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MIC2238-521YML Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC2238-521YML Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 16 page  Micrel, Inc. MIC2238 April 2010 11 M9999-040810-A Applications Information Input Capacitor A minimum 2.2µF ceramic is recommended on the VIN 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 MIC2238 was designed specifically for use with a 2.2µF or greater ceramic output capacitor. The output capacitor requires either an X7R or X5R dielectric. 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 MIC2238 was designed for use with a 1µH, 2.2µH, or 4.7µH inductor. For a better load transient response, a 1µH inductor is recommended. For better efficiency, a 4.7µH inductor 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 that the peak current will not saturate the inductor. The size requirements refer to the area and height requirements that are necessary to fit a particular design. Please refer to the inductor dimensions on their datasheet. DC resistance is also important. While DCR is inversely proportional to size, DCR can represent a significant efficiency loss. Refer to the Efficiency Considerations. Compensation The MIC2238 is an internally compensated, current mode buck regulator. Current mode is achieved by sampling the peak current and using the output of the error amplifier to pulse width modulate the switch node and maintain output voltage regulation. The MIC2238 is designed to be stable with a 1µH, 2.2µH or 4.7µH inductor with a 2.2µF ceramic (X5R) output capacitor. Feedback The MIC2238 provides a feedback pin to adjust the output voltage to the desired level. This pin connects internally to an error amplifier. The error amplifier then compares the voltage at the feedback to the internal 0.8V reference voltage and adjusts the output voltage to maintain regulation. Calculating the resistor divider network for the desired output is as follows; ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = 1 V V R1 R2 REF OUT Where VREF is 0.8V and VOUT is the desired output voltage. A 100KΩ from the output to the feedback is recommended for R1. Larger resistor values require an additional capacitor (feed- forward) from the output to the feedback. The large high side resistor value and the parasitic capacitance on the feedback pin (~10pF) can cause an additional pole in the control loop. The additional pole can create a phase loss at high frequencies. This phase loss degrades transient response by reducing phase margin. Adding feed- forward capacitance negates the parasitic capacitive effects of the feedback pin. Refer to Table 1 for recommended feedforward capacitor values. Recommended CFF Total Feedback Resistance 22pF 1M - 2MΩ 47pF 500k -1MΩ 100pF 100k - 500kΩ 180pF 10k - 100kΩ Table 1. Recommended Feed-Forward Capacitor Large feedback resistor values increase impedance, making the feedback node more susceptible to noise pick-up. A feed-forward capacitor would also reduce noise pick-up by providing a low impedance path to the output. 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 |
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