MIC26901 MICREL [Micrel Semiconductor], MIC26901 Datasheet - Page 21

• Manufacturer Part Number: MIC26901
• 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 MIC26603 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.
div
OUT
inj
=
=
=
V
=
(R1//R2)
ΔV
OUT
V
inj
V
inj
FB
ff
V
FB
FB(pp)
as 100nF, which could be considered
inj
IN
according to the expected feedback
×
−
to feed all output ripples into the
×
is obtained as:
R1
V
(1
FB
×
+
×
D
R2
R1
(
f
K
×
SW
(1
1
)
div
×
−
τ
D)
−
1)
ff
is 1nF to
Eq. 21
Eq. 22
Eq. 23
Eq. 24
21
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 MIC26901, 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, the maximum output voltage of the MIC26901
should be limited to 5.5V to avoid this problem.
Figure 6. Voltage-Divider Configuration
OUT
Figure 7. Internal Ripple Injection
is increased, the swing of V
INJ
reduces the line regulation
M9999-071311-A
INJ
is clamped to
MIC26901
INJ
will be