MIC2128 Datasheet, PDF (22/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

MIC2128
The RMS and saturation current ratings of the selected
inductor should be at least equal to the RMS current
and saturation current calculated in the Equation 5-16
and Equation 5-17.
EQUATION 5-16:
I_{L_RMS} =
 I _LOAD(MAX) ²
+
_---_----I_{--L---_--P---P---}_--²_-
12
Where:
I_LOAD(MAX) = Maximum load current
EQUATION 5-17:
Where:
R_CL
I_CL
R_{DS (ON)}
I_{L_SAT}
=
R_{----C----L----}_-----I_{--C----L----+-----1--5----}m_-----V_--
R _{D S (ON)}
= Current limit resistor
= Current-Limit Source Current (96 µA typical)
= On-resistance of low-side power MOSFET
Maximizing the efficiency requires the proper selection
of core material and minimizing the winding resistance.
Use of ferrite materials is recommended in the higher
switching frequency applications. 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 a significant
contributor. Core loss information is usually available
from the magnetic’s vendor.
The amount of copper loss in the inductor is calculated
by Equation 5-18.
EQUATION 5-18:
P_{INDUCTORCU} = I_{L_RMS}²  R_DCR
5.6 Output Capacitor Selection
The main parameters for selecting the output capacitor
are capacitance value, voltage rating and RMS current
rating. The type of the output capacitor is usually
determined by its equivalent series resistance (ESR).
Recommended capacitor types are ceramic, tantalum,
low-ESR aluminum electrolytic, OS-CON and
POSCAP. The output capacitor ESR also affects the
control loop from a stability point of view. The maximum
value of ESR can be calculated using Equation 5-19.
DS20005620A-page 22
EQUATION 5-19:
Where:
V_{OUT_PP}
I_{L_PP}
ESR  ^-----^V_---^O-_I---_LU--_{_}--^TP---^_P--^P---^P-
= Peak-to-peak output voltage ripple
= Peak-to-peak inductor current ripple
The required output capacitance to meet steady state
output ripple can be calculated using Equation 5-20.
EQUATION 5-20:
C_OUT = _8----------_f--_S---_W--^---_-^I---^L-_-^_--^P-_V--^P--_O---_U----_T---_{_}--_P---_P-
Where:
C_OUT
f_SW
= Output capacitance value
= Switching frequency
As described in Section 4.1 “Theory of Operation”,
the MIC2128 requires at least 20 mV peak-to-peak
ripple at the FB pin to ensure that the gm amplifier and
the comparator behave properly. Also, the output
voltage ripple should be in phase with the inductor
current. Therefore, the output voltage ripple caused by
the output capacitor’s value should be much smaller
than the ripple caused by the output capacitor ESR. If
low-ESR capacitors, such as ceramic capacitors, are
selected as the output capacitors, a ripple injection
circuit should be used to provide the enough
feedback-voltage ripple. Refer to the Section 5.8
“Ripple Injection” for details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for alu-
minum electrolytic, ceramic or OS-CON. The output
capacitor RMS current is calculated in Equation 5-21.
EQUATION 5-21:
I_{C_OUT(RMS)}
=
_-----I_{--L--_---P---P-}
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The power dissipated in the output capacitor is shown
in Equation 5-22.
EQUATION 5-22:
P_{DIS(C_OUT)} = I_{C_OUT(RMS)}²  ESR_{C_OUT}
5.7 Input Capacitor Selection
The input capacitor reduces peak current drawn from
the power supply and reduces noise and voltage ripple
on the input. The input voltage ripple depends on the
input capacitance and ESR. The input capacitance and
ESR values can be calculated using Equation 5-23.
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