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LM4755 数据表(PDF) 11 Page - National Semiconductor (TI) |
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LM4755 数据表(HTML) 11 Page - National Semiconductor (TI) |
11 / 18 page Application Information (Continued) threshold voltage is desired. One example is a design requir- ing a high operating supply voltage, with large supply and bias capacitors, and there is little or no other circuitry con- nected to the main power supply rail. In this circuit, when the power is disconnected, the supply and bias capacitors will discharge at a slower rate, possibly resulting in audible out- put distortion as the decaying voltage begins to clip the out- put signal. An external circuit may be used to sense for the desired threshold, and pull the bias line (pin 6) to ground to disable the input preamp. Figure 6 shows an example of such a circuit. When the voltage across the zener diode drops below its threshold, current flow into the base of Q1 is interrupted. Q2 then turns on, discharging the bias capacitor. This discharge rate is governed by several factors, including the bias capacitor value, the bias voltage, and the resistor at the emitter of Q2. An equation for approximating the value of the emitter discharge resistor, R, is given below: R = (0.7v) / (Cb • (V CC/2) / 0.1s) Note that this is only a linearized approximation based on a discharge time of 0.1s. The circuit should be evaluated and adjusted for each application. As mentioned earlier in the Built-in Mute Circuit section, when using an external circuit to pull down the bias line, the rate of discharge will have an effect on the turn-off induced distortions. Please refer to the Built-in Mute Circuit section for more information. THERMAL CONSIDERATIONS Heat Sinking Proper heatsinking is necessary to ensure that the amplifier will function correctly under all operating conditions. A heat- sink that is too small will cause the die to heat excessively and will result in a degraded output signal as the thermal pro- tection circuitry begins to operate. The choice of a heatsink for a given application is dictated by several factors: the maximum power the IC needs to dissi- pate, the worst-case ambient temperature of the circuit, the junction-to-case thermal resistance, and the maximum junc- tion temperature of the IC. The heat flow approximation equation used in determining the correct heatsink maximum thermal resistance is given below: T J–TA =PDMAX • (θJC + θCS + θSA) where: P DMAX = maximum power dissipation of the IC T J(˚C) = junction temperature of the IC T A(˚C) = ambient temperature θ JC(˚C/W) = junction-to-case thermal resistance of the IC θ CS(˚C/W) = case-to-heatsink thermal resistance (typically 0.2 to 0.5 ˚C/W) θ SA(˚C/W) = thermal resistance of heatsink When determining the proper heatsink, the above equation should be re-written as: θ SA ≤ [(TJ–TA)/PDMAX]- θJC– θCS TO-263 HEATSINKING Surface mount applications will be limited by the thermal dis- sipation properties of printed circuit board area. The TO-263 package is not recommended for surface mount applications with V S > 16V due to limited printed circuit board area. There are TO-263 package enhancements, such as clip-on heatsinks and heatsinks with adhesives, that can be used to improve performance. Standard FR-4 single-sided copper clad will have an ap- proximate Thermal resistance ( θ SA) ranging from: 1.5 x 1.5 in. sq. 20–27˚C/W (T A=28˚C, Sine wave testing, 1 oz. Copper) 2 x 2 in. sq. 16–23˚C/W The above values for θ SA vary widely due to dimensional proportions (i.e. variations in width and length will vary θ SA). For audio applications, where peak power levels are short in duration, this part will perform satisfactory with less heatsinking/copper clad area. As with any high power design proper bench testing should be undertaken to assure the de- sign can dissipate the required power. Proper bench testing requires attention to worst case ambient temperature and air flow. At high power dissipation levels the part will show a ten- dency to increase saturation voltages, thus limiting the un- distorted power levels. DETERMINING MAXIMUM POWER DISSIPATION For a single-ended class AB power amplifier, the theoretical maximum power dissipation point is a function of the supply voltage, V S, and the load resistance, RL and is given by the following equation: (single channel) P DMAX (W)=[VS 2 /(2 • π2 • R L)] The above equation is for a single channel class-AB power amplifier. For dual amplifiers such as the LM4755, the equa- tion for calculating the total maximum power dissipated is: (dual channel) P DMAX (W)=2 • [VS 2 /(2 • π2 • R L)] or V S 2 /( π2 • R L) (Bridged Outputs) P DMAX (W) = 4[VS 2 /(2 π2 • R L)] HEATSINK DESIGN EXAMPLE: Determine the system parameters: V S = 24V Operating Supply Voltage R L =4Ω Minimum Load Impedance T A = 55˚C Worst Case Ambient Temperature Device parameters from the datasheet: T J = 150˚C Maximum Junction Temperature DS100059-32 FIGURE 6. External Undervoltage Pull-Down www.national.com 11 |
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类似说明 - LM4755 |
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