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LM4913 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4913 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to stabi- lize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their presence does not eliminate the need for a local 0.47µF tantalum bypass capacitance connected between the LM4913’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 LM4913’s power supply pin and ground as short as possible. Connecting a 0.47µF capacitor, CB, 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 CB, 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 LM4913 features an active-low micro-power shutdown mode. When active, the LM4913’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The logic threshold is typically VDD/2. The low 0.1µA 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 100kW pull-up resistor between the SHUTDOWN pin and VDD and the SPST switch between the SHUTDOWN pin and GND. Select normal amplifier op- eration by opening the switch. Closing the switch applies GND to the SHUTDOWN pin, activating micro-power shut- down. The switch and resistor guarantee that the SHUT- DOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or a microcon- troller, use a digital output to apply the active-state voltage to the SHUTDOWN pin. HEADPHONE (SINGLE-ENDED) AMPLIFIER OPERATION BTL/SE [Mono (BTL)/Stereo (SE)] Function Applying a voltage greater than 0.9VDD to the LM4913’s BTL/SE headphone control pin switches the amplifier’s op- eration from mono BTL to stereo SE. Applying a voltage less than 0.55VDD to the LM4913’s BTL/SE headphone control pin switches the amplifier’s operation from stereo SE to mono BTL. Figure 3 shows how to control the LM4913’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. {Resume here and check other Fig. 3 below.} With no head- phones 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 0.55VDD logic- low threshold). This 50mV tells the LM4913 to select the signal applied to the MONO-IN input and places the LM4913 in mono BTL operation. When stereo SE operation is de- sired, both headphone jack switches are opened with a headphone plug. Opening the lower one allows R5 to apply VDD 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, 200617A1 FIGURE 3. Headphone Operation and BTL - SE Mode Switching www.national.com 11 |
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