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LT1580C Datasheet(PDF) 11 Page - Linear Technology |
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LT1580C Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1580/LT1580-2.5 APPLICATIONS INFORMATION low dropout applications the power dissipation will be less than 12W. The power in the device is made up of two main compo- nents: the power in the output transistor and the power in the drive circuit. The additional power in the control circuit is negligible. The power in the drive circuit will be equal to: PDRIVE = (VCONTROL – VOUT)(ICONTROL) where ICONTROL is equal to between IOUT/100 (typ) and IOUT/58 (max). ICONTROL is a function of output current. A curve of ICONTROL vs IOUT can be found in the Typical Performance Characteristics curves. The power in the output transistor is equal to: POUTPUT = (VPOWER – VOUT)(IOUT) The total power is equal to: PTOTAL = PDRIVE + POUTPUT Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is the lowest resistance path for heat flow. Proper mounting is required to ensure the best possible thermal flow from this area of the package to the heat sink. Thermal compound at the case-to-heat sink interface is strongly recommended. If the case of the device must be electroni- cally isolated, a thermally conductive spacer can be used as long as the added contribution to thermal resistance is considered. Please consult Linear Technology’s “ Mount- ing Considerations for Power Semiconductors,” 1990 Linear Applications Handbook, Volume 1, Pages RR3-1 to RR3-20. Note that the case of the LT1580 is electrically connected to the output. The following example illustrates how to calculate maximum junction temperature. Using an LT1580 and assuming: VCONTROL (max continuous) = 5.25V (5V + 5%), VPOWER (max continuous) = 3.465V (3.3V + 5%), VOUT = 2.5V, Iout = 4A, TA = 70°C, θHEATSINK = 4°C/W, θCASE-HEATSINK = 1°C/W (with thermal compound) Power dissipation under these conditions is equal to: Total Power Dissipation = PDRIVE + POUTPUT PDRIVE = (VCONTROL – VOUT) (ICONTROL) ICONTROL = IOUT/58 = 4A/58 = 69mA PDRIVE = (5.25V – 2.5V)(69mA) = 190mW POUTPUT = (VPOWER – VOUT)(IOUT) = ( 3.465V – 2.5V)(4A) = 3.9W Total Power Dissipation = 4.05W Junction temperature will be equal to: TJ = TA + PTOTAL (θHEATSINK + θCASE-HEATSINK + θJC) For the Control section: TJ = 70°C + 4.05W(4°C/W +1°C/W + 0.65°C/W) = 93°C 93 °C < 125°C = TJMAX for Control Section For the Power section: TJ = 70°C + 4.05W (4°C/W + 1°C/W + 2.7°C/W) = 101°C 101 °C < 150°C = TJMAX for Power Section In both cases the junction temperature is below the maximum rating for the respective sections, ensuring reliable operation. |
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