LM3481MMX National Semiconductor, LM3481MMX Datasheet - Page 10

• Manufacturer Part Number: LM3481MMX
• Description: High Efficiency Low-Side N-Channel Controller for Switching Regulators
• Manufacturer: National Semiconductor

www.national.com
To prevent the sub-harmonic oscillations, a compensation
ramp is added to the control signal, as shown in Figure 3.
With the compensation ramp, ΔV
criteria are expressed by,
The compensation ramp has been added internally in the
LM3481. The slope of this compensation ramp has been se-
lected to satisfy most applications, and it's value depends on
the switching frequency. This slope can be calculated using
the formula:
In the above equation, V
compensation ramp and f
quency. Limits for V
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
have been specified in the electrical
M
C
SL
S
= V
is the controller's switching fre-
is the amplitude of the internal
FIGURE 3. Compensation Ramp Avoids Sub-Harmonic Oscillation
SL
x f
samp1
S
SL
(as shown in Figure 5)
and the convergence
FIGURE 2. Sub-Harmonic Oscillation for D>0.5
10
increases the amplitude of the compensation ramp as shown
in Figure 4.
FIGURE 4. Additional Slope Compensation Added Using
Where,
K = 40 µA typically and changes slightly as the switching fre-
quency changes. Figure 6 shows the effect the current K has
on ΔV
changes.
A more general equation for the slope compensation ramp,
M
R
SL
C
, is shown below to include ΔV
.
SL
and different values of R
External Resistor R
M
C
ΔV
= (V
SL
SL
= K x R
+ ΔV
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SL
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as the switching frequency
SL
SL
SL
) x f
caused by the resistor
SL
S
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