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MIC5239-3.3BM Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC5239-3.3BM Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 14 page  Micrel MIC5239 December 2007 10 M9999-121007 Current Limit Figure 4 displays a method for reducing the steady state short-circuit current. The duration that the supply delivers current is set by the time required for the error flag output to discharge the 4.7µF capacitor tied to the enable pin. The off time is set by the 200kΩ resistor as it recharges the 4.7µF capacitor, enabling the regulator. This circuit reduces the short-circuit current from 800mA to 40mA while allowing for regulator restart once the short is removed. COUT VOUT VIN 5V VERR IN MIC5239 EN 200k 1N4148 200k 4.7µF OUT GND SHUTDOWN ENABLE FLG Figure 4. Remote Enable with Short-Circuit Current Foldback Thermal Characteristics The MIC5239 is a high input voltage device, intended to provide 500mA of continuous output current in two very small profile packages. The power MSOP-8 allows the device to dissipate about 50% more power than their standard equivalents. Power MSOP-8 Thermal Characteristics One of the secrets of the MIC5239’s performance is its power MSOP-8 package featuring half the thermal resistance of a standard MSOP-8 package. Lower thermal resistance means more output current or higher input voltage for a given package size. Lower thermal resistance is achieved by joining the four ground leads with the die attach paddle to create a single piece electrical and thermal conductor. This concept has been used by MOSFET manufacturers for years, proving very reliable and cost effective for the user. Thermal resistance consists of two main elements, θJC (junction-to-case thermal resistance) and θCA (case-to- ambient thermal resistance). See Figure 5. θJC is the resistance from the die to the leads of the package. θCA is the resistance from the leads to the ambient air and it includes θCS (case-to-sink thermal resistance) and θSA (sink-to-ambient thermal resistance). Figure 5. Thermal Resistance Using the power MSOP-8 reduces the θJC dramatically and allows the user to reduce θCA. The total thermal resistance, θJA (junction-to-ambient thermal resistance) is the limiting factor in calculating the maximum power dissipation capability of the device. Typically, the power MSOP-8 has a θJC of 80°C/W, this is significantly lower than the standard MSOP-8 which is typically 200°C/W. θCA is reduced because pins 5 through 8 can now be soldered directly to a ground plane which significantly reduces the case-to-sink thermal resistance and sink to ambient thermal resistance. Low-dropout linear regulators from Micrel are rated to a maximum junction temperature of 125°C. It is important not to exceed this maximum junction temperature during operation of the device. To prevent this maximum junction temperature from being exceeded, the appropriate ground plane heatsink must be used. Figure 6. Copper Area vs. Power-MSOP Power Dissipation (∆TJA) Figure 6 shows copper area versus power dissipation with each trace corresponding to a different temperature rise above ambient. From these curves, the minimum area of copper necessary for the part to operate safely can be determined. The maximum allowable temperature rise must be calculated to determine operation along which curve. |
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