Micrel, Inc.

MIC2251

February 2011

10

M9999-021511-A

current limit for the MIC2251 is typically 2A. The

saturation current rating of the selected inductor should

be 20-30% higher than the 2A specification for proper

operation.

Figure 2. Efficiency Comparison between Lower

and Higher Inductor Values

Input Capacitor

The boost converter exhibits a triangular current

waveform at its input, so an input capacitor is required to

decouple this waveform and thereby reduce the input

voltage ripple. A 10uF to 22uF ceramic capacitor should

be sufficient for most applications. A minimum input

capacitance of 1uF is recommended. The input capacitor

should be as close as possible to the inductor and the

MIC2251, with short PCB traces for good noise

performance.

Output Capacitor

Output capacitor selection is also a trade-off between

an improved transient response however the size and

cost also increase. X5R and X7R ceramic capacitors are

recommended. For most applications, 2.2uF to 22uF

should be sufficient.

Diode

The MIC2251 requires an external diode for operation.

The diode must be rated for the peak inductor current,

and its reverse voltage rating must be greater than the

output voltage. A Schottky diode is recommended for

lower output voltages due to its lower forward voltage

drop and reverse recovery time. However, at higher

output voltages (>10V), a high speed diode such as

LS4148 can be more efficient as it has the advantage of

considerably lower leakage currents, especially at higher

temperatures. This will greatly improve light load

efficiency when compared to a Schottky diode.

For example: At 70

Input current (Vishay SL04 Schottky) = 2.1mA

Input current (Generic LS4148) = 0.37mA