MIC2285A Datasheet, PDF (11/14 Pages) Micrel Semiconductor – 8MHz PWM Synchronous Buck Regulator with LDO Standby Mode

Micrel, Inc.
Applications Information
The MIC2285A 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 18µA 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 MIC2285A 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 600mA 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.
MIC2285A
Output Capacitor
The MIC2285A is optimized for a 10µF output
capacitor. A larger value can be used to improve
transient response. The MIC2285A 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 10µF, a small 10nF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to their
lower ESR and ESL.
Inductor Selection
The MIC2285A is designed for use with a 0.47µH
inductor. Proper selection should ensure that 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
⎟⎟⎠⎞
August 2007
11
M9999-083107-B