MIC2203 Datasheet, PDF (7/13 Pages) Micrel Semiconductor – High Efficiency 1MHz Synchronous Buck Regulator

Micrel, Inc.
MIC2203
Application Information
Input Capacitor
A minimum 1µF ceramic capacitor is recommended
on the V pin for bypassing. X5R or X7R dielectrics
IN
are recommended for the input capacitor. Y5V
dielectrics, aside from losing most of their
capacitance over temperature, also become resis-
tive at high frequencies. This reduces their ability to
filter out high frequency noise.
Output Capacitor
The MIC2203 was designed specifically for the use
of a 2.2µF ceramic output capacitor. Since the
MIC2203 is voltage mode regulator, the control loop
relies on the inductor and output capacitor for
compensation. For this reason, do not use
excessively large output capacitors. The output
capacitor requires either an X7R or X5R dielectric.
Y5V and Z5U dielectric capacitors, aside from the
undesirable effect of their wide variation in
capacitance over temperature, become resistive at
high frequencies. Using Y5V or Z5U capacitors will
cause instability in the MIC2203.
Total output capacitance should not exceed 3µF.
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 MIC2203 is designed for use with a 10µH
inductor.
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% 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 that the peak current will not
saturate the inductor.
The size requirements refer to the area and height
requirements that are necessary to fit a particular
design. Please refer to the inductor dimensions on
their datasheet.
DC resistance is also important. While DCR is
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the “Efficiency
Considerations” below for a more detailed
description.
Bias Capacitor
A small 10nF ceramic capacitor is required to
bypass the bias pin. The use of low ESR ceramics
provides improved filtering for the bias supply.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed.
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 drawn from a battery increases the device’s
operating time, which is critical in hand held devices.
There are two loss terms in switching converters: DC
losses and switching losses. DC losses are simply
2
the power dissipation of I R. Power is dissipated in
the high side switch during the on cycle. Power loss
is equal to the high side MOSFET R_DS(ON) multiplied
2
by the (Switch Current) . 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 to drive the gates on
and off at a constant 1MHz frequency and the
switching transitions make up the switching losses.
Figure 2 shows an efficiency curve. The non-shaded
portion, from 0mA to 100mA, efficiency losses are
dominated by quiescent current losses, gate drive
and transition losses. In this case, lower supply
voltages yield greater efficiency in that they require
less current to drive the MOSFETs and have
reduced input power consumption.
Figure 2. Efficiency Curve
The shaded region, 100mA to 300mA, efficiency
loss is dominated by MOSFET R_DS(ON) and inductor
DC losses. Higher input supply voltages will increase
December 2004
7
M9999-120604
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