MIC2208 Datasheet, PDF (12/19 Pages) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
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 MIC2208 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 is available in the bill of materials on page
x. 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.
Bias filter
A small 10 Ohm resistor is recommended from the
input supply to the bias pin along with a small 0.1uF
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.
MIC2208
Dominant
Pole
Zero
LC
Frequency
20dB/Decade
Compensation
The MIC2208 utilizes voltage mode compensation
and has the error amplifier pin (COMP) pinned out to
allow it to be compensated using external
components. This allows the MIC2208 to be stable
with a wide range of inductor and capacitor values.
TYPE II compensation
Type II compensation can be expressed as pole-
zero-pole. In our case, a dominant pole (R1 and C3)
followed by a zero (C3 and R4) , allowing the final
pole to be provided by the output inductor and
output capacitor (L and C_OUT). This mode of
compensation works well when using higher ESR
output capacitors, such as tantalum and electrolytic
dielectrics. The ESR of the capacitor, along with the
output capacitance provides a zero (C_OUT and ESR)
that negates one of the two poles created by the
inductor-output capacitor filter. This allows the gain
to cross the 0dB point with a -1 slope
(-20dB/decade).
September 2005
12
M9999-092905
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