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LT3012HFE-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3012HFE-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT3012 11 3012fd 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 50mA, 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) = 50mA VIN(MAX) = 30V IGND at (IOUT = 50mA, VIN = 30V) = 1mA So: P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W The thermal resistance 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: 1.31W • 50°C/W = 65.5°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 + 65.5°C = 115.5°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 50mA 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) + (200μA • 48V) = 0.23W P2(48V in, 50mA load) = 50mA • (48V – 5V) + (1mA • 48V) = 2.20W APPLICATIONS INFORMATION P3(72V in, 5mA load) = 5mA • (72V – 5V) + (200μA • 72V) = 0.35W P4(72V in, 50mA load) = 50mA • (72V – 5V) + (1mA • 72V) = 3.42W 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%(0.23W) + 19%(2.20W) + 4%(0.35W) + 1%(3.42W) = 0.64W 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 26°C to 38°C. High Temperature Operation Care must be taken when designing LT3012 applications to operate at high ambient temperatures. The LT3012 works at elevated temperatures but erratic operation can occur due to unforeseen variations in external components. Some tantalum capacitors are available for high temperature operation, but ESR is often several ohms; capacitor ESR above 3Ω is unsuitable for use with the LT3012. Ceramic capacitor manufacturers (Murata, AVX, TDK, and Vishay Vitramon at this writing) now offer ceramic capacitors that are rated to 150°C using an X8R dielectric. Device instability will occur if output capacitor value and ESR are outside design limits at elevated temperature and operating DC voltage bias (see information on capacitor characteristics under Output Capacitance and Transient Response). Check each passive component for absolute value and voltage ratings over the operating temperature range. Leakages in capacitors or from solder flux left after insuficient board cleaning adversely affects low quiescent current operation. The output voltage resistor divider should use a maximum bottom resistor value of 124k to compensate for high temperature leakage, setting divider current to 10μA. Consider junction temperature increase due to power dissipation in both the junction and nearby components to ensure maximum specifications are not violated for the device or external components. |
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