MIC2103 Datasheet, PDF (25/36 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

Micrel, Inc.
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 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)}

ΔI_L(PP)
12
(Eq. 21)
The power dissipated in the output capacitor is:
P_{DISS(COUT )}  I_{COUT (RMS)} ²  ESR _COUT
(Eq. 22)
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:
ΔV_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)  D  (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 MIC2103 requires two resistors to set the output
voltage as shown in Figure 7:
MIC2103/04
Figure 7. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT  V_FB  (1 _RR21)
(Eq. 26)
where, V_FB = 0.8V. A typical value of R1 can be between
3kΩ and 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small in value, it will decrease the efficiency of the power
supply, especially at light loads. Once R1 is selected, R2
can be calculated using:
R2

V_FB
V_OUT
 R1
 V_FB
(Eq. 27)
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2103/04 g_m amplifier and error comparator is 20mV
to 100mV. However, the output voltage ripple is
generally designed as 1% to 2% of the output voltage.
For a low output voltage, such as a 1V, the output
voltage ripple is only 10mV to 20mV, and the feedback
voltage ripple is less than 20mV. If the feedback voltage
ripple is so small that the g_m amplifier and error
comparator cannot sense it, then the MIC2103/04 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:
1. Enough ripple at the feedback voltage due to the
large ESR of the output capacitors.
As shown in Figure 8a, the converter is stable
without any ripple injection. The feedback voltage
ripple is:
August 2012
25
M9999-080712-A