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MSK590XRHG Fiches technique(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK590XRHG Fiches technique(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 6 page APPLICATION NOTES HEAT SINKING To determine if a heat sink is required for your application and if so, what type, refer to the thermal model and govern- ing equation below. 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 Tc = Case Temperature Ta = Ambient Temperature Ts = Heat Sink Temperature EXAMPLE: This example demonstrates an analysis where the output cur- rents are at 0.5 amp each and both inputs are 8V. Conditions for MSK 5901RH: +Vin = +8.0V; Iout = 0.5A Positive Regulator 1.) Assume 45° heat spreading model. 2.) Find positive regulator power dissipation: Pd = (Vin - Vout)(Iout) Pd = (+8V-5V)(0.5A) Pd = 1.5W 3.) For conservative design, set Tj = +125°C Max. 4.) For this example, worst case Ta = +90°C. 5.) Rθjc = 10.5°C/W from the Electrical Specification Table. 6.) Rθcs = 0.15°C/W for most thermal greases. 7.) Rearrange governing equation to solve for Rθsa: Rθsa = ((Tj - Ta)/Pd) - (Rθjc) - (Rθcs) = (125°C-90°C)/1.5W - 10.5°C/W - 0.15°C/W = 12.7°C/W The same exercise must be performed for the negative regula- tor. CAPACITOR SELECTION INPUT CAPACITOR: An input bypass capacitor is recommenced when using the MSK 5901 series regulators. This is especially true if the regu- lator is located farther than 6 inches from the power supply filter capacitors. For most applications a 1µF solid tantalum capacitor will be suitable. OUTPUT CAPACITOR: A minimum of a 1µF solid tantalum capacitor should also be used at the output to insure stability. Any increase of this out- put capacitor larger than 10µF will only improve output imped- ance. +CCOMP CAPACITOR: For improved ripple rejection, +Ccomp can be bypassed to ground with a 10µF tantalum capacitor. This bypass capacitor will provide 80dB ripple rejection. Increased capacitance above 10µF does not improve the ripple rejection at frequencies above 120Hz. If the Ccomp bypass capacitor is used, it may be neces- sary to add a protection diode to protect the regulator from ca- pacitor discharge damage. See Typical Applications Circuit for clarification. If the bypass capacitor is not used, it should be left open since it is internally connected to the regulator. 3 POSITIVE REGULATOR INPUT CAPACITOR: Once again, if the regulator will be farther than 6 inches from power supply filter capacitors, then an input capacitor will be required on the negative regulator. It is recommended that a 1µF solid tantalum capacitor be used. OUTPUT CAPACITOR: A minimum of a 1µF solid tantalum capacitor should also be used at the output to insure stability. Any increase of this out- put capacitor larger than 10µF will only improve output imped- ance. -CCOMP CAPACITOR: For improved ripple rejection, -Ccomp can be bypassed to ground with a 10µF tantalum capacitor. This bypass capacitor will provide 80dB ripple rejection. Increased capacitance above 10µF does not improve the ripple rejection at frequencies above 120Hz. If the Ccomp bypass capacitor is used, it may be neces- sary to add a protection diode to protect the regulator from ca- pacitor discharge damage. See Typical Applications Circuit for clarification. If the bypass capacitor is not used, it should be left open since it is internally connected to the regulator. NEGATIVE REGULATOR It is important to keep the output connection between the regu- lator and the load as short as possible since this directly affects the load regulation. For example, if 20 gauge wire were used which has a resistance of about 0.008 ohms per foot, this would result in a drop of 8mV/ft at 1Amp of load current. It is also important to follow the capacitor selection guidelines to achieve best performance. TYPICAL APPLICATION CIRCUIT LOAD REGULATION Rev. D 7/05 |
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