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LM4961 데이터시트(PDF) 8 Page - National Semiconductor (TI) |
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LM4961 데이터시트(HTML) 8 Page - National Semiconductor (TI) |
8 / 16 page Application Information BRIDGE CONFIGURATION EXPLANATION The Audio Amplifier portion of the LM4961 has two internal amplifiers allowing different amplifier configurations. The first amplifier’s gain is externally configurable, whereas the sec- ond amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri while the second amplifier’s gain is fixed by the two internal 20k Ω resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two. This results in both amplifiers producing sig- nals identical in magnitude, but out of phase by 180˚. Con- sequently, the differential gain for the Audio Amplifier is A VD = 2 *(Rf/Ri) By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classic single-ended amplifier configura- tion where one side of the load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration. It provides differential drive to the load, thus doubling the output swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended amplifier under the same con- ditions. The bridge configuration 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. This eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configuration. Without an output cou- pling capacitor, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage. AMPLIFIER POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the amplifier portion of the LM4961 has two operational amplifiers, the maximum inter- nal power dissipation is 4 times that of a single-ended am- plifier. The maximum power dissipation for a given BTL application can be derived from Equation 1. P DMAX(AMP) = 4(VDD) 2 /(2 π2Z L) (1) where Z L =Ro1+Ro2 +1/2 πfc BOOST CONVERTER POWER DISSIPATION At higher duty cycles, the increased ON-time of the switch FET means the maximum output current will be determined by power dissipation within the LM4961 FET switch. The switch power dissipation from ON-time conduction is calcu- lated by Equation 2. P DMAX(SWITCH) =DCxIIND(AVE) 2 xR DS(ON) (2) where DC is the duty cycle. There will be some switching losses as well, so some derat- ing needs to be applied when calculating IC power dissipa- tion. TOTAL POWER DISSIPATION The total power dissipation for the LM4961 can be calculated by adding Equation 1 and Equation 2 together to establish Equation 3: P DMAX(TOTAL) = [4*(VDD) 2/ 2 π2Z L]+[DCxIIND(AVE) 2xR DS(ON)] (3) The result from Equation 3 must not be greater than the power dissipation that results from Equation 4: P DMAX =(TJMAX -TA)/ θJA (4) For the LQA28A, θ JA = 66˚C/W. TJMAX = 125˚C for the LM4961. Depending on the ambient temperature, T A,ofthe system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage must be in- creased, the load impedance increased or T A reduced. For the typical application of a 4.2V power supply, with a 2uF+30 Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is ap- proximately 109˚C provided that device operation is around the maximum power dissipation point. Thus, for typical ap- plications, power dissipation is not an issue. Power dissipa- tion is a function of output power and thus, if typical opera- tion is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output levels. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4961’s exposed-DAP (die attach paddle) package (LD) provides a low thermal resistance between the die and the PCB to which the part is mounted and soldered. The low thermal resistance allows rapid heat transfer from the die to the surrounding PCB copper traces, ground plane, and sur- rounding air. The LD package should have its DAP soldered to a copper pad on the PCB. The DAP’s PCB copper pad may be connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink, and radiation area. Further detailed and specific information con- cerning PCB layout, fabrication, and mounting an LD (LLP) package is found in National Semiconductor’s Package En- gineering Group under application note AN1187. SHUTDOWN FUNCTION In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch connected between V DD and Shutdown pins. BAND SWITCH FUNCTION The LM4961 features a Band Switch function which allows the user to use one amplifier for both receiver (earpiece) mode and ringer/loudspeaker mode. When a logic high www.national.com 8 |
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