load After
Decoupling Capacitor
Figure 4. TPA1517 Pop Caused by the Outputs
SLOA108 – August 2004
Pop Reduction for the TPA1517 Audio Power Amplifier
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Reducing the Pop
3
Reducing the Pop
The pop is caused by dc bias issues at the input stage and output stage of the TPA1517. In order to
minimize the pop as much as possible, it is necessary to find a solution to solve both the input and output
bias problems. Essentially, this amounts to two separate solutions, because either one can be used
individually.
3.1
Quieting the Input Stage
The dc input bias problem does not contribute to the pop as much as the output dc bias, but it is more
complicated and therefore is addressed first.
Because the pop generated by the input dc bias voltage is caused by the significant drop in the input dc
bias voltage when the device goes into standby mode, the obvious solution is to force the inputs to 2.1 V,
regardless of the state of the device.
This solution is not as simple as it first appears to be. Simply putting a resistor divider at the inputs to
generate the 2.1 V (desired on the input pins) from the supply is not a good solution. Although it provides
the constant dc bias required, it also requires that two resistors be permanently placed on the device-side
of the input capacitor. This has the effect of greatly attenuating the input signal.
A solutiont is required in which the inputs are biased by an external source when the device is in standby
mode, but where the external source is disconnected during normal operation. To accomplish this, a
series of switches must be used with a resistor divider, sized appropriately for the supply voltage. The first
switch is connected to the STANDBY pin and acts as an inverter. The second switch then serves to
connect or disconnect the INPUT pin from the 2.1 V formed by the resistor divider.
The TPA1517 has a relatively large input bias current; so, it is necessary to use resistors of lower values
in the resistor divider. This is so that the input bias current has as little effect as possible on the 2.1 V
generated by the divider. Using resistors whose total series value is greater than 10 kΩ is unwise because
the input bias current is large enough to significantly alter the divider voltage. However, to go too low in
value results in high current through the resistors, and that can generate unwanted heat. For example,
when R1 is 1 kΩ it dissipates about 100 mW and R2 about 25 mW with a divider current of 9.84 mA. If R1
is lowered to 100 Ω, instead of 1 kΩ, the divider current at 12 V jumps from 9.84 mA to 98.4 mA. That
means that R1 is dissipating nearly 1 W of power and R2 nearly 1/4 W! See Table 1 for the recommended
resistor values for the input voltage divider. Exercise care in choosing resistors with the appropriate power
ratings.
Table 1. Recommended Resistor Values for the Input Voltage Divider
3.2
VCC
R1
R2
10 V
1 kΩ
270 Ω
12 V
1 kΩ
220 Ω
14.5 V
1 kΩ
180 Ω
18 V
1 kΩ
140 Ω
Quieting the Output Stage
The impact of the output transients on pop is enormous. As can be seen in Figure 2, the output stages are
responsible for the largest spike, which can easily be correlated with what the ear hears.
The solution to pop caused by the output stage is to quickly (but not instantaneously) pull the outputs to
ground when the device goes into standby mode, then allow the outputs to return to midrail when the
device returns to active mode.
When the outputs are intentionally brought to ground, they cannot fluctuate when the device first returns to
active mode. The outputs return to their proper level and are able to drive speakers only when the output
switch turns off (transistors Q2 and Q3 are output switches, see Figure 5).
4
Pop Reduction for the TPA1517 Audio Power Amplifier
SLOA108 – August 2004
www.ti.com
Reducing the Pop
3.3
Tying it all Together
Both the inputs and outputs must have the proper circuitry around them for the best pop solution. In
addition, because the TPA1517 is a stereo amplifier, the pop-reduction circuitry must be adapted to work
for both channels with a minimal component count. This can be accomplished by using just one inverter to
drive both left and right input switches and both left and right output switches.
Figure 5 is a detailed schematic for a full stereo solution. The circuit depicted in Figure 5 uses bipolar
transistors, which are generally less expensive than FETs. If FETs are preferred, a similar circuit is
depicted in Figure 6. Standby Control should be pulled to ground for a low. This ensures that variations in
VBE do not accidentally activate the circuit.
VCC
VCC
R1
1 k
R2
220 GND/HS
B
B
R7
100 k
2N3904
1 F
A
SVRR
R8
100 k
C5
2.2 F
TPA1517
In1
Input 1
470 F
VO1
B
2N3904
1 F
A
470 F
VO2
In2
Input 2
R6
100 k
Q3
2N3904
Q4
C4
100 F
PGND
SGND
Q5
C1
1 F
VCC
M/SB
Q2
2N3904
C3
10 F
R5
100 k
A
C2
10 F
VCC
R3
10 k
R4
100 k
A
2N3904
Standby Control
Q1
See Table 1 for recommended R2 values for different V
CC
Figure 5. Detailed Schematic for Full Stereo Solution Using Bipolar Transistors
SLOA108 – August 2004
Pop Reduction for the TPA1517 Audio Power Amplifier
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Audio Performance
VCC
VCC
R1
1 k
R2
220 GND/HS
SGND
B
2N7000
SVRR
C2
2.2 F
TPA1517
470 F
In1
Input 1
1 F
VO1
B
2N7000
A
M5
C4
100 F
PGND
B
M4
C3
1 F
VCC
M3
A
2N7000
470 F
VO2
In2
Input 2
1 F
M/SB
2N7000
M2
A
VCC
R3
100 k
A
M1
Standby Control
2N7000
C1
1 F
See Table 1 for recommended R2 values for different V
CC
Figure 6. Detailed Schematic for Full Stereo Solution Using FETs
4
Audio Performance
The TPA1517 pop solution presented in this document does not add to the total harmonic distortion pluse
noise (THD + N) of the overall system. Figure 7 and Figure 8 each contain two THD + N sweeps taken
with a TPA1517 EVM. Figure 7 is a THD +N vs output power sweep, whereas Figure 8 is a THD + N vs
frequency sweep. The higher distortion at lower frequencies, exhibited in Figure 8, is due to the high-pass
filter formed by the input capacitor and input resistance.
6
Pop Reduction for the TPA1517 Audio Power Amplifier
SLOA108 – August 2004
www.ti.com
Power-Up and Power-Down Pop Reduction
THD+N − Total Harmonic Distortion + Noise − %
10
RL = 4,
VCC = 12 V,
f = 1 kHz
Red − Pop Reduction Circuitry Connected
1
Blue − Pop Reduction Circuitry
Not Connected
0.1
100m
500m
1
PO − Output Power − W
5
THD+N − Total Harmonic Distortion + Noise − %
Figure 7. Comparison of THD+N vs PO Sweeps
1
RL = 4,
VCC = 12 V,
PO = 2 W
Red − Pop Reduction Circuitry Connected
Blue − Pop Reduction Circuitry Not Connected
0.1
20
100 200
1k 2k
f − Frequency − Hz
10k 20k
Figure 8. Comparison of THD+N vs Frequency Sweeps
5
Power-Up and Power-Down Pop Reduction
The pop reduction solution presented in this application report can also be used to lessen the effects of
the power-up and power-down sequences.
During normal operation, the TPA1517 is subject to loud pops during power up and power down. The pop
reduction circuitry can be easily adapted to overcome this annoyance as well. Left alone, the pop
reduction circuitry does not help much during power up and power down because power is being removed
SLOA108 – August 2004
Pop Reduction for the TPA1517 Audio Power Amplifier
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Trademarks
from the pop-reducing circuitry as well as the device. However, the TPA1517 can be powered up and
powered down in standby mode. Keeping the TPA1517 in standby mode while powering up gives the
pop-reducing circuitry enough time to properly bias, so that when the device is placed into an active state
the pop is greatly reduced. Likewise, the pop-reducing circuitry holds the output to ground when in
standby mode, so that when the device is powered off, it is virtually popless.
6
Trademarks
Audio Precision is a trademark of Audio Precision, Inc.
8
Pop Reduction for the TPA1517 Audio Power Amplifier
SLOA108 – August 2004
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