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Texas Instruments Minimize Idle Current in Portable Audio With TAS5805M Hybrid Mode Application notes
Application Report
SLAA836 – May 2018
Minimize Idle Current in Portable Audio With TAS5805M
Hybrid Mode
Shawn Zheng, Peter Cao
ABSTRACT
The Class-D audio amplifier is an important piece of the portable audio design, and is confronted with
many challenges such as smaller form factors, lower power, and higher performance. To achieve low
power losses, multiple factors must be taken into consideration, including the PWM switching frequency,
the inductors in the output LC filter, and the modulation mode of the amplifier. This application note
explains and compares how each factor affects the idle current using the TAS5805M, which features the
ultra-low-idle-current Hybrid Mode Modulation.
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Contents
General Overview ...........................................................................................................
Ripple Current on the Inductors............................................................................................
LC Filter Configuration ......................................................................................................
PWM Frequency .............................................................................................................
Modulation Mode.............................................................................................................
THD Performance Measurement ..........................................................................................
Conclusion ....................................................................................................................
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List of Figures
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PVDD/2 Common-Mode Voltage .......................................................................................... 2
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PWM Voltage Waveform.................................................................................................... 2
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Inductor Voltage and Current
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..............................................................................................
Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, Hybrid Mode) ................................................
Idle Current vs PVDD on TAS5805M (FSW = 768 kHz, BD Mode) ....................................................
Idle Current vs PVDD on TAS5805M (10 µH + 0.68 µF LC Filter, BD Mode) .......................................
PWM Voltage Waveform (15% Duty-Cycle) ..............................................................................
Inductor Voltage and Current (15% Duty-Cycle) ........................................................................
Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, 10 µH + 0.68 µF LC Filter) .................................
Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, 22 µH + 0.68 µF LC Filter) .................................
Idle Current vs PVDD on TAS5805M (FSW = 768 kHz, 10 µH + 0.68 µF LC Filter) .................................
THD vs Input Frequency (PVDD = 12 V, Load = 6 Ω, 10 µH + 0.68 µF, 1-W Output Power) .....................
THD vs Input Frequency (PVDD = 12 V, Load = 6 Ω, 10 µH + 0.68 µF, 5-W Output Power) .....................
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List of Tables
Trademarks
All trademarks are the property of their respective owners.
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Minimize Idle Current in Portable Audio With TAS5805M Hybrid Mode
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1
General Overview
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General Overview
The low idle current helps to extend the battery life and reduce the power consumption in consumer
electronics. To achieve lower idle current, the TAS5805M supports different LC filter configurations, PWM
frequency and modulation mode configurations.
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Ripple Current on the Inductors
Ripple current on the inductors is defined as the alternating current flowing through the output inductor of
a class-D amplifier. The ripple current contributes the most to the total idle current. Lower ripple current on
the inductors makes lower idle current. This application report analyzes the ripple current in idle state.
Figure 1 shows that when in BD modulation mode, class-D amplifiers produce a common-mode voltage of
PVDD/2 after the LC filter at idle, which is the average value of the 50% duty-cycle PWM switching
waveform.
I+
V = PVDD/2
Out A
I±
Out B
Figure 1. PVDD/2 Common-Mode Voltage
Output PWM voltage
Therefore, the voltage across the output inductor changes the polarity when the PWM voltage reaches
PVDD/2. The maximum voltage across the inductor is PVDD/2 and the minimum voltage is –PVDD/2 (see
Figure 2).
PVDD
PVDD/2
0V
Figure 2. PWM Voltage Waveform
Figure 3 shows the inductor voltage and current waveforms drawn using these arguments.
+I
0V
0A
-I
-PVDD/2
1/(4xFsw)
Inductor current
Inductor voltage
+PVDD/2
= Inductor Current Flows
Towards the load
= Inductor Current Flows
Away From the load
Figure 3. Inductor Voltage and Current
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Minimize Idle Current in Portable Audio With TAS5805M Hybrid Mode
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LC Filter Configuration
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At idle, the positive and negative current flow through the inductor must be symmetrical and therefore
centered around zero. Otherwise, there is a DC offset across the speaker and a constant average current
flow through the load. The shaded regions in Figure 3 indicate the direction of current flow.
Calculate the peak ripple current at idle with Equation 2:
(1)
where
•
•
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L = inductor value
FSW = PWM switching frequency
(2)
LC Filter Configuration
Equation 2 shows that the larger value inductors in the output filter make lower ripple current in idle state.
Figure 4 shows the idle current with three different LC filter configurations (22 µH, 15 µH, and 10 µH) for
FSW = 384 kHz and Hybrid mode. Figure 5 shows the idle current with 4.7-µH and 10-µH inductors for FSW
= 768 kHz and BD mode.
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10 PH + 0.68 PF
15 PH + 0.68 PF
22 PH + 0.68PF
Idle Current (mA)
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12
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PVDD (V)
18
20
22
24
D001
Figure 4. Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, Hybrid Mode)
170
4.7 PH + 0.68 PF
10 PH + 0.68 PF
150
Idle Current (mA)
130
110
90
70
50
30
10
4
6
8
10
12
14
16
PVDD (V)
18
20
22
24
D002
Figure 5. Idle Current vs PVDD on TAS5805M (FSW = 768 kHz, BD Mode)
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PWM Frequency
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PWM Frequency
Idle Current (mA)
Equation 2 shows that the idle current is lower with higher PWM frequency. Figure 6 shows the idle
current measurement for FSW = 384 kHz and 768 kHz (10-µH inductor and BD mode).
90
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60
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20
15
FSW = 384 kHz
FSW = 768 kHz
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PVDD (V)
18
20
22
24
D003
Figure 6. Idle Current vs PVDD on TAS5805M (10 µH + 0.68 µF LC Filter, BD Mode)
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Modulation Mode
The TAS5805M device supports BD mode, 1SPW mode, and Hybrid mode. The duty-cycle is lower in
1SPW mode and Hybrid mode than it is in BD mode.
Output PWM voltage
Using 15% duty-cycle as an example, the common-mode voltage after the LC filter is 0.15 × PVDD, as
Figure 7 shows. The maximum voltage across the inductor is 0.85 × PVDD and the minimum voltage is
–0.15 × PVDD, as Figure 8 shows.
PVDD
0.15*PVDD
0V
Figure 7. PWM Voltage Waveform (15% Duty-Cycle)
+I
0A
-I
0V
-0.15*PVDD
Inductor current
Inductor voltage
0.85*PVDD
0.075/Fsw
Figure 8. Inductor Voltage and Current (15% Duty-Cycle)
Apparently, the idle current is lower with the lower duty-cycle (< 50%). Figure 9, Figure 10, and Figure 11
compare the idle current between BD mode, 1SPW mode, and Hybrid mode, under different PWM
switching frequencies and LC filter configurations.
4
Minimize Idle Current in Portable Audio With TAS5805M Hybrid Mode
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Modulation Mode
www.ti.com
90
BD
1SPW
Hybrid
80
Idle Current (mA)
70
60
50
40
30
20
10
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10
12
14
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PVDD (V)
18
20
22
24
D004
Figure 9. Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, 10 µH + 0.68 µF LC Filter)
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Idle Current (mA)
30
25
20
15
10
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10
12
14
16
PVDD (V)
18
20
22
24
D005
Figure 10. Idle Current vs PVDD on TAS5805M (FSW = 384 kHz, 22 µH + 0.68 µF LC Filter)
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BD
1SPW
50
Idle Current (mA)
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30
25
20
15
10
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10
12
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PVDD(V)
18
20
22
24
D006
Figure 11. Idle Current vs PVDD on TAS5805M (FSW = 768 kHz, 10 µH + 0.68 µF LC Filter)
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5
THD Performance Measurement
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THD Performance Measurement
THD performance is measured for 1- and 5-W output power with 12-V PVDD, 6-Ω load, and a 10-µH
inductor, as Figure 12 and Figure 13 show.
The THD performance in Hybrid mode is close to BD mode. While in 1SPW mode, the THD performance
is slightly worse than BD mode or Hybrid mode. The blue (cyan) curve is for 1-W output power in 1SPW
mode, the green curve is for 1-W output power in BD mode, and the red one is 1 W in Hybrid mode.
Figure 12. THD vs Input Frequency (PVDD = 12 V, Load = 6 Ω, 10 µH + 0.68 µF, 1-W Output Power)
Figure 13. THD vs Input Frequency (PVDD = 12 V, Load = 6 Ω, 10 µH + 0.68 µF, 5-W Output Power)
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Conclusion
The ripple current on the inductor depends on the output filter configuration, PWM switching frequency,
and modulation mode. Very low idle current can be achieved with the TAS5805M Hybrid Mode, while
maintaining excellent THD performance.
6
Minimize Idle Current in Portable Audio With TAS5805M Hybrid Mode
Copyright © 2018, Texas Instruments Incorporated
SLAA836 – May 2018
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