MIC26903 MICREL [Micrel Semiconductor], MIC26903 Datasheet - Page 22

• Manufacturer Part Number: MIC26903
• Description: 28V, 9A Hyper Light Load Synchronous DC/DC Buck Regulator
• 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 MIC26903 requires two resistors to set the output
voltage as shown in Figure 7.
The output voltage is determined by the equation:
where: V
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
Figure 7. Voltage-Divider Configuration
K
R
R2
V
= 0.8V. A typical value of R1 can be between
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 MIC26903, 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 MIC26903
should be limited to 5.5V to avoid this problem.
OUT
Figure 8. Internal Ripple Injection
is increased, the swing of V
INJ
reduces the line regulation
M9999-071311-A
INJ
is clamped to
MIC26903
INJ
will be