MIC45116 Datasheet, PDF (24/42 Pages) Microchip Technology – 20V/6A DC/DC Power Module

MIC45116
5.3 Output Voltage Setting
Components
The MIC45116 requires two resistors to set the output
voltage as shown in Figure 5-1.
g_M AMP
V_REF
R_FB1
FB
R_FB2
FIGURE 5-1:
Configuration.
Voltage Divider
The output voltage is determined by Equation 5-8:
EQUATION 5-8:
Where:
V_FB
V_OUT
=
V_FB



1
+
R^{-R---FF----BB---12-}
0.8V
A typical value of R_FB1 used on the standard evaluation
board is 10 kΩ. If R_FB1 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 5-9:
EQUATION 5-9:
R_FB2 = -_VV---_O-^F---^BU----_T^----–-^R---_V-^F---_FB----¹_B-
For fixed R_FB1 = 10 kΩ, output voltage can be selected
by R_FB2. Table 5-1 provides R_FB2 values for some
common output voltages.
TABLE 5-1: V_OUT PROGRAMMING
RESISTOR LOOK-UP
R_FB2
OPEN
40.2 kΩ
20 kΩ
11.5 kΩ
8.06 kΩ
4.75 kΩ
3.24 kΩ
1.91 kΩ
V_OUT
0.8V
1.0V
1.2V
1.5V
1.8V
2.5V
3.3V
5.0V
5.4 Ripple Injection
The V_FB ripple required for proper operation of the
MIC45116 g_m amplifier and error comparator is 20 mV
to 100 mV. 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 MIC45116 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:
• Enough ripple at the feedback voltage due to the
large ESR of the output capacitors (Figure 5-2).
The converter is stable without any ripple
injection.
V_OUT
MIC45116
FB
P_GND
R_FB1
C_OUT
R_FB2
ESR
FIGURE 5-2:
Enough Ripple at FB.
The feedback voltage ripple is:
EQUATION 5-10:
V_{FBPP} = _R----_F----_B--^R-₁---^F-+--^B---_R²---_F----_B---₂-  ESR_COUT  I_{LPP}
Where:
∆I_L(PP) Peak-to-Peak Value of the Inductor
Current Ripple
DS20005571A-page 24
 2016 Microchip Technology Inc.