MIC2101_13 Datasheet, PDF (27/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 20:
ǻ9_{OUT(pp)
  ¨¨©§}
C
Ç»,_L(PP)
OUT ^{u f}SW
u 8 ^{¸¸¹·2}

Ç»,_L(PP) u ESR _COUT
2
Eq. 20
where:
D = duty cycle
C_OUT = output capacitance value
f_sw = switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC2101/02 requires at least
20mV peak-to-peak ripple at the FB pin to make the g_m
amplifier and the error 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 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. Please refer to the “Ripple
Injection” subsection for more 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 21:
I_{COUT (RMS)}
Ç»,_L(PP)
12
Eq. 21
The power dissipated in the output capacitor is:
P_{DISS(COUT )} I_{COUT (RMS)2} u ESR _COUT
Eq. 22
MIC2101/02
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:
ǻV_IN = I_L(pk) × ESR_CIN
Eq. 23
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:
I_CIN(RMS) | I_OUT(max) u D u (1 D)
Eq. 24
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × ESR_CIN
Eq. 25
Voltage Setting Components
The MIC2101/02 requires two resistors to set the output
voltage as shown in Figure 7:
Figure 7. Voltage-Divider Configuration
November 13, 2013
27
Revision 2.0