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LT1764EQ 数据表(PDF) 12 Page - Linear Technology |
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LT1764EQ 数据表(HTML) 12 Page - Linear Technology |
12 / 16 page 12 LT1764 Series 1764fa APPLICATIONS INFORMATION Thermal Considerations The power handling capability of the device is limited by the maximum rated junction temperature (125 °C). The power dissipated by the device is made up of two compo- nents: 1. Output current multiplied by the input/output voltage differential: (IOUT)(VIN – VOUT), and 2. GND pin current multiplied by the input voltage: (IGND)(VIN). The GND pin current can be found using the GND Pin Current curves in the Typical Performance Characteris- tics. Power dissipation will be equal to the sum of the two components listed above. The LT1764 series regulators have internal thermal limit- ing designed to protect the device during overload condi- tions. For continuous normal conditions, the maximum junction temperature rating of 125 °C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be considered. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Surface mount heatsinks and plated through-holes can also be used to spread the heat gener- ated by power devices. The following table lists thermal resistance for several different board sizes and copper areas. All measurements were taken in still air on 1/16" FR-4 board with one ounce copper. Table 1. Q Package, 5-Lead DD COPPER AREA THERMAL RESISTANCE TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm 2 2500mm 2 2500mm 2 23 °C/W 1000mm 2 2500mm 2 2500mm 2 25 °C/W 125mm 2 2500mm 2 2500mm 2 33 °C/W *Device is mounted on topside. T Package, 5-Lead TO-220 Thermal Resistance (Junction-to-Case) = 2.5 °C/W Calculating Junction Temperature Example: Given an output voltage of 3.3V, an input voltage range of 4V to 6V, 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) = 6V IGND at (IOUT = 500mA, VIN = 6V) = 10mA So, P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W Using a DD package, the thermal resistance will be in the range of 23 °C/W to 33°C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 1.41W(28 °C/W) = 39.5°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 + 39.5°C = 89.5°C Protection Features The LT1764 regulators incorporate several protection features which make them ideal for use in battery-powered circuits. In addition to the normal protection features associated with monolithic regulators, such as current limiting and thermal limiting, the devices are protected against reverse input voltages, reverse output voltages and reverse voltages from output to input. 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 20V. Current flow into the device will be limited to less than 1mA and no negative voltage will appear at the output. The |
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类似说明 - LT1764EQ |
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