Datasheet

<1
M
2
- M
C
M
1
+ M
C
-
M
2
- M
C
'V
samp1
= -
M
1
+ M
C
'V
samp0
+
_
PWM
Comparator
Control Signal
Compensation Ramp
-M
2
-M
C
DT
S
(1-D)T
S
M
1
'
V
samp1
'
V
samp0
Steady State
Signal V
samp
Perturbed Signal
V
SL
Control
Signal
V
samp
+
_
PWM
Comparator
Control Signal
-M
2
DT
S
(1-D)T
S
M
1
M
C
= 0
Steady State
Signal V
sam
p
Perturbed Signal
'
V
samp1
'
V
samp
0
<
1
M
2
M
1
-
LM3481
SNVS346E –NOVEMBER 2007–REVISED APRIL 2012 www.ti.com
From the above equation, when D > 0.5, ΔV
samp1
will be greater than ΔV
samp0
. In other words, the disturbance is
divergent. So a very small perturbation in the load will cause the disturbance to increase. To ensure that the
perturbed signal converges we must maintain:
Figure 19. Sub-Harmonic Oscillation for D>0.5
Figure 20. Compensation Ramp Avoids Sub-Harmonic Oscillation
To prevent the sub-harmonic oscillations, a compensation ramp is added to the control signal, as shown in
Figure 20.
With the compensation ramp, ΔV
samp1
and the convergence criteria are expressed by,
The compensation ramp has been added internally in the LM3481. The slope of this compensation ramp has
been selected to satisfy most applications, and it's value depends on the switching frequency. This slope can be
calculated using the formula:
M
C
= V
SL
x f
S
In the above equation, V
SL
is the amplitude of the internal compensation ramp and f
S
is the controller's switching
frequency. Limits for V
SL
have been specified in the electrical characteristics section.
In order to provide the user additional flexibility, a patented scheme has been implemented inside the IC to
increase the slope of the compensation ramp externally, if the need arises. Adding a single external resistor,
R
SL
(as shown in Figure 22) increases the amplitude of the compensation ramp as shown in Figure 21.
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