MIC2125 Datasheet, PDF (21/34 Pages) Microchip Technology – 28V Synchronous Buck Controllers Featuring Adaptive ON-Time Control

EQUATION 5-6:
t_F
=
Q_{-----S---W------H----S-----}_-----_---R_{---H----S---D-----P----U---L---L----–----U---P-------+-----}R_{---H----S-----G---A----T---E-----}
V_TH
Where:
R_HSD(PULL-UP)
High-Side Gate Driver Pull-Up
Resistance
R_{HSD(PULL-DOWN)} High-Side Gate Driver Pull-Down
Resistance
R_HS(GATE)
High-Side MOSFET Gate
Resistance
Q_SW(HS)
Switching Gate Charge of the
High-Side MOSFET
V_TH
Gate Threshold Voltage
The high-side MOSFET switching losses increase with
the switching frequency and the input voltage. The
low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
5.3 Inductor Selection
Inductance value, saturation, 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. Larger
peak-to-peak ripple current increases the power
dissipation in the inductor and MOSFETs. Larger
output ripple current also requires more output
capacitance to smooth out the larger ripple current.
Smaller peak-to-peak ripple current requires 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 40% of the
maximum output current.
The inductance value is calculated by Equation 5-7.
EQUATION 5-7:
L
=
---------^V----^O---^U----^T----^-----^--^V----^I--^N----^--^M----^A---^X---^---–-----^V----^O---^U----^T---^---------
V_{INMAX}  f_SW  0.4  I_{OUTMAX}
Where:
f_SW
0.4
V_IN(MAX)
Switching Frequency
Ratio of AC Ripple Current to DC Output
Current
Maximum Power Stage Input Voltage
The peak-to-peak inductor current ripple is:
EQUATION 5-8:
I _{L  P P }
=
-^V---^O----^U----^T----^-----^--^V----^I--^N---^--^M-----^A---^X---^---–-----^V---^O----^U----^T---^
V_{INMAX}  f_SW  L
 2015 Microchip Technology Inc.
MIC2125/6
The peak inductor current is equal to the average
output current plus one half of the peak-to-peak
inductor current ripple.
EQUATION 5-9:
I_{LPK} = I_{OUTMAX} + 0.5  I_{LPP}
The saturation current rating is given by:
EQUATION 5-10:
Where:
I_{LSAT}
=
-^--^R----^C---^L----^-----^I--^C----^L---^---–-----^V----^O----^F---^F---^S---^E---^T-
R_{DSON}
R_CL
I_CL
V_OFFSET
R_DS(ON)
Current-Limit Resistor
Current-Limit Source Current
Current-Limit Comparator Offset
On-Resistance of Low-Side Power
MOSFET
The RMS inductor current is used to calculate the I²R
losses in the inductor.
EQUATION 5-11:
I_{LRMS} =
I_O
U
T
M
A
2
X
+
_----I_{--L-----P---P-----}²_-
12
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance.
The high-frequency operation of the MIC2125/6
requires the use of ferrite materials. Lower cost iron
powder cores may be used, but the increase in core
loss reduces 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 significant.
Core loss information is usually available from the
magnetics vendor.
The amount of copper loss in the inductor is calculated
by Equation 5-12:
EQUATION 5-12:
P_{INDUCTORCU}
=
I
LRMS
2


R_WINDING
DS20005459B-page 21