MIC28513 Datasheet, PDF (23/34 Pages) Microchip Technology – 45V, 4A Synchronous Buck Regulator

As described in the Theory of Operation subsection of
the Functional Description, the MIC28513 requires at
least 20 mV peak-to-peak ripple at the FB pin for the g_M
amplifier and the error comparator to operate properly.
Also, the ripple on FB pin should be in phase with the
inductor current. Therefore, the output voltage ripple
caused by the output capacitors 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 method should be applied to provide the
enough feedback voltage ripple. Refer to the Ripple
Injection subsection for details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated in Equation 5-11.
EQUATION 5-11:
I_{COUTRMS}
=
_----I_{--L-----P---P----}
12
The power dissipated in the output capacitor is:
EQUATION 5-12:
P_{DISSCOUT}
=
I_COUT
R
2
MS

ESR_COUT
5.5 Input Capacitor Selection
The input capacitor for the power stage input V_IN
should be selected for ripple current rating and voltage
rating. Tantalum input capacitors may fail when
subjected to high inrush currents, caused by turning the
input supply on. A tantalum input capacitor’s voltage
rating should be at least two times the maximum input
voltage to maximize reliability. Aluminum electrolytic,
OS-CON, and multilayer polymer film capacitors can
handle the higher inrush currents without voltage
de-rating. The input voltage ripple will primarily depend
on the input capacitor’s ESR. The peak input current is
equal to the peak inductor current, so:
EQUATION 5-13:
V_IN = I_{LPK}  ESR_CIN
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
MIC28513
EQUATION 5-14:
I_{CINRMS}  I_{OUTMAX}  D  1 – D
The power dissipated in the input capacitor is:
EQUATION 5-15:
P_{DISSCIN}
=
I_C
IN
R
MS
2


ESR_CIN
5.6 Ripple Injection
The V_FB ripple required for proper operation of the
MIC28513’s g_M amplifier and error comparator is
20 mV to 100 mV. However, the output voltage ripple is
generally designed as 1% to 2% of the output voltage.
If the feedback voltage ripple is so small that the g_M
amplifier and error comparator can’t sense it, then the
MIC28513 will lose control and the output voltage is not
regulated. In order to have some amount of V_FB ripple,
a ripple injection method is applied for low output
voltage ripple applications.
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
• Enough ripple at the feedback voltage due to the
large ESR of the output capacitors (Figure 5-4).
The converter is stable without any ripple
injection.
SW
L
MIC28513 _FB
R1
R2
C_OUT
ESR
FIGURE 5-4:
Enough Ripple at FB.
The feedback voltage ripple is:
EQUATION 5-16:
V _{F B  P P }
=
-------^R----2--------
R1 + R2

ESR_COUT

I _{L  P P }
Where:
∆I_L(PP)
Peak-to-Peak Value of the Inductor
Current Ripple
• Inadequate ripple at the feedback voltage due to
the small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin
through a feed-forward capacitor, C_FF in this
situation, as shown in Figure 5-5. The typical C_FF
value is selected by using Equation 5-17.
 2016 Microchip Technology Inc.
DS20005522A-page 23