Micrel, Inc.

MIC2177

April 2008

13

M9999-042108

Application Information

Feedback Resistor Selection (Adjustable Version)

The output voltage is configured by connecting an

external resistive divider to the FB pin as shown in

“MIC2177 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 20kΩ. However, to prevent feedback error due

to input bias current at the FB pin, R2 should not be

greater than 100kΩ. After selecting R2, calculate R1

using the following formula:

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

1

1.245V

V

R2

R1

OUT

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

RMS current rating must be equal or greater than the

maximum RMS input ripple current. A simple, worst-case

formula for calculating this RMS current is:

2

I

I

LOAD(max)

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

Inductor Selection

The inductor must be at least a minimum value in order

for the MIC2177 to change from PWM to skip mode at

the correct value of output current. This minimum value

ensures the inductor ripple current never exceeds

600mA, and is calculated using the following formula:

8.3µ.3µ

V

V

1

V

L

IN(max)

OUT

OUT

MIN

×

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

=

Where:

be 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 inducto rcurrent is:

To maximize efficiency, the inductor’s resistance must

be less than the output switch on-resistance (preferably

50mΩor less).

Output Capacitor Selection

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

This parameter determines a regulator’s output ripple

mentioned in “Inductor Selection,” the maximum value

Therefore, the maximum value of ESR is:

RIPPLE

MAX

V

600mA

ESR

=

Where:

Typically, capacitors in the range of 100µF to 220µF

have ESR less than this maximum value. The output

capacitor can be either a low ESR electrolytic or

tantalum capacitor, but tantalum is a better choice for

compact layout and operation at temperatures below

0°C. The voltage rating of a tantalum capacitor must be

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 and 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 conditions is a 1A diode with a

minimize power dissipation from the diode’s 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

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 – 10nF for the

capacitor and 5kΩ – 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