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MIC49300-1.2WR Datasheet(PDF) 6 Page - Micrel Semiconductor |
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MIC49300-1.2WR Datasheet(HTML) 6 Page - Micrel Semiconductor |
6 / 8 page MIC49300 Micrel, Inc. M9999-082605-B 6 August 2005 Applications Information The MIC49300 is an ultra-high performance, low dropout linear regulator designed for high current applications requiring fast transient response. The MIC49300 utilizes two input supplies, significantly reducing dropout voltage, perfect for low-voltage, DC-to-DC conversion. The MIC49300 requires a minimum of external components and obtains a bandwidth of up to 10MHz. As a µCap regulator, the output is tolerant of virtually any type of capacitor including ceramic and tantalum. The MIC49300 regulator is fully protected from damage due to fault conditions, offering linear current limiting and thermal shutdown. Bias Supply Voltage VBIAS, requiring relatively light current, provides power to the control portion of the MIC49300. VBIASrequiresapproximately 33mA for a 1.5A load current. Dropout conditions require higher currents. Most of the biasing current is used to supply the base current to the pass transistor. This allows the pass element to be driven into saturation, reducing the dropout to 300mV at a 1.5A load current. Bypassing on the bias pin is recommended to improve performance of the regulator dur- ing line and load transients. Small ceramic capacitors from VBIAS to ground help reduce high frequency noise from being injected into the control circuitry from the bias rail and are good design practice. Good bypass techniques typically in- clude one larger capacitor such as a 1µF ceramic and smaller valued capacitors such as 0.01µF or 0.001µF in parallel with that larger capacitor to decouple the bias supply. The VBIAS input voltage must be 1.6V above the output voltage with a minimum VBIAS input voltage of 3V. Input Supply Voltage VIN provides the high current to the collector of the pass transistor. The minimum input voltage is 1.4V, allowing con- version from low voltage supplies. Output Capacitor The MIC49300 requires a minimum of output capacitance to maintain stability. However, proper capacitor selection is important to ensure desired transient response. The MIC49300 is specifically designed to be stable with virtually any capacitance value and ESR. A 1µF ceramic chip capaci- tor should satisfy most applications. Output capacitance can be increased without bound. See typical characteristics for examples of load transient response. X7R dielectric ceramic capacitors are recommended because oftheirtemperatureperformance.X7R-typecapacitorschange capacitance by 15% over their operating temperature range and are the most stable type of ceramic capacitors. Z5U and Y5V dielectric capacitors change value by as much as 50% and 60%, respectively, over their operating temperature ranges. To use a ceramic chip capacitor with Y5V dielectric, the value must be much higher than an X7R ceramic or a tantalum capacitor to ensure the same capacitance value over the operating temperature range. Tantalum capacitors have a very stable dielectric (10% over their operating temperature range) and can also be used with this device. Input Capacitor An input capacitor of 1µF or greater is recommended when the device is more than 4 inches away from the bulk supply capacitance, or when the supply is a battery. Small, surfac- emount, ceramic chip capacitors can be used for the bypass- ing. The capacitor should be placed within 1" of the device for optimal performance. Larger values will help to improve ripple rejection by bypassing the input to the regulator, further improving the integrity of the output voltage. 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 • Maximum ambient temperature (T • Maximum ambient temperature (T ) • 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. PD = VIN × IIN + VBIAS × IBIAS – VOUT × IOUT The input current will be less than the output current at high output currents as the load increases. The bias current is a sum of base drive and ground current. Ground current is constant over load current. Then the heat sink thermal resistance is determined with this formula: θSA J(MAX) A D T T J(MAX) J(MAX) A A PDD = ( ) θ θ θ θ JC JC θ θ θ θ θ θ θ θCS CS θ θ θ θ θ θ θ θ T T T T – The heat sink may be significantly reduced in applications where the maximum input voltage is known and 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 the MIC49300 allow significant reductions in regulator power dis- sipation 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 regula- tor ground. Refer to Application Note 9 for further details and examples on thermal design and heat sink specification. Minimum Load Current The MIC49300, unlike most other high current regulators, does not require a minimum load to maintain output voltage regulation. Power Sequencing There is no power sequencing requirement for VIN and VBIAS, giving more flexibility to the user. |
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