October 2005

9

MIC2198

MIC2198

Micrel, Inc.

Applications Information

The following applications information includes component

selection and design guidelines.

Inductor Selection

Values for inductance, peak, and RMS currents are required

to select the output inductor. The input and output voltages

and the inductance value determine the peak to peak induc-

tor ripple current. Generally, higher inductance values are

used with higher input voltages. Larger peak to peak ripple

currents will increase the power dissipation in the inductor

and MOSFETs. Larger output ripple currents will also require

more output capacitance to smooth out the larger ripple cur-

rent. Smaller peak to peak ripple currents require a larger

inductance value and therefore a larger and more expensive

inductor. A good compromise between size, loss and cost is

to set the inductor ripple current to be equal to 20% of the

maximum output current.

The inductance value is calculated by the equation below.

L

=

where:

0.2 = ratio of AC ripple current to DC output current

The peak-to-peak inductor current (AC ripple current) is:

I

Thepeakinductorcurrentisequaltotheaverageoutputcurrent

plus one half of the peak to peak inductor ripple current.

I

I

0.5 I

PK

OUT(max)

PP

=

+

×

in the inductor.

I

I

1 1

3

I

I

INDUCTOR(rms)

OUT(max)

P

OUT(max)

2

=

×

+













Maximizing efficiency requires the proper selection of core

material and minimizing the winding resistance. The high

frequency operation of the MIC2198 requires the use of fer-

rite materials for all but the most cost sensitive applications.

Lower cost iron powder cores may be used but the increase

in core loss will reduce the efficiency of the power supply.

This is especially noticeable at low output power. The winding

resistance decreases efficiency at the higher output current

levels. The winding resistance must be minimized although

this usually comes at the expense of a larger inductor.

The power dissipated in the inductor is equal to the sum

of the core and copper losses. At higher output loads, the

core losses are usually insignificant and can be ignored. At

lower output currents, the core losses can be a significant

contributor. Core loss information is usually available from

the magnetics vendor.

Copper loss in the inductor is calculated by the equation

below:

temperature.The value of the winding resistance used should

be at the operating temperature.













where:

resistance (usually specified by the manufacturer)

Current-Sense Resistor Selection

Low inductance power resistors, such as metal film resistors

should be used. Most resistor manufacturers make low induc-

tance resistors with low temperature coefficients, designed

specifically for current-sense applications. Both resistance

and power dissipation must be calculated before the resis-

maximum output current and the maximum threshold level.

The power dissipated is based on the maximum peak output

current at the minimum overcurrent threshold limit.

R

55mV

I

SENSE

OUT(max)

=

The maximum overcurrent threshold is:

I

95mV

R

OVERCURRENT(max)

CS

=

The maximum power dissipated in the sense resistor is:

MOSFET Selection

External N-Channel logic-level power MOSFETs must be

used for the high- and low-side switches. The MOSFET

gate-to-source drive voltage of the MIC2198 is regulated by

It is important to note the on-resistance of a MOSFET in-

creases with increasing temperature. A 75°C rise in junction

temperature will increase the channel resistance of the MOS-

FET by 50% to 75% of the resistance specified at 25°C. This

change in resistance must be accounted for when calculating

MOSFET power dissipation.

Total gate charge is the charge required to turn the MOSFET

circuit. At 500kHz switching frequency, the gate charge can

be a significant source of power dissipation in the MIC2198.

At low output load this power dissipation is noticeable as a

reduction in efficiency. The average current required to drive

the high-side MOSFET is:

I

Q

f

G[high-side](avg)

G

S

=

×