MIC4682_07 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – Precision Current Limit SOIC-8 SuperSwitcher™ Buck Regulator

Micrel, Inc.
Application 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 recomm-
ended 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 capacitors 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 operate 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.
MIC4682
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
requirement can use lower inductor value and output
capacitor. See “Typical Application Circuits” for an
example. A 0.1µF ceramic 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, between 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
incurrent 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 determines at which input and output voltage
the MIC4682 exhibits this condition.
(V_OUT + 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.
June 2007
10
M9999-061507