MIC26903ZA_14 Datasheet, PDF (18/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Application Information
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 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 equal to 20% of the maximum
output current. The inductance value is calculated in
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
MIC26903-ZA
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC26903-ZA 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.
2
P_INDUCTOR(CU) = I_{L(RMS )} × 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.
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. The output capacitor ESR also affects the
stability of the control loop.
July 22, 2014
18
Revision 1.1