MIC24053 Datasheet, PDF (17/30 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

Micrel, Inc.
Application Information
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 3:
L = ^{VOUT × (VIN(max) − VOUT )}
V_IN(max) × f_sw × 20% × I_OUT(max)
Eq. 3
where:
f_SW = switching frequency, 600kHz
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:
∆I_L(pp)
=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_sw × L
Eq. 4
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor current
ripple.
I_L(pk) =I_OUT(max) + 0.5 × ΔI_L(pp)
Eq. 5
The RMS inductor current is used to calculate the I²R
losses in the inductor.
I_L(RMS) =
I_OUT(max) ²
+
ΔI_L(PP) ²
12
Eq.6
MIC24053
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 7:
P_INDUCTOR(Cu) = I_L(RMS)2 × R_WINDING
Eq. 7
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.
P_WINDING(Ht) = R_{WINDING(20°C)} × (1 + 0.0042 × (T_H – T_20°C))
Eq. 8
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)
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.
November 2012
17
M9999-110712-A