MIC28513 Datasheet, PDF (22/34 Pages) Microchip Technology – 45V, 4A Synchronous Buck Regulator

MIC28513
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 by:
EQUATION 5-4:
L = ^V----^O----^U----^T----^-----^--^V----^I--^N---^--^M-----^A---^X---^---–-----^V---^O----^U----^T---^
V_{INMAX}  I_{LPP}  f_SW
Where:
f_SW
∆I_L(PP)
Switching Frequency
The peak-to-peak inductor current
ripple; typically 20% of the maximum
output current
In continuous conduction mode, the peak inductor
current is equal to the average output current plus one
half of the peak-to-peak inductor current ripple.
EQUATION 5-5:
I_{LPK} = I_OUT + 0.5  I_{LPP}
The RMS inductor current is used to calculate the I²R
losses in the inductor.
EQUATION 5-6:
I_{LRMS} =
I_O
U
T

M
A
X
2

+
_----I_{--L-----P---P-----}²_-
I²
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance.
The high frequency operation of the MIC28513
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 5-7:
DS20005522A-page 22
EQUATION 5-7:
P_{LCU}
=
I_LR
MS
2


DCR
The resistance of the copper wire, DCR, increases with
the temperature. The value of the winding resistance
used should be at the operating temperature.
EQUATION 5-8:
DCR_{HT} = DCR_{20C}  1 + 0.0042  T_H – T_20C
Where:
T_H
T_20C
DCR_(20C)
Temperature of wire under full load
Ambient temperature
Room temperature winding resistance
(usually specified by the manufacturer)
5.4 Output Capacitor Selection
The type of the output capacitor is usually determined
by its equivalent series resistance (ESR). Voltage and
RMS current capability are also important factors in
selecting an output capacitor. Recommended capacitor
types are ceramic, tantalum, low-ESR aluminum
electrolytic, OS-CON and POSCAP. For high ESR
electrolytic capacitors, ESR is the main cause of the
output ripple. The output capacitor ESR also affects the
control loop from a stability point of view. For a low ESR
ceramic output capacitor, ripple is dominated by the
reactive impedance. The maximum value of ESR is
calculated by Equation 5-9.
EQUATION 5-9:
Where:
ESR_COUT

_----V_{----O----U----T----P----P---}
I _{L  P P }
∆V_OUT(PP) Peak-to-Peak Output Voltage Ripple
∆I_L(PP) Peak-to-Peak Inductor Current Ripple
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated by
Equation 5-10.
EQUATION 5-10:
V_{OUTPP} =


-_C----_O-2--_U--^--_T---^-_---^I--^L_f--_S-^--_WP---^P--_-^----8-_
+
 I_{L  P P }

ESR_COUT²
Where:
D
C_OUT
f_SW
Duty Cycle
Output Capacitance Value
Switching Frequency
 2016 Microchip Technology Inc.