 
PAGE 2 of 4  MCP1640 Parts Selection Parts Selection  Input and Output Capacitors Let's see the components now. First, the input (C2) and output (C1) capacitors. The input capacitor can be anything between 4.7uF and above. Microchip recommends that you use a ceramic X5R or X7R, but you can also use electrolytic capacitors. I've ordered already some ceramic capacitors for test, but in the meanwhile I used SMD electrolytic capacitors which by the way are HUGE compared to the ceramic ones. As a typical rule of the thumb, the input capacitance can be 4.7uF for low current applications, and 10uF for higher currents. If the chip is located far from the batteries, higher capacitor should be used. The output capacitor (C1) has to be selected according to your current and ripple requirements. The range is from 10 to 100uF. Again, a ceramic X5R or X7R should be used, but electrolytic will work as well. To calculate the capacitance, Microchip gives a complicated formula: dV/dt is the product of the ripple voltage divided by the ON time of the inductor mosfet. Since the PWM frequency is fixed at 500KHz, dt be calculated by this formula: dt = 0.000002 x D Typically, a 10uF output capacitor will fit to most applications. a 4.7UF can also be used but may result into an unstable circuit. You should first consult the application note 1311 page 8 before you use a 4.7uF capacitor (check the references tab). Parts Selection  Inductor As always, choosing the proper inductor is vital for the efficiency and the stable operation of an SMPS. An inductor from 2.2 to 10uH can be used. A 4.7uH is recommended to achieve a good balance between inductor size, converter load transient response and noise. You can consult the datasheet at page 16 for the inductor part selection. It is important that the inductors' saturation current must be higher than the PEAK current, and NOT the average current. As inductance decreases, the inductor ripple current increases. Therefore, if size does not matter, use a 10uH instead. Continue reading Page 3 to learn about Output Voltage Set, Efficiency and Power Dissipation.
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