MIC45208-1 Datasheet, PDF (19/35 Pages) Micrel Semiconductor – 26V 10A DC-to-DC Power Module

Micrel, Inc.
Output Voltage Setting Components
The MIC45208 requires two resistors to set the output
voltage as shown in Figure 8:
Figure 8. Voltage-Divider Configuration
The output voltage is determined by Equation 14:
V_OUT
=
V_FB
× _{}1 +
R_FB1
R_FB2

Eq. 14
Where:
V_FB = 0.8V
A typical value of R_FB1 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 R_FB1 is too
small in value, it will decrease the efficiency of the power
supply, especially at light loads. Once R_FB1 is selected,
R_FB2 can be calculated using Equation 15:
MIC45208
Table 1. VOUT Programming Resistor Look-Up Table
RFB2
VOUT
OPEN
0.8V
40.2kΩ
1.0V
20kΩ
1.2V
11.5kΩ
1.5V
8.06kΩ
1.8V
4.75kΩ
2.5V
3.24kΩ
3.3V
1.91kΩ
5.0V
Ripple Injection
The V_FB ripple required for proper operation of the
MIC45208 g_M amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
too small to provide enough ripple amplitude at the FB
pin and this issue is more visible in lower output voltage
applications. If the feedback voltage ripple is so small that
the g_M amplifier and error comparator cannot sense it,
then the MIC45208 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 two 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 9, the converter is stable without
any ripple injection.
R_FB2
=
V_FB × R_FB1
V_OUT − V_FB
Eq. 15
For fixed R_FB1 = 10kΩ, output voltage can be selected by
R_FB2. Table 1 provides R_FB2 values for some common
output voltages.
Figure 9. Enough Ripple at FB from ESR
February 6, 2015
19
Revision 2.0