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LT1763 Datasheet(PDF) 11 Page - Linear Technology |
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LT1763 Datasheet(HTML) 11 Page - Linear Technology |
11 / 12 page LT1964 11 1964f Calculating Junction Temperature Example: Given an output voltage of –5V, an input voltage range of –6V to –8V, an output current range of 0mA to –100mA, 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) = –100mA VIN(MAX) = –8V IGND at (IOUT = –100mA, VIN = –8V) = –2mA so, P = –100mA • (–8V + 5V) + (–2mA • –8V) = 0.32W The thermal resistance (junction to ambient) will be in the range of 125 °C/W to 150°C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 0.32W • 140 °C/W = 44.2°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 + 44.2°C = 94.2°C Protection Features The LT1964 incorporates several protection features which make it ideal for use in battery-powered circuits. In addi- tion to the normal protection features associated with monolithic regulators, such as current limiting and ther- mal limiting, the device is protected against reverse input voltages and reverse output voltages. Current limit protection and thermal overload protection are intended to protect the device against current overload conditions at the output of the device. For normal opera- tion, the junction temperature should not exceed 125 °C. The output of the LT1964 can be pulled above ground without damaging the device. If the input is left open circuit or grounded, the output can be pulled above ground by 20V. For fixed voltage versions, the output will act like a large resistor, typically 500k Ω or higher, limiting current flow to less than 40 µA. For adjustable versions, the output will act like an open circuit, no current will flow into the pin. If the input is powered by a voltage source, the output will sink the short-circuit current of the device and will protect itself by thermal limiting. In this case, grounding the SHDN pin will turn off the device and stop the output from sinking the short-circuit current. Like many IC power regulators, the LT1964 series have safe operating area protection. The safe area protection activates at input-to-output differential voltages greater than –7V. The safe area protection decreases the current limit as the input-to-output differential voltage increases and keeps the power transistor inside a safe operating region for all values of forward input to-output voltage. The protection is designed to provide some output current at all values of input-to-output voltage up to the device breakdown. A 50 µA load is required at input-to-output differential voltages greater than –7V. When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads. During start-up, as the input voltage is rising, the input-to-output voltage differential is small, allowing the regulator to supply large output currents. With a high input voltage, a problem can occur wherein removal of an output short will not allow the output voltage to fully recover. Other regulators, such as the LT1175, also exhibit this phenomenon, so it is not unique to the LT1964 series. The problem occurs with a heavy output load when the input voltage is high and the output voltage is low. Common situations are immediately after the removal of a short-circuit or when the SHDN pin is pulled high after the input voltage has already been turned on. The load line for such a load may intersect the output current curve at two points. If this happens, there are two stable operating points for the regulator. With this double intersection, the input supply may need to be cycled down to zero and brought up again to make the output recover. APPLICATIO S I FOR ATIO Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. |
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