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LM4876MX Datasheet(PDF) 10 Page - Texas Instruments |
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LM4876MX Datasheet(HTML) 10 Page - Texas Instruments |
10 / 19 page LM4876 SNAS054E – FEBRUARY 2000 – REVISED MAY 2013 www.ti.com SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4876's performance requires properly selecting external components. Though the LM4876 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. The LM4876 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to- noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the AUDIO POWER AMPLIFIER DESIGN section for more information on selecting the proper gain. Input Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitor (Ci in Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150 Hz. Applications using speakers with this limited low frequency response reap little improvement by using a large input capacitor. Besides affecting system cost and size, Ci also affects the LM4876's click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor's size. Higher value capacitors need more time to reach a quiescent DC voltage (usually VCC/2) when charged with a fixed current. The amplifier's output charges the input capacitor through the feedback resistor, Rf. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired -3dB frequency. As shown in Figure 1, the input resistor (RI) and the input capacitor, CI produce a -3dB high pass filter cutoff frequency that is found using Equation 7. f-3dB = 2πRINCI (7) As an example when using a speaker with a low frequency limit of 150Hz, Equation 7 gives a value of Ci equal to 0.1µF. The 0.22µF Ci shown in Figure 1 allows for a speaker whose response extends down to 75Hz. Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consideration should be paid to value of, CB, the capacitor connected to the BYPASS pin. Since CB determines how fast the LM4876 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4876's outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci as small as possible helps minimize clicks and pops. AUDIO POWER AMPLIFIER DESIGN Audio Amplifier Design: Driving 1W into an 8 Ω Load The following are the desired operational parameters: Power Output 1WRMS Load Impedance 8 Ω Input Level 1VRMS Input Impedance 20k Ω Bandwidth 100Hz–20kHz ± 0.25dB 10 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM4876 |
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