June 1998

11

MIC2178

MIC2178

Micrel

Application Information

Feedback Resistor Selection (Adjustable Version)

The output voltage is programmed by connecting an external

resistive divider to the FB pin as shown in “MIC2178 Block

Diagram.”

The ratio of R1 to R2 determines the output

voltage. To optimize efficiency during low output current

operation, R2 should not be less than 20k

Ω. However, to

prevent feedback error due to input bias current at the FB pin,

R2 should not be greater than 100k

Ω. After selecting R2,

calculate R1 with the following formula:

R1 = R2

V

1.245V

OUT

















– 1

Input Capacitor Selection

The input capacitor is selected for its RMS current and

voltage rating and should be a low ESR (equivalent series

resistance) electrolytic or tantalum capacitor. As a rule of

thumb, the voltage rating for a tantalum capacitor should be

twice the value of V

should be 40% higher than V

be equal or greater than the maximum RMS input ripple

current. A simple, worst case formula for calculating this

RMS current is:

I

=

I

RMS(max)

LOAD(max)

2

Tantalum capacitors are a better choice for applications that

require the most compact layout or operation below 0

°C. The

input capacitor must be located very close to the VIN pin

(within 0.2in, 5mm). Also, place a 0.1

µF ceramic bypass

capacitor as close as possible to VIN.

Inductor Selection

The MIC2178 is a current-mode controller with internal slope

compensation. As a result, the inductor must be at least a

minimum value to prevent subharmonic oscillations. This

minimum value is calculated by the following formula:

L

= V

3.0 H/V

MIN

In general, a value at least 20% greater than L

selected because inductor values have a tolerance of

±20%.

Two other parameters to consider in selecting an inductor are

winding resistance and peak current rating. The inductor

must have a peak current rating equal or greater than the

peak inductor current. Otherwise, the inductor may saturate,

causing excessive current in the output switch. Also, the

inductor’s core loss may increase significantly. Both of these

effects will degrade efficiency. The formula for peak inductor

current is:

I

= I

I

2

L(peak)

LOAD(max)

L(max)

+

∆

Where:

∆I

= V

1

V

V

5s

L

L(max)

OUT

OUT

IN(max)

−











 ×

µ

To maximize efficiency, the inductor’s resistance must be

less than the output switch on-resistance (preferably,

50m

Ω or less).

Output Capacitor Selection

Select an output capacitor that has a low value of ESR. This

parameter determines a regulator’s output ripple voltage

(V

must be equal or less than a maximum value calculated for a

specified V

age) and

∆I

ESR

=

V

I

MAX

RIPPLE

L(max)

∆

Typically, capacitors in the range of 100 to 220

µF have ESR

less than this maximum value. The output capacitor can be

a low ESR electrolytic or tantalum capacitor, but tantalum is

a better choice for compact layout and operation at tempera-

tures below 0

°C. The voltage rating of a tantalum capacitor

must be 2

× V

must be 1.4

× V

Output Diode Selection

In PWM operation, inductor current flows through the output

diode approximately 50ns during the dead time when one

output MOSFET turns off the other turns on. In skip mode, the

inductor current flows through the diode during the entire P-

channel off time. The correct diode for both of these condi-

tions is a 1A diode with a reverse voltage rating greater than

V

It must be a Schottky or ultrafast-recovery diode

(t

reverse-recovery charge.

Compensation

Compensation is provided by connecting a series RC load to

the COMP pin. This creates a pole-zero pair in the regulator

control loop, allowing the regulator to remain stable with

enough low frequency loop-gain for good load and line

regulation. At higher frequencies, the pole-zero reduces

loop-gain to a level referred to as the mid-band gain. The mid-

band gain is low enough so that the loop gain crosses 0db

with sufficient phase margin. Typical values for the RC load

are 4.7nF to 10nF for the capacitor and 5k

Ω to 20kΩ for the

resistor.

Printed Circuit Board Layout

A well designed PC board will prevent switching noise and

ground bounce from interfering with the operation of the

MIC2178. A good design takes into consideration compo-

nent placement and routing of power traces.

The first thing to consider is the locations of the input

capacitor, inductor, output diode, and output capacitor. The

input capacitor must be placed very close to the VIN pin, the

inductor and output diode very close to the SW pin, and the

output capacitor near the inductor. These components pass

large high-frequency current pulses, so they must use short,

wide power traces. In addition, their ground pins and PGND

are connected to a ground plane that is nearest the power

supply ground bus.