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LM4867MTE bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LM4867MTE bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
11 / 26 page Application Information (Continued) STEREO-INPUT MULTIPLEXER (STEREO MUX) The LM4867 has two stereo inputs. The MUX CTRL Pin controls which stereo input is active. As shown in the Truth Table for Logic Inputs, applying 0V to the MUX CTRL input activates stereo input 1, whereas applying V DD to the MUX CTRL inputs activates stereo input 2. To ensure correct amplifier operation, unused MUX inputs must be tied to GND. Typical LM4867 applications use the MUX to switch between two stereo input signals. Each stereo channel’s gain can be tailored to produce the required output signal level by choos- ing the appropriate input and feedback resistor ratio. Another configuration uses the MUX to select two different gains or frequency compensated gains that amplify a single pair of stereo input signals. Figure 5 shows two different feedback networks, Network 1 and Network 2. Network 1 produces increasing gain as the input signal’s frequency decreases. This can be used to compensate a small, full-range speaker’s low frequency response roll-off. Network 2 sets the gain for an alternate load such as headphones. The circuit in Figure 6 uses Network 1 when driving external speakers, switching to Network 2 when headphones are connected. The normally closed control switch in Figure 6’s headphone jack connects to the MUX CTRL pin. When headphones are connected, the LM4867’s internal pull-up that applies V DD to the HP-IN and the external 100kΩ resis- tor applies V DD to MUX CTRL pin. Simultaneously applying these control voltages automatically selects the amplifier (headphone or bridge) and switches the gain (MUX channel selection). Alternatively, leaving the MUX CTRL pin indepen- dently accessible allows a user to select bass boost as needed. This alternative user-selectable bass-boost scheme requires connecting equal ratio resistor feedback networks to each MUX input channel. The value of the resistor in the RC network is chosen to give a gain that is necessary to achieve the desired bass-boost. Switching between the MUX channels may change the input signal source or the feedback resistor network. During the channel switching transition, the average voltage level present on the internal amplifier’s input may change. This change can slew at a rate that may produce audible voltage transients or clicks in the amplifier’s output signal. Using the MUX to select between two vastly dissimilar gains is a typical transient-producing situation. As the MUX is switched, an audible click may occur as the gain suddenly changes. PIN OUT COMPATIBILITY WITH THE LM4863 The LM4867 pin out was designed to simplify replacing the LM4863: except for the four Pins(-IN A 2, MUX CTRL, -IN B2, and NC) that implement the LM4867’s extra functionality, the LM4867MT/MTE and LM4863MT/MTE pin outs match. (Note 22) Note 22: If the LM4867 replaces an LM4863 and the input MUX circuitry is not being used, the LM4867 MUX CTRL pin must be tied to VDD or GND and the unused MUX inputs must be connected to GND. EXPOSED-DAP MOUNTING CONSIDERATIONS The LM4867’s exposed-DAP (die attach paddle) packages (MTE and LQ) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surround- ing PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a low voltage audio power amplifier that produces 2.4W dissipation in a 4 Ω load at ≤ 1% THD+N and over 3W in a 3Ω load at 10% THD+N. This high power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4867’s high power performance and activate unwanted, though necessary, thermal shutdown protection. DS200013-70 FIGURE 5. Input MUX Example DS200013-39 FIGURE 6. As configured, connecting headphones to this jack automatically selects the stereo headphone amplifier and, with the additional NC switch, changes MUX channels (Network 2 in Figure 2 ) www.national.com 11 |
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