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LT1229CJ8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1229CJ8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT1229/LT1230 8 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 ±12V supplies that delivers a maximum of 2V into 150 Ω. Capacitive Loads The LT1229/LT1230 can drive capacitive loads directly when the proper value of feedback resistor is used. The graph Maximum Capacitive Load vs Feedback Resistor should be used to select the appropriate value. The value shown is for 5dB peaking when driving a 1k load at a gain of 2. This is a worst case condition; the amplifier is more stable at higher gains and driving heavier loads. Alterna- tively, a small resistor (10 Ω to 20Ω) can be put in series with the output to isolate the capacitive load from the amplifier output. This has the advantage that the amplifier bandwidth is only reduced when the capacitive load is present, and the disadvantage that the gain is a function of the load resistance. Power Supplies The LT1229/LT1230 amplifiers will operate from single or split supplies from ±2V (4V total) to ±15V (30V total). It is not necessary to use equal value split supplies, however, the offset voltage and inverting input bias current will change. The offset voltage changes about 350 µV per volt of supply mismatch, the inverting bias current changes about 2.5 µA per volt of supply mismatch. Power Dissipation The LT1229/LT1230 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 ensure that the LT1229 and LT1230 remain within their absolute maximum ratings, 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 LT1229/LT1230 has a strong nega- tive temperature coefficient. The supply current of each S APPLICATI I FOR ATIO Now if that is the dual LT1229, the total power in the package is twice that, or 0.602W. We now must calcu- late 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 tempera- ture rise. For the above example, if we use the SO8 surface mount package, the thermal resistance is 150 °C/W junction to ambient in still air. Temperature Rise = Pd (MAX) 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. If we must use this package at the maximum ambient we must lower the supply voltage or reduce the output swing. As a guideline to help in the selection of the LT1229/ LT1230 the following table describes the maximum sup- ply voltage that can be used with each part in cable driving applications. PV I V V V R PV mA V V V W per Amp d MAX S S MAX S O MAX O MAX L d MAX () () () () () =+ () =× × + () × =+= 2 2 12 7 12 2 2 150 0 168 0 133 0 301 – – ... Ω |
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