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LCXW Arkusz danych(PDF) 10 Page - Linear Technology |
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LCXW Arkusz danych(HTML) 10 Page - Linear Technology |
10 / 16 page LT1965 10 1965f Other regulators, such as the LT1083/LT1084/LT1085 family, also exhibit this phenomenon, so it is not unique to the LT1965. The problem occurs with a heavy output load when the input voltage is high and the output voltage is low. Com- mon situations include immediately after the removal of a short-circuit or if the shutdown 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 output operating points for the regulator. With this double intersection, the input power supply may need to be cycled down to zero and brought up again to make the output recover. Output Voltage Noise The LT1965 regulator’s design provides low output voltage noise over the 10Hz to 100kHz bandwidth while operating at full load. Output voltage noise is approximately 80nV/√⎯H⎯z over this frequency bandwidth for the LT1965. For higher output voltages (generated by using a resistor divider), the output voltage noise gains up accordingly. Higher values of output voltage noise may be measured if care is not exercised with regard to circuit layout and testing. Crosstalk from nearby traces can induce unwanted noise onto the LT1965’s output. Power supply ripple rejec- tion must also be considered; the LT1965 regulator does not have unlimited power supply rejection and will pass a small portion of the input noise through to the output. APPLICATIONS INFORMATION Thermal Considerations The LT1965’s maximum rated junction temperature of 125°C limits its power handling capability. Two compon- ents comprise the power dissipated by the device: 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 GND pin current is determined using the GND Pin Current curves in the Typical Performance Characteristics section. Power dissipation equals the sum of the two components listed. The LT1965 regulator has internal thermal limiting that protects the device during overload conditions. For con- tinuous normal conditions, do not exceed the maximum junction temperature rating of 125°C. Carefully consider all sources of thermal resistance from junction to ambi- ent including other heat sources mounted in proximity to the LT1965. The underside of the LT1965 DFN package has exposed metal (4mm2) from the lead frame to the die attachment. The underside of the LT1965 MSOP package also has ex- posed metal (2mm2). Both packages allow heat to directly transfer from the die junction to the printed circuit board metal to control maximum operating junction temperature. The dual-in-line pin arrangement allows metal to extend beyond the ends of the package on the topside (component side) of a PCB. Connect this metal to GND on the PCB. The multiple IN and OUT pins of the LT1965 also assist in spreading heat to the PCB. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the heat gener- ated by power devices. |
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