MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 21

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node SW via a resistor R
capacitor C
is:
where:
V
D = Duty cycle
f
τ = (R1//R2//R
In Equations 21 and 22, it is assumed that the time
constant associated with C
the switching period:
SW
November 2010
IN
= Power stage input voltage
= Switching frequency
Figure 6b. Inadequate Ripple at FB
inj
K
ΔV
Figure 6c. Invisible Ripple at FB
, as shown in Figure 6c. The injected ripple
div
inj
FB(pp)
) × C
f
=
SW
R
1
inj
ff
×
=
R1//R2
τ
V
+
IN
=
R1//R2
×
T
τ
K
ff
<<
div
must be much greater than
1
×
D
×
(1
-
D)
×
f
SW
1
×
τ
inj
and a
(31)
(33)
(32)
21
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C
time constant requirements. Also, a 100nF injection
capacitor C
for a wide range of the frequencies.
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 24:
Then the value of R
Step 3. Select C
as short for a wide range of the frequencies.
ff
inj
of 1nF to 100nF can easily satisfy the large
K
R
is used in order to be considered as short
div
inj
=
=
inj
inj
ff
(R1//R2)
ΔV
as 100nF, which could be considered
inj
according to the expected feedback
to feed all output ripples into the
V
FB(pp)
is obtained as:
IN
×
×
D
(
f
K
SW
×
(1
1
div
×
−
τ
D)
−
1)
M9999-111710-A
ff
MIC2176
is 1nF to
(34)
(35)