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MSK5150-3.3ZD Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK5150-3.3ZD Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 5 page First, the power dissipation must be calculated as follows: Power Dissipation = (Vin - Vout) x Iout Next, the user must select a maximum junction temperature. The absolute maximum allowable junction temperature is 125°C. The equation may now be rearranged to solve for the required heat sink to ambient thermal resistance (R θsa). EXAMPLE: An MSK 5150-3.3 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 tem- perature is 125°C. R θjc = 1.5°C/W and Rθcs = 0.15°C/W for most thermal greases Power Dissipation= (5V - 3.3V) x (1A) = 1.7 Watts Solve for R θsa: R θsa = 125°C - 25°C - 1.5°C/W - 0.15°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 junction tem- perature of no more than 125°C. APPLICATION NOTES HEAT SINK SELECTION: To select a heat sink for the MSK 5150, the following formula for convective heat flow may be used. Governing Equation: Tj = Pd x (R θjc + Rθcs + Rθsa) + Ta WHERE: Tj = Junction Temperature Pd = Total Power Dissipation R θjc = Junction to Case Thermal Resistance R θcs = Case to Heat Sink Thermal Resistance R θsa = Heat Sink to Ambient Thermal Resistance Ta = Ambient Temperature REGULATOR PROTECTION: The MSK 5150 series is fully protected against reversed input polarity, overcurrent faults, overtemperature conditions (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. MSK5150-00 OUTPUT ADJUSTMENT: The MSK 5150-00 is an adjustable version in the series of high performance regulators. The diagram below illustrates proper adjustment technique for the output voltage. The series resis- tance of R1+R2 should be selected to pass the minimum regu- lator output current requirement of 10mA. DEVICE/CASE CONNECTION: The MSK 5150 series are highly thermally conductive devices and the thermal path from the package heat sink to the internal junctions is very short. Since the case is electrically isolated from the internal circuitry, the package can be directly con- nected to a heat sink. FLAG OUTPUT PIN: All of the fixed output voltage versions of the MSK 5150 series are equipped with a flag output pin. Since the flag pin is an open collector configuration it can be pulled up to any voltage between 3V and 26V. This feature allows direct interfacing to practically any logic. This active low output has a typical level of 0.22V when the flag comparator detects an "out of regula- tion" condition. Flag states include low input voltage, out of regulation and output current limit. Extremely high level input voltage transients will also cause the flag output pin to acti- vate. ENABLE PIN: The MSK 5150 series of voltage regulators are equipped with a TTL compatible ENABLE pin. A TTL high level on this pin acti- vates the internal bias circuit and powers up the device. A TTL low level on this pin places the controller in shutdown mode and the device draws approximately 10µA of quiescent current. If the enable function is not used, simply connect the enable pin to the input. LOAD CONNECTIONS: In voltage regulator applications where very large load cur- rents are present, the load connection is very important. The path connecting the output of the regulator to the load must be extremely low impedance to avoid affecting the load regulation specifications. Any impedance in this path will form a voltage divider with the load. The MSK 5150 series requires a mini- mum of 10mA of load current to stay in regulation. OUTPUT CAPACITOR: The output voltage ripple of the MSK 5150 series voltage regu- lators can be minimized by placing a filter capacitor from the output to ground. The optimum value for this capacitor may vary from one application to the next, but a minimum of 20µF is recommended for optimum performance. This capacitor need not be an expensive low ESR type: aluminum electrolytics are adequate. In fact, extremely low ESR capacitors may contrib- ute to instability. Tantalum capacitors are recommended for systems where fast load transient response is important. Tran- sient load response can also be improved by placing a capaci- tor directly across the load. 1.7W 3 Rev. F 1/06 = 57.17°C/W |
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