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MIC39500-1.8WU Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC39500-1.8WU Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 10 page August 2005 7 M9999-082605-B MIC39500/39501 Micrel Applications Information The MIC39500/1 is a high-performance low-dropout voltage regulator suitable for moderate to high-current voltage regula- tor applications. Its 400mV dropout voltage at full load makes it especially valuable in battery-powered systems and as a high-efficiency noise filter in post-regulator applications. Un- like older NPN-pass transistor designs, where the minimum dropout voltage is limited by the base-to-emitter voltage drop and collector-to-emitter saturation voltage, dropout perfor- mance of the PNP output of these devices is limited only by the low VCE saturation voltage. A trade-off for the low dropout voltage is a varying base drive requirement. Micrel’s Super βeta PNP™ process reduces this drive requirement to only 2% to 5% of the load current. The MIC39500/1 regulator is fully protected from damage due to fault conditions. Current limiting is provided. This limiting is linear; output current during overload conditions is constant. Thermal shutdown disables the device when the die tempera- ture exceeds the maximum safe operating temperature. Tran- sient protection allows device (and load) survival even when the input voltage spikes above and below nominal. The output structureoftheseregulatorsallowsvoltagesinexcessofthede- sired output voltage to be applied without reverse current flow. MIC39500-x.x IN OUT GND CIN COUT VIN VOUT Figure 1. Capacitor Requirements Thermal Design Linear regulators are simple to use. The most complicated design parameters to consider are thermal characteristics. Thermal design requires four application-specific param- eters: • Maximum ambient temperature (TA • Maximum ambient temperature (T • Maximum ambient temperature (T ) • Output Current (IOUT) • Output Voltage (VOUT) • Input Voltage (VIN) • Ground Current (IGND) Calculate the power dissipation of the regulator from these numbers and the device parameters from this datasheet, where the ground current is taken from data sheet. PD = (VIN – VOUT) × IOUT + VIN × IGND The heat sink thermal resistance is determined by: where: TJ (max) ≤ 125°C and θCS is between 0° and 2°C/W. The heat sink may be significantly reduced in applications where the minimum input voltage is known and is large com- pared with the dropout voltage. Use a series input resistor to drop excessive voltage and distribute the heat between this resistor and the regulator. The low-dropout properties of Micrel Super βeta PNP regulators allow significant reductions in regulator power dissipation and the associated heat sink without compromising performance. When this technique is employed, a capacitor of at least 1µF is needed directly between the input and regulator ground. Refer to Application Note 9 for further details and examples on thermal design and heat sink specification. Output capacitor The MIC39500/1 requires an output capacitor to maintain stability and improve transient response. Proper capaci- tor selection is important to ensure proper operation. The MIC39500/1 output capacitor selection is dependent upon the ESR (equivalent series resistance) of the output capacitor to maintain stability. When the output capacitor is 47µF or greater, the output capacitor should have less than 1Ω of ESR. This will improve transient response as well as promote stability. Ultra-low-ESR capacitors, such as ceramic chip capacitors may promote instability. These very low ESR levels may cause an oscillation and/or underdamped transient response. When larger capacitors are used, the ESR requirement ap- proaches zero. A 100µF ceramic capacitor can be used on the output while maintaining stability. A low-ESR 47µF solid tantalum capacitor works extremely well and provides good transient response and stability over temperature. Aluminum electrolytics can also be used, as long as the ESR of the capacitor is < 1Ω. The value of the output capacitor can be increased without limit. Higher capacitance values help to improve transient response and ripple rejection and reduce output noise. Input capacitor An input capacitor of 1µF or greater is recommended when the device is more than 4 inches away from the bulk ac supply capacitance, or when the supply is a battery. Small surface- mount ceramic chip capacitors can be used for bypassing. Larger values will help to improve ripple rejection by bypass- ing the input to the regulator, further improving the integrity of the output voltage. Transient Response and 3.3V to 2.5V or 2.5V to 1.8V Conversion The MIC39500/1 has excellent transient response to varia- tions in input voltage and load current. The device has been designed to respond quickly to load current variations and input voltage variations. Large output capacitors are not re- quired to obtain this performance. A standard 47µF output capacitor, preferably tantalum, is all that is required. Larger values improve performance even further. By virtue of its low-dropout voltage, this device does not saturate into dropout as readily as similar NPN-based de- signs. When converting from 3.3V to 2.5V, or 2.5V to 1.8V, the NPN-based regulators are already operating in dropout, with typical dropout requirements of 1.2V or greater. To con- vert down to 2.5V without operating in dropout, NPN-based regulators require an input voltage of 3.7V at the very least. The MIC39500/1 regulator provides excellent performance |
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