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MIC39151-1.65WT Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC39151-1.65WT Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 14 page  Micrel, Inc. MIC39150/39151/39152 October 2009 10 M9999-102309-A Application Information The MIC39150/1/2 are high-performance, low-dropout voltage regulators suitable for moderate to high-current voltage regulator applications. Its 500mV dropout voltage at full load and overtemperature makes it especially valuable in battery-powered systems and as high-efficiency noise filters in post-regulator applications. Unlike 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 performance 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 MIC39150/1/2 regulators are 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 temperature exceeds the maximum safe operating temperature. Transient protection allows device (and load) survival even when the input voltage spikes above and below nominal. The output structure of these regulators allows voltages in excess of the desired output voltage to be applied without reverse current flow. Thermal Design Linear regulators are simple to use. The most complicated design parameters to consider are thermal characteristics. Thermal design requires the following application-specific parameters: • Maximum ambient temperature (TA) • Output Current (IOUT) • Output Voltage (VOUT) • Input Voltage (VIN) • Ground Current (IGND) First, calculate the power dissipation of the regulator from these numbers and the device parameters from this datasheet. GND IN OUT OUT IN D I V I ) V (V P + − = where the ground current is approximated by using numbers from the “Electrical Characteristics” or “Typical Characteristics.” Then the heat sink thermal resistance is determined with this formula: () CS JC D A J(max) SA θ θ P T T θ + − − = 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 compared 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. With no heat sink in the application, calculate the junction temperature to determine the maximum power dissipation that will be allowed before exceeding the maximum junction temperature of the MIC39152. The maximum power allowed can be calculated using the thermal resistance (θJA) of the TO-252 (D-Pak) adhering to the following criteria for the PCB design: 2 oz. copper and 100mm 2 copper area for the MIC39152. For example, given an expected maximum ambient temperature (TA) of 75°C with VIN = 2.25V, VOUT = 1.75V, and IOUT = 1.5A, first calculate the expected PD using Equation (1); PD = (2.25V – 1.75V)1.5A + (2.25V)(0.017A) = 0.788W Next, calcualte the junction temperature for the expected power dissipation. TJ = (θJA × PD) + TA = (56°C/W × 0.788W) + 75°C = 119.14 °C Now determine the maximum power dissipation allowed that would not exceed the IC’s maximum junction temperature (125 °C) without the use of a heat sink by PD(MAX) = (TJ(MAX) – TA)/θJA = (125°C – 75°C)/(56°C/W) = 0.893W MIC39150-x.x IN OUT GND CIN COUT VIN VOUT Figure 1. Capacitor Requirements Output Capacitor The MIC39150/1/2 requires an output capacitor to maintain stability and improve transient response. See Figure 1. Proper capacitor selection is important to ensure proper operation. TheMIC39150/1/2 output capacitor selection is dependent upon the ESR (equivalent series resistance) of the output capacitor to maintain stability. When the output capacitor is 10µF or greater, the output capacitor should have an ESR less than 2Ω. This will improve transient response as well as |
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