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LM4914 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4914 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) LM4914’s supply pins and ground. Do not substitute a ce- ramic capacitor for the tantalum. Doing so may cause oscil- lation. Keep the length of leads and traces that connect capacitors between the LM4914’s power supply pin and ground as short as possible. Connecting a 0.47µF capacitor, C B, be- tween the BYPASS pin and ground improves the internal bias voltage’s stability and improves the amplifier’s PSRR. The PSRR improvements increase as the bypass pin ca- pacitor value increases. Too large, however, increases turn-on time and can compromise the amplifier’s click and pop performance. The selection of bypass capacitor values, especially C B, depends on desired PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. MICRO-POWER SHUTDOWN The LM4914 features an active-low micro-power shutdown mode. When active, the LM4914’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The logic threshold is typically V DD/2. The low 0.03µA typical shutdown current is achieved by applying a voltage to the SHUTDOWN pin that is as near to GND as possible. A voltage that is greater than GND may increase the shutdown current. There are a few methods to control the micro-power shut- down. These include using a single-pole, single-throw switch (SPST), a microprocessor, or a microcontroller. When using a switch, connect a 100k Ω pull-up resistor between the SHUTDOWN pin and V DD and the SPST switch between the SHUTDOWN pin and GND. Select normal amplifier opera- tion by opening the switch. Closing the switch applies GND to the SHUTDOWN pin, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or a microcontroller, use a digital output to apply the active-state voltage to the SHUT- DOWN pin. HEADPHONE (SINGLE-ENDED) AMPLIFIER OPERATION BTL/SE [Mono (BTL)/Stereo (SE)] Function Applying a voltage greater than 0.9V DD to the LM4914’s BTL/SE headphone control pin switches the amplifier’s op- eration from mono BTL to stereo SE. Applying a voltage less than 0.55V DD to the LM4914’s BTL/SE headphone control pin switches the amplifier’s operation from stereo SE to mono BTL. Figure 3 shows how to control the LM4914’s headphone function using four external resistors and a dual-switch ste- reo headphone jack. External resistors R4 - R6 provide the control voltages that are applied through the upper head- phone jack switch. R6 and R7 provide a DC return path for the SE coupling capacitors. With no headphones connected to the headphone jack, the R5-R6 voltage divider sets the voltage applied to the BTL/SE pin (pin 7) at approximately 50mV (comfortably below the 0.55V DD logic-low threshold). This 50mV tells the LM4914 to select the signal applied to the MONO-IN input and places the LM4914 in mono BTL operation. When stereo SE opera- tion is desired, both headphone jack switches are opened with a headphone plug. Opening the lower one allows R5 to apply V DD to the BTL/SE pin. This switches the amplifier’s inputs to the stereo signal. Opening the lower one breaks the connection between AMP4’s output and the BTL speaker, muting it. The output coupling capacitors block the amplifi- er’s half supply DC voltage, protecting the headphones from the V DD/2 DC output voltage. Figure 3 also shows the suggested headphone jack electri- cal connections. The jack is designed to mate with a three- wire plug. The plug’s tip and adjacent ring should carry the left and right channel stereo signals, respectively. The sleeve furthest from the tip should carry the ground return. The Switchcraft 35RAPC4BH3 five-terminal headphone jack 200634A1 FIGURE 3. Headphone Operation and BTL - SE Mode Switching www.national.com 11 |
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