MIC261203 MICREL [Micrel Semiconductor], MIC261203 Datasheet - Page 22

• Manufacturer Part Number: MIC261203
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C
100nF if R1 and R2 are in kΩ range.
Step 2. Select R
voltage ripple using Equation 22:
Then the value of R
Step 3. Select C
as short for a wide range of the frequencies.
Setting Output Voltage
The MIC261203 requires two resistors to set the output
voltage as shown in Figure 7.
The output voltage is determined by Equation 23:
where V
A typical value of R1 can be between 3kΩ and 10kΩ. If
R1 is too large, it may allow noise to be introduced into
the voltage feedback loop. If R1 is too small, it will
decrease the efficiency of the power supply, especially
at light loads. Once R1 is selected, R2 can be calculated
using:
July 2011
FB
K
R
R2
V
= 0.8V.
OUT
div
inj
=
=
=
V
=
(R1//R2)
OUT
V
ΔV
inj
V
inj
FB
ff
FB
as 100nF, which could be considered
V
FB(pp)
inj
according to the expected feedback
IN
×
−
to feed all output ripples into the
×
is obtained as:
R1
V
(1
FB
+
×
×
R2
R1
(
D
K
f
×
SW
1
)
div
(1
×
-
−
D)
τ
1)
ff
is 1nF to
Eq. 21
Eq. 22
Eq. 23
Eq. 24
22
In addition to the external ripple injection added at the
FB pin, internal ripple injection is added at the inverting
input of the comparator inside the MIC261203, as shown
in Figure 8. The inverting input voltage V
1.2V. As V
clamped. The clamped V
because it is reflected as a DC error on the FB terminal.
Therefore,
MIC261203 should be limited to 5.5V to avoid this
problem.
Figure 7. Voltage-Divider Configuration
OUT
Figure 8. Internal Ripple Injection
the
is increased, the swing of V
maximum
INJ
reduces the line regulation
output
voltage
M9999-071311-A
INJ
is clamped to
MIC261203
INJ
of
will be
the