Datasheet LT1253, LT1254 (Analog Devices) - 6

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