MIC4744 Datasheet, PDF (13/23 Pages) Micrel Semiconductor – 4 MHz Dual 2A Integrated Switch Buck Regulator

Micrel, Inc.
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 MIC4744 is designed for a 4.7µF output capacitor and
a 1µH inductor or a 10µF output capacitor and a 0.47µH
inductor. 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 a 10µF, 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 MIC4744 utilizes type III voltage mode internal
compensation and utilizes an internal zero to compensate
for the double pole roll off of the LC filter. For this reason,
larger output capacitors can create instabilities. In cases
where a 4.7µF output capacitor or a 10µF output capacitor
is not sufficient, other values of capacitance can be used
but the original LC filter pole frequency determined by C_OUT
= 10µF and L = 0.47µH (which is approximately 73.4KHz)
must remain fixed. Increasing C_OUT forces L to decrease
and vice versa.
Inductor Selection
The MIC4744 is designed for use with a 0.47µH inductor
and a 10µF output capacitor or a 1µH inductor and a 4.7µF
output capacitor. 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.
MIC4744
Diode Selection
Since the MIC4744 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 17. Although the range of
resistance for the FB resistors is very wide, R1 is
recommended to be 10K. This minimizes the effect the
parasitic capacitance 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.
March 2009
13
M9999-030209-C