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MSK5020-3.3B Datasheet(数据表) 3 Page - M.S. Kennedy Corporation |
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MSK5020-3.3B Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
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3 page 
To select a heat sink for the MSK 5020, the following formula for convective heat flow must be used. APPLICATION NOTES The output voltage ripple of the MSK 5020 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 and is best determined by experimentation. Transient load response can also be improved by placing a 33uF or larger capacitor directly across the load. MINIMIZING OUTPUT RIPPLE: To implement current limiting, a sense resistor (Rsc) must be placed from pin 5 to pins 8,9 and 10 as shown in the typical connection diagram. When the voltage drop across the sense resistor reaches 35mV, the internal control loop limits the out- put current only enough to maintain 35mV across the sense resistor. The device is not disabled. The following formula may be used to find the correct value of sense resistance: RSC=35mV/ILIM If current limit is not required simply connect the Vsc pins di- rectly to the input voltage along with the sense pin. Refer to the typical connection diagram for an illustration. CURRENT LIMIT CONNECTIONS The output pass transistors in the MSK 5020 are rated to dissi- pate nearly 200 watts. The limiting factor of this device is effective dissipation of heat generated under such conditions. For example, to dissipate 200 watts, calculations show that the MSK 5020 would have to be bolted to the underbelly of a submarine submerged in the Arctic Ocean! Careful consider- ation must be paid to heat dissipation and junction temperature when applying this device. POWER DISSIPATION: 37W = 37 Watts 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 175°C. The equation may now be rearranged to solve for the required heat sink to ambient thermal resistance (R θsa). EXAMPLE; An MSK 5020-3.3 is configured for Vin = +7V and Vout = +3.3V. Iout is a continuous 10A DC level. The ambient tem- perature is +25°C. The maximum desired junction tempera ture is 150°C. R θjc = 1.0°C/W and Rθcs = 0.15°C/W typi- cally. Power Dissipation = (7V - 3.3V) x (10A) Solve for R θsa: R θsa = 150°C - 25°C - 1.0°C/W - 0.15°C/W In this example, a heat sink with a thermal resistance of no more than 2.23°C/W must be used to maintain a junction tem- perature of no more than 150°C. 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 HEAT SINK SELECTION: For all applications, the user must connect a 1.0uF capacitor from pin 4 directly to ground. This capacitor is part of the circuit which drives the gate of the internal MOSFETS. Ap- proximately three times the voltage seen on the input will ap- pear across this capacitor. Careful attention must be paid to capacitor voltage rating since voltages larger than the power supply are present. FILTER CAPACITOR: Pin 6 of the MSK 5020 series is the Fault pin. When the out- put voltage drops 6% or more below its nominal value, the voltage level on the Fault pin drops to a logic low (typically less than 0.1 volts). This pin can be used to drive a light emitting diode or other external circuitry as long as the current is limited to less than 10.0mA (see typical connection diagram). The Fault pin is an open collector output so the high state output voltage will be equal to the pull up voltage since no current flows under these conditions. FAULT PIN CONNECTIONS The MSK 5020 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 only 10µA of quiescent current. This pin can be connected to VIN if the enable function is not desired. ENABLE/DISABLE PIN: 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 same holds true for the connection from the low end of the load to ground. For best load regula- tion, the low end of the load must be connected directly to pin 3 of the MSK 5020 and not to a ground plane inches away from the hybrid. LOAD CONNECTIONS: For best results, the ground pin should be connected directly to the load (see next note). 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 be- tween 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 80mV/ft at a load current of 10 amps. LOAD REGULATION: The case of the MSK 5020 is connected to pin seven of the package but isolated from the internal circuitry allowing direct attachment of the heat sink to the case. It may be necessary in some applications to ground the case to limit noise or elimi- nate oscillations on the output. Pin seven can be left as a no connect if the designer chooses. CASE CONNECTIONS: 3 Rev. F 1/05 |
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