MIC2245 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – 4MHz PWM Synchronous Buck Regulator with LDO Standby Mode

Micrel, Inc.
Applications Information
The MIC2245 is a 500mA PWM power supply that
utilizes a LOWQ™ light load mode to maximize
battery efficiency in light load conditions. This is
achieved with a LOWQ control pin that when pulled
low, shuts down all the biasing and drive current for
the PWM regulator, drawing only 20µA of operating
current. This allows the output to be regulated
through the LDO output, capable of providing 60mA
of output current. This method has the advantage of
producing a clean, low current, ultra-low noise
output in LOWQ™ mode. During LOWQ™ mode,
the SW node becomes high impedance, blocking
current flow. Other methods of reducing quiescent
current, such as pulse frequency modulation (PFM),
or bursting techniques, create large amplitude, low
frequency ripple voltages that can be detrimental to
system operation.
When more than 60mA is required, the LOWQ pin
can be forced high, causing the MIC2245 to enter
PWM mode. In this case, the LDO output makes a
"hand-off" to the PWM regulator with virtually no
variation in output voltage. The LDO output then
turns off allowing up to 500mA of current to be
efficiently supplied through the PWM output to the
load.
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. Please refer to layout
recommendation section of data sheet for proper
layout of the input capacitor.
MIC2245
Output Capacitor
The MIC2245 is optimized for a 4.7µF output
capacitor. The MIC2245 utilizes type III internal
compensation and utilizes an internal high frequency
zero 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 4.7µF, 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 MIC2245 is designed for use with a 1.0µH
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 that 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:
I_PK
= I_OUT
+
V_{OUT ⎜⎜⎝⎛}1−
V_OUT
V_IN
2×f ×L
⎟⎟⎠⎞
January 2006
11
M9999-012406
www.micrel.com