MIC23150 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – 4MHz PWM 2A Buck Regulator with HyperLight Load

Micrel Inc.
MIC23150
Application Information
The MIC23150 is a high performance DC-to-DC step
down regulator offering a small solution size. Supporting
an output current up to 2A inside a tiny 2mm x 2mm Thin
MLF^® package, the IC requires only three external
components while meeting today’s miniature portable
electronic device needs. Using the HyperLight Load™
switching scheme, the MIC23150 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 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 MIC23150 is 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 also 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)
The MIC23150 is designed for use with a 0.47µH to
2.2µH inductor. For faster transient response, a 0.47µH
inductor will yield the best result. For lower output ripple,
a 2.2µH 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:
I _PEAK
⎡
= _⎢I_OUT
⎣
+
V_OUT
⎜⎛
⎝
1
− V_OUT /V_IN
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 the 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 MIC23150 is designed to be stable with a 0.47µH to
2.2µH inductor with a minimum of 2.2µF ceramic (X5R)
output capacitor.
Duty Cycle
The typical maximum duty cycle of the MIC23150 is
80%.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞
× 100
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
battery-powered applications. Reduced current draw
from a battery increases the devices operating time and
is critical in hand held devices.
There are two types of losses in switching converters;
DC losses and switching losses. DC losses are simply
the power dissipation of I²R. Power is dissipated in the
high side switch during the on cycle. Power loss is equal
to the high side MOSFET R_DSON multiplied by the Switch
Current squared. During the off cycle, the low side N-
channel MOSFET conducts, also dissipating power.
Device operating current also reduces efficiency. The
product of the quiescent (operating) current and the
supply voltage represents another DC loss. The current
required driving the gates on and off at a constant 4MHz
frequency and the switching transitions make up the
switching losses.
February 2009
11
M9999-082908-A