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LT1129CQ-5 Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LT1129CQ-5 Datasheet(HTML) 11 Page - Linear Integrated Systems |
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11 / 16 page  11 LT1129/LT1129-3.3/LT1129-5 112935fc Calculating Junction Temperature Example: Given an output voltage of 3.3V, an input voltage range of 4.5V to 5.5V, an output current range of 0mA to 500mA, 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 = 500mA VIN MAX = 5.5V IGND at (IOUT = 500mA, VIN = 5.5V) = 25mA so, P = 500mA • (5.5V – 3.3V) + (25mA • 5.5V) = 1.24W If we use a DD package, then the thermal resistance will be in the range of 25°C/W to 35°C/W depending on copper area. So the junction temperature rise above ambient will be approximately equal to: 1.24W • 30°C/W = 37.2°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 + 37.2°C = 87.2°C Output Capacitance and Transient Performance The LT1129 is designed to be stable with a wide range of output capacitors. The minimum recommended value is 3.3µF with an ESR of 2Ω or less. The LT1129 is a micropower device and output transient response will be a function of output capacitance. See the Transient Re- sponse curves in the Typical Performance Characteristics. Larger values of output capacitance will decrease the peak deviations and provide improved output transient re- sponse. Bypass capacitors, used to decouple individual components powered by the LT1129, will increase the effective value of the output capacitor. Protection Features The LT1129 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 voltages from output to input. For fixed voltage devices the output and sense pins are tied together at the output. 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 input of the device will withstand reverse voltages of 30V. Current flow into the device will be limited to less than 1mA (typically less than 100µA) and no negative voltage will appear at the output. The device will protect both itself and the load. This provides protection against batteries that can be plugged in backwards. For fixed voltage versions of the device, the sense pin is internally clamped to one diode drop below ground. For the adjustable version of the device, the output pin is internally clamped at one diode drop below ground. If the output pin of an adjustable device, or the sense pin of a fixed voltage device, is pulled below ground, with the input open or grounded, current must be limited to less than 5mA. In circuits where a backup battery is required, several different input/output conditions can occur. The output voltage may be held up while the input is either pulled to ground, pulled to some intermediate voltage, or is left open circuit. Current flow back into the output will vary depending on the conditions. Many battery-powered cir- cuits incorporate some form of power management. The following information will help optimize battery life. Table 3 summarizes the following information. The reverse output current will follow the curve in Figure 3 when the input pin is pulled to ground. This current flows through the output pin to ground. The state of the shut- down pin will have no effect on output current when the input pin is pulled to ground. APPLICATIO S I FOR ATIO |
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