MIC4950 Datasheet, PDF (12/18 Pages) Micrel Semiconductor – Hyper Speed Control 5A Buck Regulator

Micrel, Inc.
Applications Information
The MIC4950 is a highly efficient, 5A synchronous buck
regulator ideally suited for supplying processor core and
I/O voltages from a 5V or 3.3V bus.
Input Capacitor
A 10µF ceramic capacitor or greater should be placed
close to the PVIN pin and PGND pin for bypassing. A
X5R or X7R temperature rating is recommended for the
input capacitor. Take into account C versus bias effect to
estimate the effective capacitance and the input ripple at
the V_IN voltage.
Output Capacitor
The MIC4950 is designed for use with a 10µF 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
on performance, size, and cost. Ceramic capacitors with
X5R or X7R temperature ratings are recommended.
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 MIC4950 is designed for use with a 1µH to 2.2µH
inductor. For faster transient response, a 1µH inductor
will yield the best result. For lower output ripple, a 2.2µH
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. Make sure 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-DC
converters, since core losses increase with at least the
square of the excitation frequency. For more accurate
core loss estimation, 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.
MIC4950
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 in Equation 2:
I_PEAK


^IOUT


V_OUT


1
 V_OUT /V_IN
2f L


Eq. 2
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.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied (see the
Typical Characteristics curves):
Ef
f
iciency%



V_OUT
V_IN


I_OUT
I_IN


 100
Eq. 3
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 to drive the gates on and off at high frequency
and the switching transitions make up the switching
losses.
March 20, 2014
12
Revision 1.1