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LM4867LQ Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM4867LQ Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 26 page External Components Description ( Refer to Figure 1. ) Components Functional Description 1. R i Inverting input resistance which sets the closed-loop gain in conjunction with R f. This resistor also forms a high pass filter with C i at fc = 1/(2πRiCi). 2. C i Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a highpass filter with R i at fc = 1/(2πRiCi). Refer to the section, Proper Selection of External Components, for an explanation of how to determine the value of C i. 3. R f Feedback resistance which sets the closed-loop gain in conjunction with R i. 4. C s Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of the supply bypass capacitor. 5. C B Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External Components, for information concerning proper placement and selection of C B. Application Information ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that can switch be- tween driving bridge-tied-load (BTL) speakers and single-ended (SE) headphones use a coupling capacitor on each SE output. This capacitor blocks the half-supply volt- age to which the output amplifiers are typically biased and couples the audio signal to the headphones. The signal return to circuit ground is through the headphone jack’s sleeve. The LM4867 eliminates these coupling capacitors. Amp2A is internally configured to apply V DD/2 to a stereo headphone jack’s sleeve. This voltage matches the quiescent voltage present on the Amp1A and Amp1B outputs that drive the headphones. The headphones operate in a manner very similar to a bridge-tied-load (BTL). The same DC voltage is applied to both headphone speaker terminals. This results in no net DC current flow through the speaker. AC current flows through a headphone speaker as an audio signal’s output amplitude increases on the speaker’s terminal. When operating as a headphone amplifier, the headphone jack sleeve is not connected to circuit ground. Using the headphone output jack as a line-level output will place the LM4867’s one-half supply voltage on a plug’s sleeve con- nection. Driving a portable notebook computer or audio-visual display equipment is possible. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DC-coupled, the LM4867 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mA PK, the amplifier is shutdown, pro- tecting the LM4867 and the external equipment. For more information, see the section titled ’Single-Ended Output Power Performance and Measurement Considerations’. OUTPUT TRANSIENT (’POPS AND CLICKS’) ELIMINATED The LM4867 contains advanced circuitry that eliminates out- put transients (’pop and click’). This circuitry prevents all traces of transients when the supply voltage is first applied, when the part resumes operation after shutdown, or when switching between BTL speakers and SE headphones. Two circuits combine to eliminate pop and click. One circuit mutes the output when switching between speaker loads. Another circuit monitors the input signal. It maintains the muted condition until there is sufficient input signal magni- tude to mask any remaining transient that may occur. Figure 2 shows the LM4867’s lack of transients in the differ- ential signal (Trace B) across a BTL 8 Ω load. The LM4867’s active-high SHUTDOWN pin is driven by the logic signal shown in Trace A. Trace C is the VOUT- output signal and trace D is the VOUT+ output signal. The shutdown signal frequency is 1Hz with a 50% duty cycle. Figure 3 is gener- ated with the same conditions except that the output drives a 32 Ω single-ended (SE) load. Again, no trace of output tran- sients is seen. USING THE LM4867 TO UPGRADE LM4863 AND LM4873 DESIGNS The LM4867’s noise-free operation plus coupling-capacitorless headphone operation and functional compatibility with the LM4873 and the LM4863 simplifies upgrading systems using these parts. Upgrading older de- signs that use either the LM4863 or the LM4873 is easy. Simply remove and short the coupling capacitors located between the LM4873’s or LM4863’s Amp1A and Amp1B outputs and the headphone connections. Also remove the 1k Ω resistor between each headphone connection and ground. Finally, remove any resistors connected to the HP-IN pin (typically two 100k Ω resistors). Connect the HP-IN pin directly to the headphone jack control pin as shown in Figure 4. DS200013-56 FIGURE 2. Differential output signal (Trace B) is devoid of transients. The SHUTDOWN pin is driven by a shutdown signal (Trace A). The inverting output (Trace C) and the non-inverting output (Trace D) are applied across an 8 Ω BTL load. www.national.com 9 |
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