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LM4947 Datasheet(PDF) 27 Page - Texas Instruments

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Part No. LM4947
Description  Mono Class D and Stereo Audio Sub-System
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM4947 Datasheet(HTML) 27 Page - Texas Instruments

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LM4947
C3DR
C3DL
R3DL
R3DR
(internal resistors)
LM4947, LM4947TLEVAL
www.ti.com
SNAS349D – JUNE 2006 – REVISED MAY 2013
Figure 89. External RC Network with Optional R3DL and R3DR Resistors
f3DL(-3dB) = 1 / 2π * (20kΩ + R3DL) * C3DL
(3)
f3DR(-3dB) = 1 / 2π * 20kΩ + R3DR) * C3DR
(4)
ΔAV (change in AC gain) = 1 / 1 + M, where M represents some ratio of the nominal internal resistor, 20kΩ (see
example below).
f3dB (3D) = 1 / 2π (1 + M)(20kΩ * C3D)
(5)
CEquivalent (new) = C3D / 1 + M
(6)
Table 7. Pole Locations
R3D (kΩ)
C3D (nF)
M
ΔAV (dB)
f-3dB (3D)
Value of C3D
new Pole
(optional)
(Hz)
to keep same
Location
pole location
(Hz)
(nF)
0
68
0
0
117
1
68
0.05
–0.4
111
64.8
117
5
68
0.25
–1.9
94
54.4
117
10
68
0.50
–3.5
78
45.3
117
20
68
1.00
–6.0
59
34.0
117
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 8
Ω LOAD
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance. As load
impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and
wire) resistance between the amplifier output pins and the load's connections. Residual trace resistance causes
a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1
Ω
trace resistance reduces the output power dissipated by an 8
Ω load from 158.3mW to 156.4mW. The problem of
decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load
dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide
as possible.
Poor power supply regulation adversely affects maximum output power. A poorly regulated supply's output
voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output
signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the
same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps
maintain full output voltage swing.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required
to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For
audio systems, the energy delivered in the audible bands is considered useful including the distortion products of
the input signal. Sub-sonic (DC) and super-sonic components (>22kHz) are not useful. The difference between
the power flowing from the power supply and the audio band power being transduced is dissipated in the
Copyright © 2006–2013, Texas Instruments Incorporated
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Product Folder Links: LM4947 LM4947TLEVAL


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