MIC24045 Datasheet, PDF (30/46 Pages) Microchip Technology – I2C Programmable, 4.5V-19V Input, 5A Step-Down Converter

MIC24045
From the switching frequency, input voltage, output
voltage setting, and load current, the peak-to-peak
inductor current ripple and the peak inductor current
can be calculated as:
EQUATION 7-4:
I_{L_PP}
=
V_O
U
T


1-----–_{-----V--f-OS----U----T-L---}_{---V---I--N--}
EQUATION 7-5:
I_L,PEAK
=
I_OUT
+
_{-----I--L--_---P---P-}
2
The capacitive ripple V_R,C and the ESR ripple
V_R,ESR are given by:
EQUATION 7-6:
V_R,C = _8---------^-f--_S-I--^L_---^_--^P_C---^P--_O---_U----_T-
EQUATION 7-7:
V_R,ESR = ESR  I_{L_PP}
The total peak-to-peak output ripple is then
conservatively estimated as:
EQUATION 7-8:
V_R  V_R,C + V_R,ESR
The output capacitor value and the ESR should be
chosen so that V_R is within specifications. Capacitor
tolerance should be considered for worst-case calcula-
tions. In the case of ceramic output capacitors, factor
into account the decrease of effective capacitance
versus applied DC bias.
The worst-case load transient for output capacitor cal-
culation is an instantaneous 100% to 0% load release
when the inductor current is at its peak value. In this
case, all the energy stored in the inductor is absorbed
by the output capacitor while the converter stops
switching and keeps the low-side FET ON.
The peak output voltage overshoot (V_OUT) happens
when the inductor current has decayed to zero. This
can be calculated with Equation 7-9:
EQUATION 7-9:
V_OUT =
V_{O2 UT}
+
C-----_O^-L---_U^---_T^-
I²
L,PEAK
–
V_OUT
Equation 7-10 calculates the minimum output
capacitance value (C_OUT(MIN)) needed to limit the
output overshoot below V_OUT.
EQUATION 7-10:
2
L  ^IL,PEAK
C_OUT(MIN) = _---_----V----_O----_U---_T----+------V----_O----_U---_T---_---₂---_–-----V----_O2---_U----_T---
The result from the minimum output capacitance value
for load transient is the most stringent requirement
found for capacitor value in most applications. Low
equivalent series resistance (ESR) ceramic output
capacitors, with X5R or X7R temperature characteris-
tics, are recommended.
For low-output voltage applications with demanding
load transient requirements, using a combination of
polarized and ceramic output capacitors may be most
convenient for smallest solution size.
7.6 Input Capacitor Selection
Two main requirements determine the size and charac-
teristics of the input capacitor:
• Steady-state ripple
• RMS current
The buck converter input current is a pulse train with
very fast rising and falling times so low-ESR ceramic
capacitors are recommended for input filtering,
because of their good high-frequency characteristics.
For ideal input filtering (assuming a DC input current
feeding the filtered buck power stage) and by neglect-
ing the capacitor ESR contribution to the input ripple
(typically possible for ceramic input capacitors), the
minimum capacitance value C_IN(MIN) needed for a
given input peak-to-peak ripple voltage V_r,IN can be
estimated as shown in Equation 7-11:
EQUATION 7-11:
where:
C_IN(MIN) = ^I---O---^U----^T-_---^-V----D_r--_,-_I--_N-^---_--^--1-f--_S--^–----D-----^
D = the duty cycle at the given operating point
DS20005568A-page 30
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