Capacitor selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in

series with the supply source impedance and lowers overall efficiency. This capacitor has to

supply the relatively high peak current to the coil and smooth the current ripple on the input

supply. A minimum value of 4.7 uF is acceptable if the input source is close to the device, but

higher values will improve performance at lower input voltages, especially when the source

impedance is high. The input capacitor should be placed as close as possible to the IC.

Inductor selection

Recommended inductor values for the FC1360 are in the range 33 uH to 100 uH. Higher values

of inductance are recommended at higher supply voltages in order to minimize errors due to

switching delays, which result in increased ripple and lower efficiency. Higher values of

inductance also result in a smaller change in output current over the supply voltage range. The

inductor should be mounted as close to the device as possible with low resistance connections

to the LX and VIN pins.

Note that, in practice, the duty cycle and operating frequency will deviate from the calculated

values due to dynamic switching delays, switch rise/fall times and losses in the external

components.

Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal

supply voltage. This helps to equalize the undershoot and overshoot and improves temperature

stability of the output current.

Diode selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance

Schottky diode with low reverse leakage at the maximum operating voltage and temperature.

It is important to select parts with a peak current rating above the peak coil current and a

continuous current rating higher than the maximum output load current. It is very important to

consider the reverse leakage of the diode when operating above 85°C. Excess leakage will

increase the power dissipation in the device and if close to the load may create a thermal

runaway condition.

FC1360

2009. 07. 10

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Revision No : 0