MIC4930 Datasheet, PDF (14/24 Pages) Micrel Semiconductor – Hyper Speed Control 3A Buck Regulator

MIC4930
5.0 APPLICATION INFORMATION
The MIC4930 is a highly efficient, 3A synchronous
buck regulator ideally suited for supplying processor
core and I/O voltages from a 5V or 3.3V bus.
5.1 Input Capacitor
A 10 μF ceramic capacitor or greater should be placed
close to the PV_IN pin and PGND pin for bypassing. A
X5R or X7R temperature rating is recommended for the
input capacitor. Take into account C vs. bias effect in
order to estimate the effective capacitance and the
input ripple at the V_IN voltage.
5.2 Output Capacitor
The MIC4930 is designed for use with a 10 μF or
greater ceramic output capacitor. Increasing the output
capacitance will lower output ripple and improve load
transient response. A low equivalent series resistance
(ESR) ceramic output capacitor is recommended
based upon performance, size, and cost. Ceramic
capacitors with X5R or X7R temperature ratings are
recommended.
5.3 Inductor Selection
When selecting an inductor, it is important to consider
the following factors:
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
• Core losses
The MIC4930 is designed for use with a 1 μH to 2.2 μH
inductor. For faster transient response, a 1 μH inductor
will yield the best result. For lower output ripple, a 2.2
μH inductor is recommended.
Inductor current ratings are generally given in two
methods: permissible DC current, and saturation
current. Permissible DC current can be rated for a 20°C
to 40°C temperature rise. Saturation current can be
rated for a 10% to 30% loss in inductance. Ensure that
the nominal current of the application is well within the
permissible DC current ratings of the inductor, also
depending on the allowed temperature rise. Note that
the inductor permissible DC current rating typically
does not include inductor core losses. These are a very
important contribution to the total inductor core loss
and temperature increase in high-frequency DC-to-DC
converters, since core losses increase with at least the
square of the excitation frequency. For more accurate
core loss estimation, it is recommended to refer to
manufacturers’ datasheets or websites.
When saturation current is specified, make sure that
there is enough design margin, so that the peak current
does not cause the inductor to enter saturation.
DS20005669A-page 14
Also pay attention to the inductor saturation
characteristic in current limit. The inductor should not
heavily saturate even in current limit operation,
otherwise the current might instantaneously run away
and reach potentially destructive levels. Typically,
ferrite-core inductors exhibit an abrupt saturation
characteristic, while powdered-iron or composite
inductors have a soft-saturation characteristic.
Peak current can be calculated by using Equation 5-1.
EQUATION 5-1:
I_PEAK =
I_OUT
+
V _OUT



1-----–---2--V_{----O----Uf----T----}_--L-V_{----I--N--}
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 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 subsection.
5.4 Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied. (See
Typical Performance Curves section).
EQUATION 5-2:
Efficiency%
=


-^V----^O_V----^U-_I--_N-^T----^-----^II---^OI--_N-^U----^T--_
 100
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 RDSON multiplied by
the switch current squared. During the off cycle, the low
side N-channel MOSFET conducts, also dissipating
power. The 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 high frequency and the switching transitions make
up the switching losses.
At the higher currents for which the MIC4930 is
designed, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply
voltages will increase the gate-to-source threshold on
the internal MOSFETs, thereby reducing the internal
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