Micrel, Inc. MIC2224
September 2006 11
M9999-091106
Application Information
The MIC2224 is a 600mA PWM power supply that
utilizes a dynamically adjustable output voltage for
powering RF power amplifiers. By dynamically adjusting
the output power as necessary, battery life can be
dramatically improved in battery powered RF
applications. For instances where high power is
required, the MIC2224 also has a bypass mode. Bypass
mode is enabled by driving the DAC pin above 1.2V.
This bypasses the input supply directly to the output
through a highly efficient, short circuit protected, 40m P
channel MOSFET.
Input Capacitor
A minimum 1µF ceramic is recommended on the 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. A minimum 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.
Output Capacitor
The MIC2224 is optimized for a 1µF ceramic output
capacitor. The MIC2224 utilizes type III internal
compensation and utilizes internal high frequency zeros
to compensate for the double pole roll off of the LC filter.
For this reason, larger output capacitors can create
instabilities. 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 1µF, a small 10nF is
recommended close to the load for high frequency
filtering. Smaller case size capacitors are recommended
due to there lower ESR and ESL.
Inductor Selection
The MIC2224 is designed for use with a 4.7µH 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. Peak inductor current can be calculated as
follows:
Lf2
V
V
1V
II
IN
OUT
OUT
OUTPK
××
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
+=
