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MIC23051-CGYML Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC23051-CGYML Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 13 page Micrel, Inc. MIC23051 October 2007 9 M9999-101207-C Applications Information Input Capacitor A minimum of 2.2µF ceramic capacitor should be placed close to the VIN pin and PGND pin for bypassing. X5R or X7R dielectrics are recommended for the input capacitor. Y5V dielectrics, aside from losing most of their capacitance over temperature, they also become resistive at high frequencies. This reduces their ability to filter out high frequency noise. Output Capacitor The MIC23051 was designed for use with a 2.2µF or greater ceramic output capacitor. A low equivalent series resistance (ESR) ceramic output capacitor either X7R or X5R is recommended. Y5V and Z5U dielectric capacitors, aside from the undesirable effect of their wide variation in capacitance over temperature, become resistive at high frequencies. Inductor Selection Inductor selection will be determined by the following (not necessarily in the order of importance); • Inductance • Rated current value • Size requirements • DC resistance (DCR) The MIC23051 was designed for use with an inductance range from 0.47µH to 2.2µH. Typically, a 1µH inductor is recommended for a balance of transient response, efficiency and output ripple. For faster transient response a 0.47µH inductor may be used. For lower output ripple, a 2.2µH is recommended. Maximum current ratings of the inductor are generally given in two methods; permissible DC current and saturation current. Permissible DC current can be rated either for a 40°C temperature rise or a 10% to 20% loss in inductance. Ensure the inductor selected can handle the maximum operating current. When saturation current is specified, make sure that there is enough margin so that the peak current of the inductor does not cause it to saturate. Peak current can be calculated as follows: IPK = IOUT + VOUT (1-VOUT/VIN)/2fL As shown by the previous calculation, the peak inductor current is inversely proportional to the switching frequency and the inductance; the lower the switching frequency or the inductance the higher the peak current. As input voltage increases the peak current also increases. The size of the inductor depends on the requirements of the application. Refer to the Application Circuit and Bill of Material for details. DC resistance (DCR) is also important. While DCR is inversely proportional to size, DCR can represent a significant efficiency loss. Refer to the Efficiency Considerations. Compensation The MIC23051 is designed to be stable with a 0.47µH to 2.2µH inductor with a 2.2µF ceramic (X5R) output capacitor. Efficiency Considerations Efficiency is defined as the amount of useful output power, divided by the amount of power supplied. 100 I V I V _% Efficiency IN IN OUT OUT × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × = Maintaining high efficiency serves two purposes. It reduces power dissipation in the power supply, reducing the need for heat sinks and thermal design considerations and it reduces consumption of current for battery powered applications. Reduced current draw from a battery increases the devices operating time and is critical in hand held devices. There are two types of losses in switching converters; DC losses and switching losses. DC losses are simply the power dissipation of I 2R. Power is dissipated in the high side switch during the on cycle. Power loss is equal to the high side MOSFET RDSON multiplied by the Switch Current 2. During the off cycle, the low side N-channel MOSFET conducts, also dissipating power. Device operating current also reduces efficiency. The product of the quiescent (operating) current and the supply voltage is another DC loss. The current required driving the gates on and off at a constant 4MHz frequency and the switching transitions make up the switching losses. 0 20 40 60 80 100 0.1 1 10 100 1000 LOAD (mA) Efficiency VOUT = 1.8V VOUT = 1.8V L = 1µH VIN = 3.3V VIN = 2.7V VIN = 3.6V The Figure above shows an efficiency curve. From no load to 100mA, efficiency losses are dominated by quiescent current losses, gate drive and transition losses. By using the Hyper Light Load mode the MIC23051 is able to maintain high efficiency at low output currents. Over 100mA, efficiency loss is dominated by MOSFET RDSON and inductor losses. Higher input supply voltages will increase the Gate to Source threshold on the internal MOSFETs, reducing the internal RDSON. This improves |
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