LM3481 Datasheet, PDF (10/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

FIGURE 2. Sub-Harmonic Oscillation for D>0.5
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FIGURE 3. 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 3.
With the compensation ramp, ΔV_samp1 and the convergence
criteria are expressed by,
increases the amplitude of the compensation ramp as shown
in Figure 4.
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:
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 fre-
quency. 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 5)
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FIGURE 4. Additional Slope Compensation Added Using
External Resistor R_SL
Where,
ΔV_SL = K x R_SL
K = 40 µA typically and changes slightly as the switching fre-
quency changes. Figure 6 shows the effect the current K has
on ΔV_SLand different values of R_SL as the switching frequency
changes.
A more general equation for the slope compensation ramp,
M_C, is shown below to include ΔV_SL caused by the resistor
R_SL.
M_C = (V_SL + ΔV_SL) x f_S
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