Micrel, Inc.

MIC2298

October 2007

10

M9999-101707-B

Component Selection

Inductor

Inductor selection is a balance between efficiency,

stability, cost, size, and rated current. For most

applications, a 4.7µH is the recommended inductor

value. It is usually a good balance between these

considerations. Larger inductance values reduce the

peak-to-peak ripple current, affecting efficiency. This has

the effect of reducing both the DC losses and the

transition losses. There is also a secondary effect of an

inductor’s DC resistance (DCR). The DCR of an inductor

will be higher for more inductance in the same package

size. This is due to the longer windings required for an

increase in inductance. Since the majority of input

current (minus the MIC2298 operating current) is passed

through the inductor, higher DCR inductors will reduce

efficiency. To maintain stability, increasing inductor size

will have to be met with an increase in output

capacitance. This is due to the unavoidable “right half

plane zero” effect for the continuous current boost

converter topology. The frequency at which the right half

plane zero occurs can be calculated as follows:

π

2

2

⋅

⋅

⋅

=

OUT

OUT

IN

rhpz

I

L

V

V

f

The right half plane zero has the undesirable effect of

increasing gain, while decreasing phase. This requires

that the loop gain is rolled off before this has significant

effect on the total loop response. This can be

accomplished by either reducing inductance (increasing

RHPZ frequency) or increasing the output capacitor

value (decreasing loop gain).

Output Capacitor

Output capacitor selection is also a trade-off between

performance,

size,

and

cost.

Increasing

output

capacitance will lead to an improved transient response,

but also an increase in size and cost. X5R or X7R

dielectric ceramic capacitors are recommended for

designs with the MIC2298.

The output capacitor sets the frequency of the dominant

pole and zero in the power stage. The zero is given by:

π

2

1

⋅

⋅

=

esr

z

R

C

f

For ceramic capacitors, the ESR is very small. This puts

the zero at a very high frequency where it can be

ignored. Fortunately, the MIC2298 is current mode in

operation which reduces the need for this output

capacitor zero when compensating the feedback loop.

The frequency of the pole caused by the output

capacitor is given by.

π

⋅

⋅

⋅

=

2

OUT

OUT

p

V

C

I

f

Diode Selection

The MIC2298 requires an external diode for operation. A

Schottky diode is recommended for most applications

due to their lower forward voltage drop and reverse

recovery time. Ensure the diode selected can deliver the

peak inductor current and the maximum reverse voltage

is rated greater than the output voltage.

Input capacitor

A minimum 1µF ceramic capacitor with an X5R or X7R

dielectric is recommended for designing with the

MIC2298. Increasing input capacitance will improve

performance and greater noise immunity on the source.

The input capacitor should be as close as possible to the

inductor and the MIC2298, with short traces for good

noise performance.

The MIC2298 utilizes a feedback pin to compare the

LED current to an internal reference. The LED current is

adjusted by selecting the appropriate feedback resistor

value. The desired output current can be calculated as

follows:

R

V

2

.

0

=

Compensation

The comp pin is connected to the output of the voltage

error amplifier. The voltage error amplifier is a

transconductance amplifier. Adding a series RC-to-

ground adds a zero at:

4

2

2

1

C

R

f

zero

π

=

The resistor typically ranges from 1kΩ to 50kΩ. The

capacitor typically ranges from 1nF to 100nF.

Adding an optional capacitor from comp pin-to-ground

adds a pole at approximately

3

2

2

1

C

R

f

pole

π

=

This capacitor typically ranges from 100pF to 10nF.

Generally, an RC to ground is all that is needed. The RC

should be placed as close as possible to the

compensation pin. The capacitor should be a ceramic

with a X5R, X7R, or COG dielectric. Refer to the

MIC2298 evaluation board document for component

location.