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LM4870M Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM4870M Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Application Information (Continued) ceeded momentarily, due to the discharging time constant of the bias-pin voltage. This time constant is established by the two 50k resistors (in parallel) with the series bypass capaci- tor value. When a set of headphones are plugged into the system, the contact pin of the headphone jack is disconnected from the signal pin, interrupting the voltage divider set up by resistors R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, en- abling the headphone function and disabling the LM4870 amplifier. The headphone amplifier then drives the head- phones, whose impedance is in parallel with resistor R2. Since the typical impedance of headphones are 32 Ω, resis- tor R2 has negligible effect on the output drive capability. Also shown in Figure 3 are the electrical connections for the headphone jack and plug. A 3-wire plug consists of a Tip, Ring, and Sleave, where the Tip and Ring are signal carrying conductors and the Sleave is the common ground return. One control pin contact for each headphone jack is sufficient to indicate to control inputs that the user has inserted a plug into a jack and that another mode of operation is desired. For a system implementation where the headphone amplifier is designed using a split supply, the output coupling cap, C C and resistor R2 of Figure 3, can be eliminated. The function- ality described earlier remains the same, however. In addition, the HP-SENSE pin, although it may be con- nected to the SHUTDOWN pin as shown in Figure 2, may still be used as a control flag. It is capable of driving the input to another logic gate or approximately 2 mA without serious loading. AUDIO POWER AMPLIFIER DESIGN Design a 500 mW/8 Ω Audio Amplifier Given: Power Output: 1.0Wrms Load Impedance: 8 Ω Input Level: 1 Vrms(max) Input Impedance: 20 k Ω Bandwidth: 100 Hz-20 kHz ±0.25 dB A designer must first determine the needed supply rail to ob- tain the specified output power. The Output Power vs Supply Voltage graphs in the Typical Performance Characteris- tics section show how much power the LM4870 will output given different supply voltage. According to the graph, 4.6V DS100094-7 FIGURE 2. HP-SENSE Pin to SHUTDOWN Pin Connection DS100094-8 FIGURE 3. Typical Headphone Control Input Circuitry www.national.com 6 |
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