MIC4682 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – PRECISION CURRENT LIMIT SO-8 SUPERSWITCHER BUCK REGULATOR

MIC4682
Applications Information
Output Voltage
The output voltage of the MIC4682 is determined by using the
following formulas:
V_OUT
=
V_FB

R1
R2
+ 1^{}
R2= ^R1


V_OUT
V_FB


–
1
V_FB = 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(10µF/50V),
tantalum capacitors are good values to use for the conditions
listed in the SOA typical tables. A 0.1µF 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.
Micrel
Inductor and Output Capacitor
A 68µH 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 47µH 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 0.1µF 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_ISET." In addition
to the R_ISET, a resistor, ranging from 10MΩ to 15MΩ, be-
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.
( ) VOUT + 1.4 > 50%
V_IN
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.
October 2003
10
M0334-102203