Datasheet LT1229, LT1230 (Analog Devices) - 9

LT1229/LT1230
O U U W U APPLICATI S I FOR ATIO Capacitive Loads
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 LT1229/LT1230 can drive capacitive loads directly the output voltage, the supply voltage and load resistance. when the proper value of feedback resistor is used. The The worst case occurs when the output voltage is at half graph Maximum Capacitive Load vs Feedback Resistor supply, if it can go that far, or its maximum value if it should be used to select the appropriate value. The value cannot reach half supply. shown is for 5dB peaking when driving a 1k load at a gain For example, let’s calculate the worst case power dissipa- of 2. This is a worst case condition; the amplifier is more tion in a video cable driver operating on ±12V supplies that stable at higher gains and driving heavier loads. Alterna- delivers a maximum of 2V into 150Ω. tively, a small resistor (10Ω to 20Ω) can be put in series with the output to isolate the capacitive load from the VO MAX  ( ) amplifier output. This has the advantage that the amplifier P = V 2 I + V – V d MAX S S MAX S O MAX ( ) ( )  ( ) bandwidth is only reduced when the capacitive load is RL present, and the disadvantage that the gain is a function of V = 2 2 • 12 • 7 + (12 – 2 ) the load resistance. P V mA V V d MAX ( ) • 150Ω = 0.168 + 0 133 . = 0. W 301 per Amp
Power Supplies
Now if that is the dual LT1229, the total power in the The LT1229/LT1230 amplifiers will operate from single or package is twice that, or 0.602W. We now must calcu- split supplies from ±2V (4V total) to ±15V (30V total). It is late how much the die temperature will rise above the not necessary to use equal value split supplies, however, ambient. The total power dissipation times the thermal the offset voltage and inverting input bias current will resistance of the package gives the amount of tempera- change. The offset voltage changes about 350µV per volt ture rise. For the above example, if we use the SO8 of supply mismatch, the inverting bias current changes surface mount package, the thermal resistance is about 2.5µA per volt of supply mismatch. 150°C/W junction to ambient in still air.
Power Dissipation
Temperature Rise = Pd (MAX) RθJA = 0.602W • 150°C/W = 90.3°C The LT1229/LT1230 amplifiers combine high speed and large output current drive into very small packages. Be- The maximum junction temperature allowed in the plastic cause these amplifiers work over a very wide supply range, package is 150°C. Therefore, the maximum ambient al- it is possible to exceed the maximum junction temperature lowed is the maximum junction temperature less the under certain conditions. To ensure that the LT1229 and temperature rise. LT1230 remain within their absolute maximum ratings, Maximum Ambient = 150°C – 90.3°C = 59.7°C we must calculate the worst case power dissipation, define the maximum ambient temperature, select the Note that this is less than the maximum of 70°C that is appropriate package and then calculate the maximum specified in the absolute maximum data listing. If we must junction temperature. use this package at the maximum ambient we must lower the supply voltage or reduce the output swing. The worst case amplifier power dissipation is the total of the quiescent current times the total power supply voltage As a guideline to help in the selection of the LT1229/ plus the power in the IC due to the load. The quiescent LT1230 the following table describes the maximum sup- supply current of the LT1229/LT1230 has a strong nega- ply voltage that can be used with each part in cable driving tive temperature coefficient. The supply current of each applications. 9