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LT1253 Datasheet(PDF) 6 Page - Linear Technology |
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LT1253 Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page 6 LT1253/LT1254 Transient Response LT1253/54 • TPC16 Transient Response VS = ±5V AV = 1 RL = 150Ω LT1253/54 • TPC17 RF = 562Ω RG = 61.9Ω VO = 1.5V VS = ±5V AV = 10 RL = 150Ω RF = 787Ω VO = 1V Power Dissipation The LT1253/LT1254 amplifiers combine high speed and large output current drive into very small packages. Be- cause these amplifiers work over a very wide supply range, it is possible to exceed the maximum junction temperature under certain conditions. To insure that the LT1253/ LT1254 are used properly, we must calculate the worst case power dissipation, define the maximum ambient temperature, select the appropriate package and then calculate the maximum junction temperature. The worst case amplifier power dissipation is the total of the quiescent current times the total power supply voltage plus the power in the IC due to the load. The quiescent supply current of the LT1253/LT1254 has a strong nega- tive temperature coefficient. The supply current of each amplifier at 150 °C is less than 7mA and typically is only 4.5mA. The power in the IC due to the load is a function of the output voltage, the supply voltage and load resistance. The worst case occurs when the output voltage is at half supply, if it can go that far, or its maximum value if it cannot reach half supply. For example, let’s calculate the worst case power dissipa- tion in a video cable driver operating on a ±12V supply that delivers a maximum of 2V into 150 Ω. PDMAX = 2 × VS × ISMAX + (VS – VOMAX) × VOMAX/RL PDMAX = 2 × 12V × 7mA + (12V – 2V) × 2V/150 = 0.168 + 0.133 = 0.301 Watt per Amp Now if that is the dual LT1253, the total power in the package is twice that, or 0.602W. We now must calculate how much the die temperature will rise above the ambient. The total power dissipation times the thermal resistance of the package gives the amount of temperature rise. For the above example, if we use the S8 surface mount package, the thermal resistance is 150 °C/W junction to ambient in still air. Temperature Rise = PDMAX × RθJA = 0.602W × 150°C/W = 90.3°C The maximum junction temperature allowed in the plastic package is 150 °C. Therefore the maximum ambient al- lowed is the maximum junction temperature less the temperature rise. Maximum Ambient = 150 °C – 90.3°C = 59.7°C Note that this is less than the maximum of 70 °C that is specified in the absolute maximum data listing. In order to use this package at the maximum ambient we must lower the supply voltage or reduce the output swing. APPLICATIO S I FOR ATIO CC HARA TERISTICS A TYPICALPERFOR CE |
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