MIC28304 Datasheet, PDF (28/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
Equation 13 should be used to calculate the input
capacitor. Also it is recommended to keep some margin
on the calculated value:
C_IN
≈
I_OUT(max) × (1− D)
F_SW × dV
Eq. 13
Where:
dV = The input ripple and F_SW is the switching frequency
Output Voltage Setting Components
The MIC28304 requires two resistors to set the output
voltage as shown in Figure 8:
MIC28304
Ripple Injection
The V_FB ripple required for proper operation of the
MIC28304 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 MIC28304 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 table 2
summarizes the ripple injection component values for
ceramic output capacitor.
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 (Figure 9):
Figure 8. Voltage-Divider Configuration
The output voltage is determined by Equation 14:
V_OUT
=
V_FB
× 1+

R1 _
R11 _
Eq. 14
Where:
V_FB = 0.8V
A typical value of R1 used on the standard evaluation
board is 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,
R11 can be calculated using Equation 15:
R11 = ^{VFB × R1}
V_OUT − V_FB
Eq. 15
Figure 9. Enough Ripple at FB
As shown in Figure 10, the converter is stable without
any ripple injection.
Figure 10. Inadequate Ripple at FB
March 25, 2014
28
Revision 1.1