PDF LM4895LD Data sheet ( Hoja de datos )

Número de pieza	LM4895LD
Descripción	1 Watt Fully Differential Audio Power Amplifier With Shutdown Select and Fixed 6dB Gain
Fabricantes	National Semiconductor
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No Preview Available ! October 2002 LM4895 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select and Fixed 6dB Gain General Description The LM4895 is a fully differential audio power amplifier primarily designed for demanding applications in mobile phones and other portable communication device applica- tions. It is capable of delivering 1 watt of continuous average power to an 8Ω load with less than 1% distortion (THD+N) from a 5VDC power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. The LM4895 does not require output coupling capacitors or bootstrap capacitors, and therefore is ideally suited for mobile phone and other low voltage appli- cations where minimal power consumption is a primary re- quirement. The LM4895 features a low-power consumption shutdown mode. To facilitate this, Shutdown may be enabled by either logic high or low depending on mode selection. Driving the shutdown mode pin either high or low enables the shutdown select pin to be driven in a likewise manner to enable Shut- down. Additionally, the LM4895 features an internal thermal shutdown protection mechanism. The LM4895 contains advanced pop & click circuitry which eliminates noises which would otherwise occur during turn-on and turn-off transitions. The LM4895 has an internally fixed gain of 6dB. Key Specifications j Improved PSRR at 217Hz j Power Output at 5.0V & 1% THD j Power Output at 3.3V & 1% THD j Shutdown Current 80dB 1.0W(typ.) 400mW(typ.) 0.1µA(typ.) Features n Fully differential amplification n Internal-gain-setting resistors n Available in space-saving packages micro SMD, MSOP and LLP n Ultra low current shutdown mode n Can drive capacitive loads up to 500 pF n Improved pop & click circuitry eliminates noises during turn-on and turn-off transitions n 2.2 - 5.5V operation n No output coupling capacitors, snubber networks or bootstrap capacitors required n Shutdown high or low selectivity Applications n Mobile phones n PDAs n Portable electronic devices Typical Application 20023201 FIGURE 1. Typical Audio Amplifier Application Circuit Boomer® is a registered trademark of National Semiconductor Corporation. © 2002 National Semiconductor Corporation DS200232 www.national.com 1 page Typical Performance Characteristics LD Specific Characteristics (Continued) LM4895LD Power Dissipation vs Output Power LM4895LD Power Derating Curve 20023211 Typical Performance Characteristics Non-LD Specific Characteristics THD+N vs Frequency at VDD = 5V, 8Ω RL, and PWR = 400mW 20023212 THD+N vs Frequency VDD = 3V, 8Ω RL, and PWR = 250mW 20023213 THD+N vs Frequency at VDD = 3V, 4Ω RL, and PWR = 225mW 20023230 THD+N vs Frequency VDD = 2.6V, 8Ω RL, and PWR = 150mW 20023231 5 20023232 www.national.com 5 Page Application Information DIFFERENTIAL AMPLIFIER EXPLANATION The LM4895 is a fully differential audio amplifier that fea- tures differential input and output stages. Internally this is accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the output voltages so that the average value remains VDD/2. The LM4895 features precisely matched internal gain-setting re- sistors, thus eliminating the need for external resistors and fixing the differential gain at AVD = 6dB. A differential amplifier works in a manner where the differ- ence between the two input signals is amplified. In most applications, this would require input signals that are 180˚ out of phase with each other. The LM4895 provides what is known as a ’bridged mode’ output (bridge-tied-load, BTL). This results in output signals at Vo1 and Vo2 that are 180˚ out of phase with respect to each other. Bridged mode operation is different from the single-ended amplifier configuration that connects the load between the amplifier output and ground. A bridged amplifier design has distinct advantages over the single-ended con- figuration: it provides differential drive to the load, thus dou- bling maximum possible output swing for a specific supply voltage. Four times the output power is possible compared with a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. A bridged configuration, such as the one used in the LM4895, also creates a second advantage over single- ended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. BTL configuration eliminates the output coupling capacitor required in single-supply, single-ended amplifier configurations. If an output coupling capacitor is not used in a single-ended output configuration, the half-supply bias across the load would result in both increased internal IC power dissipation as well as permanent loudspeaker dam- age. Further advantages of bridged mode operation specific to fully differential amplifiers like the LM4895 include in- creased power supply rejection ratio, common-mode noise reduction, and click and pop reduction. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4895’s exposed-DAP (die attach paddle) package (LD) 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 surrounding PCB copper traces, ground plane and, finally, surrounding air. The result is a low voltage audio power amplifier that produces 1.4W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful consideration of necessary ther- mal design. Failing to optimize thermal design may compro- mise the LM4895’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The LD package must have its DAP soldered to a copper pad on the PCB. The DAP’s PCB copper pad is connected to a large plane of continuous unbroken copper. This plane forms a thermal mass and heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with 4 (2x2) vias. The via diameter should be 0.012in - 0.013in with a 0.050in pitch. Ensure efficient thermal conductivity by plating- through and solder-filling the vias. Best thermal performance is achieved with the largest prac- tical copper heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4Ω load. Heatsink areas not placed on the same PCB layer as the LM4895 should be 5in2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient temperature. In all circumstances and conditions, the junc- tion temperature must be held below 150˚C to prevent acti- vating the LM4895’s thermal shutdown protection. The LM4895’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation ver- sus temperature. Example PCB layouts for the exposed- DAP TSSOP and LLP packages are shown in the Demon- stration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mount- ing an LLP package is available from National Semiconduc- tor’s package Engineering Group under application note AN-1187. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load imped- ance decreases, load dissipation becomes increasingly de- pendent on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connec- tions. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1Ω trace resistance reduces the output power dissipated by a 4Ω load from 1.4W to 1.37W. This problem of decreased load dissipation is exac- erbated as load impedance decreases. Therefore, to main- tain the highest load dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide as possible. Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated sup- plies, trace resistance creates the same effects as poor sup-ply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifer, whether the amplifier is bridged or single-ended. Equation 2 states the maximum power dissi- pation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. PDMAX=(VDD)2/(2π2RL) Single-Ended (1) 11 www.national.com 11 Page

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