MIC24053 Datasheet, PDF (20/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

MIC24053
5.0 APPLICATION INFORMATION
5.1 Inductor Selection
Selecting the output inductor requires values for
inductance, peak, and RMS currents. 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 increase
the power dissipation in the inductor and MOSFETs.
Larger output ripple currents 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 by Equation 5-1:
EQUATION 5-1:
L = _V-----_I--_N----^V_--_m--^O--_a--^Ux---_-^T--_--^----_f-^--_S^V--_W---^I--^N-_---^--^m_2---0-^a--^x_%---^---–-_---^V--_I---_OO---_UU----_TT---_-^-_m----_a---_x--_
Where:
f_SW = Switching Frequency, 600 kHz
20% = Ratio of AC ripple current to DC
output current
V_IN(max) = Maximum power stage input voltage
The peak-to-peak inductor current ripple is:
EQUATION 5-2:
I_{L pp}
=
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 _{K  pk } = I _{OUT max} + 0.5  I _{L pp}
The RMS inductor current is used to calculate the I²R
losses in the inductor.
EQUATION 5-4:
I_{LRMS} =
I
OUT
2
max
+
_-----I_{--L--1---2P----P-----}²
The proper selection of core material and minimizing
the winding resistance is required to maximize
efficiency. The high-frequency operation of the
MIC24053 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
2
LRMS

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:
P_{WINDINGHt} =
R_{WINDING20C}  1 + 0.0042  T _H – T _20C 
Where:
T_H = Temperature of wire under full
load
T_20°C = Ambient Temperature
R_{WINDING(20°C)} = Room temperature winding
resistance (usually specified by
the manufacturer)
5.2 Output Capacitor Selection
The type of the output capacitor is usually determined
by its equivalent series resistance (ESR). Voltage and
RMS current capability are two other important factors
for selecting the output capacitor. Recommended
capacitor types are ceramic, low-ESR aluminum
electrolytic, OS-CON, and POSCAP. The output
capacitor’s ESR is usually the main cause of the output
ripple. It also affects the stability of the control loop.
DS20005668A-page 20
 2015 Microchip Technology Inc.