MIC23031_10 Datasheet, PDF (11/18 Pages) Micrel Semiconductor – 4MHz PWM 400mA Buck Regulator with HyperLight Load

Micrel, Inc.
Application Information
The MIC23031 is a high-performance DC/DC step-down
regulator offering a small solution size. Supporting an
output current up to 400mA inside a tiny 1.6mm x 1.6mm
Thin MLF^® package and requiring only three external
components, the MIC23031 meets today’s miniature
portable electronic device needs. Using the HyperLight
Load™ switching scheme, the MIC23031 is able to
maintain high efficiency throughout the entire load range
while providing ultra-fast load transient response. The
following sections provide additional device application
information.
Input Capacitor
A 2.2µF ceramic capacitor or greater should be placed
close to the VIN pin and PGND / GND pin for bypassing.
A TDK C1608X5R0J475K, size 0603, 4.7µF ceramic
capacitor is recommended based upon performance,
size and cost. A X5R or X7R temperature rating is
recommended for the input capacitor. Y5V temperature
rating capacitors, aside from losing most of their
capacitance over temperature, can also become
resistive at high frequencies. This reduces their ability to
filter out high-frequency noise.
Output Capacitor
The MIC23031 was designed for use with a 2.2µF or
greater ceramic output capacitor. Increasing the output
capacitance will lower output ripple and improve load
transient response but could increase solution size or
cost. A low equivalent series resistance (ESR) ceramic
output capacitor such as the TDK C1608X5R0J475K,
size 0603, 4.7µF ceramic capacitor is recommended
based upon performance, size and cost. Both the X7R or
X5R temperature rating capacitors are recommended.
The Y5V and Z5U temperature rating capacitors are not
recommended due to their wide variation in capacitance
over temperature and increased resistance at high
frequencies.
Inductor Selection
When selecting an inductor, it is important to consider
the following factors (not necessarily in the order of
importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
MIC23031
The MIC23031 was designed for use with an inductance
range from 0.47µH to 4.7µH. Typically, a 1µH inductor is
recommended for a balance of transient response,
efficiency and output ripple. For faster transient
response, a 0.47µH inductor will yield the best result. For
lower output ripple, a 4.7µH inductor is recommended.
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 so
that the peak current does not cause the inductor to
saturate. Peak current can be calculated as follows:
IPEAK
⎡
= _{⎢⎢⎣}IOUT
+
VOUT
⎜⎜⎝⎛
1−
VOUT /VIN
2×f ×L
⎟⎟⎠⎞⎥⎥⎦⎤
As shown by the calculation above, the peak inductor
current is inversely proportional to the switching
frequency and the inductance; the lower the switching
frequency or the inductance the higher the peak current.
As input voltage increases, the peak current also
increases.
The size of the inductor depends on the requirements of
the application. Refer to Typical Application Circuit and
Bill of Materials for details.
DC resistance (DCR) is also important. While DCR is
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the Efficiency
Considerations.
Compensation
The MIC23031 is designed to be stable with a 0.47µH to
4.7µH inductor with a minimum of 2.2µF ceramic (X5R)
output capacitor.
Duty Cycle
The typical maximum duty cycle of the MIC23031 is
80%.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency
%
=
⎜⎛
⎜⎝
VOUT
VIN
×IOUT
×IIN
⎟⎞
⎟⎠
×100
October 2010
11
M9999-102210-A