MIC4682

Micrel

October 2003

10

M0334-102203

Applications Information

Output Voltage

The output voltage of the MIC4682 is determined by using the

following formulas:

VV

R1

R2

1

OUT

FB

=+









R2

R1

V

V

OUT

FB

=









– 1

V

1.23V

For most applications, a 3.01k resistor is recommended for

R1 and R2 can be calculated.

Input Capacitor

Low ESR (Equivalent Series Resistance) capacitor should be

used for the input capacitor of the MIC4862 to minimize the

input ripple voltage. Selection of the capacitor value will

depend on the input voltage range, inductor value, and the

load. Two Vishay Sprague 593D106X9050D2T(10

µF/50V),

tantalum capacitors are good values to use for the conditions

listed in the SOA typical tables. A 0.1

µF ceramic capacitor is

recommended in parallel with the tantalum capacitors to filter

the high frequency ripple. The ceramic capacitor should be

placed close to the IN pin of the MIC4682 for optimum result.

For applications that are cost sensitive, electrolytic capaci-

tors can be used but the input ripple voltage will be higher.

Diode

A Schottky diode is recommended for the output diode. Most

of the application circuits on this data sheet specify the Diode

Inc. B340A or Micro Commercial SS34A surface mount

Schottky diode. Both diodes have forward current of 3A and

low forward voltage drop. These diodes are chosen to oper-

ate at wide input voltage range and at maximum output

current. For lower output current and lower input voltage

applications, a smaller Schottky diode such as B240A or

equivalence can be used.

Inductor and Output Capacitor

A 68

µH inductor and a 220µF tantalum output capacitor are

chosen because of their stability over the input voltage range

with maximum output current listed in the SOA typical tables.

The Sumida CDRH127-680 and Vishay Sprague

593D106X9050D2T are recommended. See “Bode Plots” for

additional information. With the same conditions, a lower

value inductor and a higher output capacitor can be used. The

disadvantages for this combination are that the output ripple

voltage will be higher and the output capacitor’s package size

will be bigger. For example, a 47

µH inductor and 330µF

output capacitor are good combination. Another option is to

use a higher value inductor and a lower output capacitor. The

advantages of this combination are that the switch peak

current and the output ripple voltage will be lower. The

disadvantage is that the inductor’s package size will be

bigger. Applications that have lower output current require-

ment can use lower inductor value and output capacitor. See

“Typical Application Circuits” for an example. A 0.1

µF ce-

ramic capacitor is recommended in parallel with the tantalum

output capacitor to reduce the high frequency ripple.

Current Limit Set Resistor

An external resistor connects between the ISET pin and

ground to control the current limit of the MIC4682 ranging

from 400mA to 2A. For resistor value selections, see the

“Typical Characteristics: Current Limit vs. R

to the R

tween the ISET and IN pin is recommended for current limit

accuracy over the input voltage range.

When the MIC4682 is in current limit, the regulator is in

current mode. If the duty cycle is equal or greater than 50%,

the regulator is in the sub-harmonic region. This lowers the

average current limit. The below simplified equation deter-

mines at which input and output voltage the MIC4682 exhibits

this condition.

V1

V

OUT

IN

+

()

>

.

%

4

50

Do not short or float the ISET pin. Shorting the ISET pin will

set a peak current limit greater than 2.1A. Floating the ISET

pin will exhibit unstable conditions. To disable the current limit

circuitry, the voltage at the ISET pin has to be between 2V

and 7V.