MIC2199

Micrel

MIC2199

10

November 2004

Applications Information

Following applications information includes component se-

lection 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

current. 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

V(V

V

)

V

f

0.2 I

OUT

IN(max)

OUT

IN(max)

S

OUT(max)

=

×

−

××

×

where:

f

0.2 = ratio of AC ripple current to DC output current

V

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

I

V(V

V

)

Vf

L

PP

OUT

IN(max)

OUT

IN(max)

S

=

×

−

××

The peak inductor current is equal to the average output

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

current.

I

I

0.5 I

PK

OUT(max)

PP

=+

×

in the inductor.

II

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 MIC2199 requires the use of ferrite

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 contribu-

tor. Core loss information is usually available from the mag-

netics vendor.

Copper loss in the inductor is calculated by the equation

below:

PI

R

INDUCTORCu

INDUCTOR(rms)

WINDING

2

=×

The resistance of the copper wire, R

temperature. The value of the winding resistance used should

be at the operating temperature.

R

R

1 0.0042 (T

T

)

WINDING(hot)

WINDING(20 C)

HOT

20 C

=×

+

×

−

()

°°

where:

T

T

R

(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

inductance resistors with low temperature coefficients, de-

signed specifically for current-sense applications. Both resis-

tance and power dissipation must be calculated before the

resistor is selected. The value of R

the 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:

PI

R

D(R

)

OVERCURRENT(max)

2

CS

SENSE

=×

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 MIC2199 is regulated by an

internal 5V V

operation is specified at V

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

increases with increasing temperature. A 75°C rise in junc-

tion temperature will increase the channel resistance of the

MOSFET 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

on and off under specified operating conditions (V

V

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

be a significant source of power dissipation in the MIC2199.

At low output load this power dissipation is noticeable as a