MIC2125 Datasheet, PDF (23/34 Pages) Microchip Technology – 28V Synchronous Buck Controllers Featuring Adaptive ON-Time Control

5.6 Output Voltage Setting
The MIC2125/26 requires two resistors to set the
output voltage, as shown in Figure 5-3.
g_m AMP
V_REF
R1
FB
R2
MIC2125/6
5.7 Output Overvoltage Limit Setting
The output overvoltage limit should be typically 20%
higher than the nominal output voltage. Set the OVP
limit by connecting a resistor divider from the output to
ground as shown in Figure 5-4.
V_REF
R1
OVP
R2
FIGURE 5-3:
Configuration.
Voltage-Divider
The output voltage is determined by Equation 5-21:
EQUATION 5-21:
Where:
V_OUT
=
V_F
B



1
+
RR_{----12--}
V_FB
0.6V
A typical value of R1 can be in the range of 3 kΩ and
15 kΩ. 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 Equation 5-22.
EQUATION 5-22:
R2 = ----^V----^F---^B----^-----^R----1-----
V_OUT – V_FB
FIGURE 5-4:
Configuration.
OVP Voltage-Divider
Choose R2 in the range of 10 kΩ to 49.9 kΩ and
calculate R1 using Equation 5-23.
EQUATION 5-23:
R1
=
R2
V_{----O----V---P--}
0.6
–
1
5.8 Ripple Injection
The V_FB ripple required for proper operation of the
MIC2125/6 g_m amplifier and comparator is 20 mV to
100 mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For low
output voltages, such as a 1V, the output voltage ripple
is only 10 mV to 20 mV, and the feedback voltage ripple
is less than 20 mV. If the feedback voltage ripple is so
small that the g_m amplifier and comparator cannot
sense it, then the MIC2125/6 loses control and the
output voltage is not regulated. In order to have
sufficient V_FB ripple, a ripple injection method should
be 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 5-5).
The converter is stable without any ripple
injection.
 2015 Microchip Technology Inc.
DS20005459B-page 23