MIC23250 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – 4MHz Dual 400mA Synchronous Buck Regulator with HYPER LIGHT LOAD™

Micrel, Inc.
MIC23250
Applications Information
The MIC23250 is designed for high performance with a
small solution size. With a dual 400mA output inside a tiny
2mm x 2mm Thin MLF^® package and requiring only six
external components, the MIC23250 meets today’s
miniature portable electronic device needs. While small
solution size is one of its advantages, the MIC23250 is big
in performance. Using the Hyper Light Load™ switching
scheme, the MIC23250 is able to maintain high efficiency
throughout the entire load range while providing ultra-fast
load transient response. Even with all the given benefits,
the MIC23250 can be as easy to use as linear regulators.
The following sections provide an over view of
implementing MIC23250 into related applications
Input Capacitor
A minimum of 2.2µF ceramic capacitor should be placed
close to the VIN pin and PGND pin for bypassing. A TDK
C1608X5R0J476K, 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 MIC23250 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 C1608X5R0J476K, size 0603,
4.7µF ceramic capacitor is recommended based upon
performance, size and cost. Either the X7R or X5R
temperature rating capacitors are recommended. The Y5V
and Z5U temperature rating capacitors, aside from the
undesirable effect of their wide variation in capacitance
over temperature, become resistive at high frequencies.
Inductor Selection
Inductor selection will be determined by the following (not
necessarily in the order of importance);
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC23250 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 may be used. For lower output ripple, a
4.7µH 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 of the inductor does not cause it 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 previous calculation, 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 Application Circuit and Bill of
Material 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 MIC23250 is designed to be stable with a 0.47µH to
4.7µH inductor with a minimum of 2.2µF ceramic (X5R)
output capacitor.
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 is
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.
December 2007
11
M9999-121707-A