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MIC5237-5.0BT Datasheet(PDF) 6 Page - Micrel Semiconductor |
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MIC5237-5.0BT Datasheet(HTML) 6 Page - Micrel Semiconductor |
6 / 8 page MIC5237 Micrel MIC5237 6 January 2000 Applications Information The MIC5237 is intended for general-purpose use and can be implemented in a wide variety of applications where 500mA of output current is needed. It is available in several voltage options for ease of use. For voltage options that are not available on the MIC5237, consult the MIC5209 for a 500mA adjustable LDO regulator, or the MIC5219 for applications that require only short-duration peak output current. Input Capacitor A 1 µF capacitor should be placed from IN to GND if there is more than 10 inches of wire between the input and the ac filter capacitor or if a battery is used as the input. Output Capacitor An output capacitor is required between OUT and GND to prevent oscillation. 1 µF minimum is recommended for stan- dard applications. Larger values improve the regulator’s transient response. The output capacitor value may be in- creased without limit. The output capacitor should have an ESR (equivalent series resistance) of about 5 Ω or less and a resonant frequency above 1MHz. Ultralow-ESR capacitors can cause low-ampli- tude oscillations and/or underdamped transient response. Most tantalum or aluminum electrolytic capacitors are ad- equate; film types will work, but are more expensive. Since many aluminum electrolytics have electrolytes that freeze at about –30 °C, solid tantalums are recommended for operation below –25 °C. At lower values of output current, less output capacitance is needed for output stability. The capacitor can be reduced to 0.47 µF for current below 10mA or 0.33µF for currents below 1mA. For 2.5V applications a 22 µF output capacitor is recom- mended to reduce startup voltage overshoot. No-Load Stability The MIC5237 will remain stable and in regulation with no load (other than the internal voltage divider) unlike many other voltage regulators. This is especially important in CMOS RAM keep-alive applications. Thermal Considerations Proper thermal design can be accomplished with some basic design criteria and some simple equations. The following information is required to implement a regulator design. V IN = input voltage V OUT = output voltage I OUT = output current T A = ambient operating temperature I GND = ground current The regulator ground current, I GND, can be measured or read from the data sheet. Assuming the worst case scenario is good design procedure, and the corresponding ground cur- rent number can be obtained from the data sheet. First, calculate the power dissipation of the device. This example uses the MIC5237-5.0BT, a 13V input, and 500mA output current, which results in 20mA of ground current, worst case. The power dissipation is the sum of two power calculations: voltage drop × output current and input voltage × ground current. P = (V – V ) I + V I DIN OUT OUT IN GND × [] × () P = [(13V – 5V) 500mA] + 13V 20mA D ×× () P = 4.260W D From this number, the heat sink thermal resistance is deter- mined using the regulator’s maximum operating junction temperature (T J(max)) and the ambient temperature (TA) along with the power dissipation number already calculated. T J(MAX) = 125°C θ JC = junction-to-case thermal resistance θ CS = case-to-sink thermal resistance θ JA = junction-to-ambient thermal resistance θ SA = sink-to-ambient thermal resistance To determine the heat sink thermal resistance, the junction- to-case thermal resistance of the device must be used along with the case-to-heat sink thermal resistance. These num- bers show the heat-sink thermal resistance required at T A = 25 °C that does not exceed the maximum operating junction temperature. θ JA J(max) A D T T P = − θθ θ SA JA JC =− θ CS is approximately 1°C/W and θJC for the TO-220 is 3°C/W in this example. θ JA = 125 – 25 4.260W θ JA = 23.5 C/W ° θ SA = 23.5 C/W – 3 C/W + 1 C/W °° ° () θ SA = 19.5 C/W ° Therefore, a heat sink with a thermal resistance of 19.5 ° C/W will allow the part to operate safely and it will not exceed the maximum junction temperature of the device. The heat sink can be reduced by limiting power dissipation, by reducing the input voltage or output current. Either the TO-220 or TO-263 package can operate reliably at 2W of power dissipation without a heat sink. Above 2W, a heat sink is recommended. For a full discussion on voltage regulator thermal effects, please refer to “Thermal Management” in Micrel’s Designing with Low-Dropout Voltage Regulators handbook. |
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