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LM4863 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM4863 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Application Information (Continued) POWER DISSIPATION Whether the power amplifier is bridged or single-ended, power dissipation is a major concern when designing the amplifier. Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified load. P DMAX = (VDD) 2/(2 π2R L): Single-Ended (1) However, a direct consequence of the increased power de- livered to the load by a bridge amplifier is an increase in in- ternal power dissipation. Equation 2 states the maximum power dissipation point for a bridge amplifier operating at the same given conditions. P DMAX = 4 * (VDD) 2/(2 π2R L): Bridge Mode (2) Since the LM4863 is a dual channel power amplifier, the maximum internal power dissipation is 2 times that of Equa- tion 1 or Equation 2 depending on the mode of operation. Even with this substantial increase in power dissipation, the LM4863 does not require heatsinking. The power dissipation from Equation 2, assuming a 5V power supply and an 8 Ω load, must not be greater than the power dissipation that re- sults from Equation 3: P DMAX = (TJMAX −TA)/θJA (3) For packages M16A and MTA20, θ JA = 80˚C/W, and for package N16A, θ JA = 63˚C/W. T JMAX = 150˚C for the LM4863. Depending on the ambient temperature, T A,ofthe system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the supply voltage must be de- creased, the load impedance increased, or the ambient tem- perature reduced. For the typical application of a 5V power supply, with an 8 Ω bridged load, the maximum ambient tem- perature possible without violating the maximum junction temperature is approximately 48˚C provided that device op- eration is around the maximum power dissipation point and assuming surface mount packaging. Internal power dissipa- tion is a function of output power. If typical operation is not around the maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical Perfor- mance Characteristics curves for power dissipation infor- mation for different output powers. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is criti- cal for low noise performance and high power supply rejec- tion. The capacitor location on both the bypass and power supply pins should be as close to the device as possible. The effect of a larger half supply bypass capacitor is improved PSRR due to increased half-supply stability. Typical applica- tions employ a 5V regulator with 10 µF and a 0.1 µF bypass capacitors which aid in supply filtering. This does not elimi- nate the need for bypassing the supply nodes of the LM4863. The selection of bypass capacitors, especially C B, is thus dependent upon desired PSRR requirements, click and pop performance as explained in the section, Proper Selection of External Components, system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4863 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the am- plifier off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high level is typically half supply. It is best to switch between ground and the supply V DD to provide maximum device performance. By switching the shutdown pin to V DD, the LM4863 supply cur- rent draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than V DD, the idle current may be greater than the typical value of 0.7 µA. In either case, the shutdown pin should be tied to a definite voltage to avoid unwanted state changes. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which pro- vides a quick, smooth transition into shutdown. Another solu- tion is to use a single-pole, single-throw switch in conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground and enables the amplifier. If the switch is open, then the external pull-up re- sistor will disable the LM4863. This scheme guarantees that the shutdown pin will not float, thus preventing unwanted state changes. HP-IN FUNCTION The LM4863 possesses a headphone control pin that turns off the amplifiers which drive +OutA and +OutB so that single-ended operation can occur and a bridged connected load is muted. Quiescent current consumption is reduced when the IC is in this single-ended mode. Figure 2 shows the implementation of the LM4863’s head- phone control function using a single-supply headphone am- plifier. The voltage divider of R1 and R2 sets the voltage at the HP-IN pin (pin 16) to be approximately 50 mV when there are no headphones plugged into the system. This logic-low voltage at the HP-IN pin enables the LM4863 and places it in bridged mode operation. Resistor R4 limits the amount of current flowing out of the HP-IN pin when the voltage at that pin goes below ground resulting from the music coming from the headphone amplifier. The output coupling capacitors pro- tect the headphones by blocking the amplifier’s half supply DC voltage. When there are no headphones plugged into the system and the IC is in bridged mode configuration, both loads are es- sentially at a 0V DC potential. Since the HP-IN threshold is set at 4V, even in an ideal situation, the output swing cannot cause a false single-ended trigger. 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-IN pin, en- abling the headphone function. This disables the second side of the amplifier thus muting the bridged speakers. The amplifier then drives the headphones, whose impedance is in parallel with resistors R2 and R3. Resistors R2 and R3 have negligible effect on output drive capability since the typical impedance of headphones are 32 Ω. Also shown in Figure 2 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 conduc- tors and the Sleave is the common ground return. One con- trol pin contact for each headphone jack is sufficient to indi- cate to control inputs that the user has inserted a plug into a jack and that another mode of operation is desired. The LM4863 can be used to drive both a pair of bridged 8 Ω speakers and a pair of 32 Ω headphones without using the HP-IN pin. In this case the HP-IN would not be connected to the headphone jack but to a microprocessor or a switch. By enabling the HP-IN pin, the 8 Ω speakers can be muted. www.national.com 9 |
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