MIC4782 Datasheet, PDF (13/23 Pages) Micrel Semiconductor – 1.8 MHz Dual 2A Integrated Switch Buck Regulator

Micrel, Inc.
MIC4782
Component Selection
Input Capacitor
A 10µF ceramic is recommended on each VIN pin for
bypassing. X5R or X7R dielectrics are recommended for
the input capacitor. Y5V dielectrics lose most of their
capacitance over temperature and are therefore, not
recommended. Also, tantalum and electrolytic capacitors
alone are not recommended due to their reduced RMS
current handling, reliability, and ESR increases.
An additional 0.1µF is recommended close to the VIN
and PGND pins for high frequency filtering. Smaller case
size capacitors are recommended due to their lower
ESR and ESL. Please refer to layout recommendation
for proper layout of the input capacitor.
Output Capacitor
The MIC4782 is designed to be stable with a 4.7µF
output capacitor. X5R or X7R dielectrics are
recommended for the output capacitor. Y5V dielectrics
lose most of their capacitance over temperature and are
therefore not recommended.
In addition to a 4.7µF or larger value output capacitor, a
small 0.1µF is recommended close to the load for high
frequency filtering. Smaller case size capacitors are
recommended due to there lower equivalent series ESR
and ESL.
The MIC4782 utilizes type III voltage mode internal
compensation and utilizes an internal zero to
compensate for the double pole roll off of the LC filter.
Inductor Selection
The MIC4782 is designed for use with a 1µH inductor.
Proper selection should ensure the inductor can handle
the maximum average and peak currents required by the
load. Maximum current ratings of the inductor are
generally given in two methods; permissible DC current
and saturation current. Permissible DC current can be
rated either for a 40°C temperature rise or a 10% to 20%
loss in inductance. Ensure the inductor selected can
handle the maximum operating current. When saturation
current is specified, make sure that there is enough
margin that the peak current will not saturate the
inductor.
Diode Selection
Since the MIC4782 is non-synchronous, a free-wheeling
diode is required for proper operation. A Schottky diode
is recommended due to the low forward voltage drop
and their fast reverse recovery time. The diode should
be rated to be able to handle the average output current.
Also, the reverse voltage rating of the diode should
exceed the maximum input voltage. The lower the
forward voltage drop of the diode the better the
efficiency. Please refer to the layout recommendation to
minimize switching noise.
Feedback Resistors
The feedback resistor set the output voltage by dividing
down the output and sending it to the feedback pin. The
feedback voltage is 0.6V. Calculating the set output
voltage is as follows;
V_OUT
=
V_FB
⎜⎛
⎝
R1
R2
+
1⎟⎞
⎠
Where R1 is the resistor from V_OUT to FB and R2 is the
resistor from FB-to-GND. The recommended feedback
resistor values for common output voltages are available
in the bill of materials on page 19 of this data sheet.
Although the range of resistance for the FB resistors is
very wide, R1 is recommended to be 10KΩ. This
minimizes the parasitic capacitance effect of the FB
node.
Feedforward Capacitor (C_FF)
A capacitor across the resistor from the output to the
feedback pin (R1) is recommended for most designs.
This capacitor can give a boost to phase margin and
increase the bandwidth for transient response. Also,
large values of feedforward capacitance can slow down
the turn-on characteristics, reducing inrush current. For
maximum phase boost, C_FF can be calculated as follows;
C_FF
=
2π
1
× 200kHz × R1
Large values of feedforward capacitance may introduce
negative FB pin voltage during load shorting, which will
cause latch-off. In that case, a Schottky diode from FB
pin to the ground is recommended.
Bias Filter
A small 10Ω resistor is recommended from the input
supply to the bias pin along with a small 0.1µF ceramic
capacitor from bias-to-ground. This will bypass the high
frequency noise generated by the violent switching of
high currents from reaching the internal reference and
control circuitry. Tantalum and electrolytic capacitors are
not recommended for the bias, these types of capacitors
lose their ability to filter at high frequencies.
Voltage Derating of Ceramic Capacitors
The capacitance of ceramic capacitors drops at high
voltage. Figure 7 shows typical voltage derating curves
of X5R 6.3V ceramic capacitors. At half of the rating
voltage and room temperature, the capacitance of 0603
X5R capacitors can drop about 30%, while the 0805
package only drops by 5%. Therefore, 0805 package
ceramic capacitors are preferred if the application
voltage is close to half of the capacitor rating voltage or
August 2009
13
M9999-081709-D