MIC2207_10 Datasheet, PDF (11/18 Pages) Micrel Semiconductor – 3mm x 3mm 2MHz 3A PWM Buck Regulator

Micrel, Inc.
Also, be aware that there are additional core losses
associated with switching current in an inductor. Since
most inductor manufacturers do not give data on the
type of material used, approximating core losses
becomes very difficult, so verify inductor temperature
rise.
Switching losses occur twice each cycle, when the
switch turns on and when the switch turns off. This is
caused by a non-ideal world where switching transitions
are not instantaneous, and neither are currents. Figure 6
demonstrates (Or exaggerates…) how switching losses
due to the transitions dissipate power in the switch.
Figure 6. Switching Transition Losses
Normally, when the switch is on, the voltage across the
switch is low (virtually zero) and the current through the
switch is high. This equates to low power dissipation.
When the switch is off, voltage across the switch is high
and the current is zero, again with power dissipation
being low. During the transitions, the voltage across the
switch (V_S-D) and the current through the switch (I_S-D) are
at midpoint of their excursions and cause the transition
to be the highest instantaneous power point. During
continuous mode, these losses are the highest. Also,
with higher load currents, these losses are higher. For
discontinuous operation, the transition losses only occur
during the “off” transition since the “on” transitions there
is no current flow through the inductor.
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 because of 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 recommendations
for proper layout of the input capacitor.
MIC2207
Output Capacitor
The MIC2207 is designed for 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, a small 0.1µF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to their
lower equivalent series ESR and ESL.
The MIC2207 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 is
not sufficient, the MIC2208 offers the ability to externally
control the compensation, allowing for a wide range of
output capacitor types and values.
Inductor Selection
The MIC2207 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 MIC2207 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 recommendations 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 1.0V. Calculating the set output
voltage is as follows;
V_OUT
=
V_FB
⎜⎛
⎝
R1
R2
+
1⎟⎞
⎠
Where R1 is the resistor from VOUT to FB and R2 is the
resistor from FB to GND. The recommended feedback
resistor values for common output voltages are available
April 2010
11
M9999-041910