Datasheet LT1581, LT1581-2.5 (Analog Devices) - 10

LT1581/LT1581-2.5
U U W U APPLICATIONS INFORMATION
where ICONTROL is equal to between IOUT/100 (typ) and Power dissipation under these conditions is equal to: IOUT/58 (max). Total Power Dissipation = PDRIVE + POUTPUT ICONTROL is a function of output current. A curve of P I DRIVE = (VCONTROL – VOUT) (ICONTROL) CONTROL vs IOUT can be found in the Typical Performance Characteristics curves. ICONTROL = IOUT/58 = 4A/58 = 69mA The power in the output transistor is equal to: PDRIVE = (5.25V – 2.5V)(69mA) = 190mW P P OUTPUT = (VPOWER – VOUT)(IOUT) OUTPUT = (VPOWER – VOUT)(IOUT) The total power is equal to: = ( 3.465V – 2.5V)(4A) = 3.9W P Total Power Dissipation = 4.09W TOTAL = PDRIVE + POUTPUT Junction-to-case thermal resistance is specified from the Junction temperature will be equal to: IC junction to the bottom of the case directly below the die. TJ = TA + PTOTAL (θHEATSINK + θCASE-HEATSINK + θJC) This is the lowest resistance path for heat flow. Proper mounting is required to ensure the best possible thermal For the Control section: flow from this area of the package to the heat sink. Thermal TJ = 70°C + 4.09W(4°C/W + 1°C/W + 0.65°C/W) = 93°C compound at the case-to-heat sink interface is strongly 93°C < 125°C = T recommended. If the case of the device must be electroni- JMAX for Control Section cally isolated, a thermally conductive spacer can be used For the Power section: as long as the added contribution to thermal resistance is T considered. Please consult Linear Technology’s “Mount- J = 70°C + 4.09W (4°C/ W + 1°C/ W + 2.5°C/ W) = 101°C ing Considerations for Power Semiconductors,” 1990 101°C < 150°C = TJMAX for Power Section Linear Applications Handbook, Volume 1, Pages RR3-1 to In both cases the junction temperature is below the RR3-20. Note that the case of the LT1581 is electrically maximum rating for the respective sections, ensuring connected to the output. reliable operation. The following example illustrates how to calculate maximum junction temperature. Using an LT1581 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) 10