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MSK5275-19 Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK5275-19 Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
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3 / 5 page  ] APPLICATION NOTES REGULATOR PROTECTION: The MSK 5275 series is fully protected against reversed input polarity, overcurrent faults, overtemperature con- ditions (Pd) and transient voltage spikes of up to 60V. If the regulator is used in dual supply systems where the load is returned to a negative supply, the output voltage must be diode clamped to ground. LOAD CONNECTIONS: In voltage regulator applications where very large load currents are present, the load connection is very impor- tant. The path connecting the output of the regulator to the load must be extremely low impedance to avoid af- fecting the load regulation specifications. Any imped- ance in this path will form a voltage divider with the load. MINIMIZING POWER DISSIPATION: Many applications can not take full advantage of the extremely low dropout specifications of the regulator due to large input to output voltage differences. The simple circuit below illustrates a method to reduce the input voltage at the regulator to just over the dropout specifi- cation to keep the internal power dissipation minimized: For a given continuous maximum load of 1 amp, R1 can be selected to drop the voltage seen at the regulator to 4V. This allows for the output tolerance and dropout specifications. Input voltage variations (5V) also should be included in the calculations. The resistor should be sized according to the power levels required for the ap- plication. OUTPUT CAPACITOR: The output voltage ripple of the MSK 5275 series volt- age regulators can be minimized by placing a filter ca- pacitor from the output to ground. The optimum value for this capacitor may vary from one application to the next, but a minimum of 33μF is recommended for opti- mum performance. Transient load response can also be improved by placing a capacitor directly across the load. The capacitor should not be an ultra-low ESR type. Tan- talum capacitors are best for fast load transients but aluminum electrolytics will work fine in most applica- tions. 3 1.7W PACKAGE CONNECTIONS: The MSK 5275 series are highly thermally conductive devices and the thermal path from the package heat sink to the internal junctions is very short. Standard surface mount soldering techniques should be used when mount- ing the device. Some applications may require additional heat sinking of the device. HEAT SINK SELECTION: To select a heat sink for the MSK 5275, the following formula for convective heat flow may be used. Governing Equation: Tj = Pd x (Rθjc + Rθcs + Rθsa) + Ta WHERE: Tj = Junction Temperature Pd = Total Power Dissipation Rθjc = Junction to Case Thermal Resistance Rθcs = Case to Heat Sink Thermal Resistance Rθsa = Heat Sink to Ambient Thermal Resistance Ta = Ambient Temperature [ First, the power dissipation must be calculated as fol- lows: Power Dissipation = (Vin - Vout) x Iout Next, the user must select a maximum junction tempera- ture. The absolute maximum allowable junction tem- perature is 125°C. The equation may now be rearranged to solve for the required heat sink to ambient thermal resistance (Rθsa). EXAMPLE: An MSK 5275-3.3 is configured for Vin=+5V and Vout=+3.3V. Iout is a continuous 1A DC level. The ambient temperature is +25°C. The maximum desired junction temperature is 125°C. Rθjc = 1.0°C/W and Rθcs = 0.5°C/W typically. Power Dissipation = (5V - 3.3V) x (1A) = 1.7 Watts Solve for Rθsa: Rθsa = 125°C - 25°C - 1.0°C/W - 0.5°C/W In this example, a heat sink with a thermal resistance of no more than 57°C/W must be used to maintain a junc- tion temperature of no more than 125°C. = 57.32°C/W 8548-42 Rev. J 4/12 |
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