MIC2101_13 Datasheet, PDF (28/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
The output voltage is determined by the equation:
V_OUT
V_FB
u (1 
R1)
R2
Eq. 26
where, V_FB = 0.8V. A typical value of R1 can be between
Nȍ DQGNȍ,I5LVWRRODUJH, 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:
R2 ^{VFB u R1}
V_OUT  V_FB
Eq. 27
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2101/02 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 can’t sense it, then the MIC2101/02 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:
1. Enough ripple at the feedback voltage due to the
large ESR of the output capacitors.
As shown in Figure 8a, the converter is stable
without any ripple injection. The feedback voltage
ripple is:
ǻ9_FB(pp)
R2
R1  R2
u ESR _COUT
u Ç»,_{L (pp)}
Eq. 28
where Ç»,_L(pp) is the peak-to-peak value of the
inductor current ripple.
MIC2101/02
2. Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin
through a feedforward capacitor C_ff in this situation,
as shown in Figure 8b. The typical C_ff value is
between 1nF and 100nF. With the feedforward
capacitor, the feedback voltage ripple is very close
to the output voltage ripple:
ǻ9_FB(pp) | ESR u ǻ,_{L (pp)}
Eq. 29
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors:
Figure 8a. Enough Ripple at FB
Figure 8b. Inadequate Ripple at FB
Figure 8c. Invisible Ripple at FB
November 13, 2013
28
Revision 2.0