MIC24052 Datasheet, PDF (21/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

5.0 APPLICATION INFORMATION
5.1 Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine
the peak-to-peak inductor ripple current. Generally,
higher inductance values are used with higher input
voltages. Larger peak-to-peak ripple currents will
increase the power dissipation in the inductor and
MOSFETs. Larger output ripple currents will also
require more output capacitance to smooth out the
larger ripple current. Smaller peak-to-peak ripple
currents require a larger inductance value and
therefore a larger and more expensive inductor. A good
compromise between size, loss and cost is to set the
inductor ripple current to be equal to 20% of the
maximum output current. The inductance value is
calculated in Equation 5-1.
EQUATION 5-1:
L = _V----_I--_N----_-^V-_M---^O-_A--^U-_X---^T_---_-^----_f-^-_S-^V--_W--^I--^N-_---^--^M_2---0-^A--%-^X---^---_^–-----_IV--_O--^O--_U-^U--_T--^T-_--_M^----_A----_X---_
Where:
f_SW
20%
V_IN(MAX)
Switching frequency, 600 kHz
Ratio of AC ripple current to DC output
current
Maximum power stage input voltage
The peak-to-peak inductor current ripple is:
EQUATION 5-2:
I _{L  P P }
=
V_{----O----U----T----}_-----_--V_{----I--N-----M-----A---X------–-----}V_{---O----U----T---}
V_{INMAX}  f_SW  L
The peak inductor current is equal to the average
output current plus one half of the peak-to-peak
inductor current ripple.
EQUATION 5-3:
I_{LPK} = I_{OUTMAX} + 0.5  I_{LPP}
MIC24052
The RMS inductor current is used to calculate the I²R
losses in the inductor.
EQUATION 5-4:
I_{LRMS} =
I_OUTM
2
AX
+
_----I_{--L--1---2P---P-----}²_-
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance.
The high-frequency operation of the MIC24052
requires the use of ferrite materials for all but the most
cost sensitive applications. Lower cost iron powder
cores may be used but the increase in core loss will
reduce the efficiency of the power supply. This is
especially noticeable at low output power. The winding
resistance decreases efficiency at the higher output
current levels. The winding resistance must be
minimized although this usually comes at the expense
of a larger inductor. The power dissipated in the
inductor is equal to the sum of the core and copper
losses. At higher output loads, the core losses are
usually insignificant and can be ignored. At lower
output currents, the core losses can be a significant
contributor. Core loss information is usually available
from the magnetics vendor. Copper loss in the inductor
is calculated by Equation 5-5:
EQUATION 5-5:
P_{INDUCTORCU}
=
I_L

R
2
MS

R_WINDING
The resistance of the copper wire, R_WINDING, increases
with the temperature. The value of the winding
resistance used should be at the operating
temperature.
EQUATION 5-6:
R_{WINDINGHT} =
R_{WINDING20C}  1 + 0.0042  T_H – T_20C
Where:
T_H
Temperature of wire under full load
T_20C
Ambient temperature
R_WINDING(20C) Room temperature winding
resistance (usually specified by
the manufacturer)
 2016 Microchip Technology Inc.
DS20005659A-page 21