MIC28303 Datasheet, PDF (26/38 Pages) Microchip Technology – 50V, 3A Power Module

MIC28303
5.5 Output Voltage Setting
Components
The MIC28303 requires two resistors to set the output
voltage, as shown in Figure 5-4:
g_m Amp
V_REF
R1
FB
R11
FIGURE 5-4:
Configuration.
Voltage-Divider
The output voltage is determined by Equation 5-10:
EQUATION 5-10:
Where:
V_OUT
=
V_F
B



1
+
R^{--R---1-1--1-}
V_FB
0.8V
A typical value of R1 used on the standard evaluation
board is 10 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, R11 can be calculated using Equation 5-11:
EQUATION 5-11:
R11 = ----^V----^F---^B----^-----^R----1-----
V_OUT – V_FB
5.6 Ripple Injection
The V_FB ripple required for proper operation of the
MIC28303 g_m amplifier and error comparator is 20 mV
to 100 mV. 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 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 error
comparator cannot sense it, then the MIC28303 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. Table 2-2 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:
• Enough ripple at the feedback voltage due to the
large ESR of the output capacitors (Figure 5-5):
VOUT
MIC28303
FB
R1
C_OUT
R11
ESR
FIGURE 5-5:
Enough Ripple at FB.
As shown in Figure 5-6, the converter is stable
without any ripple injection.
VOUT
MIC28303
FB
R1
R11
C_ff
C_OUT
ESR
FIGURE 5-6:
Inadequate Ripple at FB.
The feedback voltage ripple is:
EQUATION 5-12:
V _{F B  P P }
=
-------^R----1---1--------
R1 + R11

ESR_COUT

I _{L  P P }
Where:
∆I_L(PP)
Peak-to-Peak Value of the Inductor
Current Ripple
• Inadequate ripple at the feedback voltage due to
the small ESR of the output capacitors, such is
the case with ceramic output capacitor.
The output voltage ripple is fed into the FB pin
through a feed-forward capacitor, C_ff in this situation,
as shown in Figure 5-7. The typical C_ff value is
between 1 nF and 100 nF.
DS20005464A-page 26
 2016 Microchip Technology Inc.