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LM4947 Datasheet(PDF) 27 Page - Texas Instruments |
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LM4947 Datasheet(HTML) 27 Page - Texas Instruments |
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27 / 37 page  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 Submit Documentation Feedback 27 Product Folder Links: LM4947 LM4947TLEVAL |
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