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LT3014 Datasheet(PDF) 10 Page - Linear Technology |
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LT3014 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LT3014 10 3014fd APPLICATIONS INFORMATION Continuous operation at large input/output voltage dif- ferentials and maximum load current is not practical due to thermal limitations. Transient operation at high input/output differentials is possible. The approximate thermal time constant for a 2500sq mm 3/32" FR-4 board with maximum topside and backside area for one ounce copper is 3 seconds. This time constant will increase as more thermal mass is added (i.e. vias, larger board, and other components). For an application with transient high power peaks, average power dissipation can be used for junction temperature calculations as long as the pulse period is significantly less than the thermal time constant of the device and board. Calculating Junction Temperature Example 1: Given an output voltage of 5V, an input volt- age range of 24V to 30V, an output current range of 0mA to 20mA, and a maximum ambient temperature of 50°C, what will the maximum junction temperature be? The power dissipated by the device will be equal to: IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX)) where: IOUT(MAX) = 20mA VIN(MAX) = 30V IGND at (IOUT = 20mA, VIN = 30V) = 0.55mA So: P = 20mA • (30V – 5V) + (0.55mA • 30V) = 0.52W The thermal resistance for the DFN package will be in the range of 40°C/W to 62°C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 0.52W • 50°C/W = 26°C The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient plus the maximum ambient temperature or: TJMAX = 50°C + 26°C = 76°C Example 2: Given an output voltage of 5V, an input voltage of 48V that rises to 72V for 5ms(max) out of every 100ms, and a 5mA load that steps to 20mA for 50ms out of every 250ms, what is the junction temperature rise above ambi- ent? Using a 500ms period (well under the time constant of the board), power dissipation is as follows: P1(48V in, 5mA load) = 5mA • (48V – 5V) + (100μA • 48V) = 0.22W P2(48V in, 20mA load) = 20mA • (48V – 5V) + (0.55mA • 48V) = 0.89W P3(72V in, 5mA load) = 5mA • (72V – 5V) + (100μA • 72V) = 0.34W P4(72V in, 20mA load) = 20mA • (72V – 5V) + (0.55mA • 72V) = 1.38W Operation at the different power levels is as follows: 76% operation at P1, 19% for P2, 4% for P3, and 1% for P4. PEFF = 76%(0.22W) + 19%(0.89W) + 4%(0.34W) + 1%(1.38W) = 0.36W With a thermal resistance in the range of 40°C/W to 62°C/W, this translates to a junction temperature rise above ambient of 20°C. |
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