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MSK5332-15S Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK5332-15S Datasheet(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 regulator is at one-half of its maximum rated power dissipation, which occurs when the output current is at 1.5 amps. Conditions for MSK 5332-5: Vin = -7.0V; Iout = -1.5A 1.) Assume 45° heat spreading model. 2.) Find regulator power dissipation: Pd = (Vin - Vout)(Iout) Pd = (-7-(-5))(-1.5) = 3.0W 3.) For conservative design, set Tj = +125°C Max. 4.) For this example, worst case Ta = +90°C. 5.) Rθjc = 7.0°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)/3.0W - 7.0°C/W - 0.15°C/W = 4.5°C/W In this case the result is 4.5°C/W. Therefore, a heat sink with a thermal resistance of no more than 4.5°C/W must be used in this application to maintain the regulator junction temperature under 125°C. OVERLOAD SHUTDOWN The MSK 5332 features both power and thermal overload protection. When the maximum power dissipation is not ex- ceeded, the regulator will current limit slightly above its 3 amp rating. As the Vin-Vout voltage increases, however, shutdown occurs in relation to the maximum power dissipation curve. If the device heats enough to exceed its rated die junction tem- perature due to excessive ambient temperature, improper heat sinking etc., the regulator will shutdown until an appropriate junction temperature is maintained. It should also be noted that in the case of an extreme overload, such as a sustained direct short, the device may not be able to recover. In these instances, the device must be shut off and power reapplied to eliminate the shutdown condition. LOAD REGULATION For best results the ground pin should be connected directly to the load as shown below, this effectively reduces the ground loop effect and eliminates excessive voltage drop in the sense leg. It is also important to keep the output connection between the regulator 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 .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. Refer to Figure 1 for connection dia- gram. BYPASS CAPACITORS For most applications a 33uF minimum, low ESR (0.5-2 ohm) tantalum capacitor should be attached as close to the regulator's output as possible. This will effectively lower the regulator's output impedance, increase transient response and eliminate any oscillations that are normally associated with low dropout regu- lators. Additional bypass capacitors can be used at the remote load locations to further improve regulation. These can be either of the tantalum or the electrolytic variety. Unless the regulator is located very close to the power supply filter capacitor(s), a 4.7uF minimum low ESR (0.5-2 ohm) tantalum capacitor should also be added to the regulator's input. An electrolytic may also be substituted if desired. When substituting electrolytic in place of tantalum capacitors, a good rule of thumb to follow is to increase the size of the electrolytic by a factor of 10 over the tantalum value. Low Dropout Negative Power Supply MSK 5332 TYPICAL APPLICATION: FIGURE 1 3 Rev. B 8/05 |
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