LM3478_09 Datasheet, PDF (12/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

Sub-harmonic Oscillation can be easily understood as a ge-
ometric problem. If the control signal does not have slope, the
slope representing the inductor current ramps up until the
control signal is reached and then slopes down again. If the
duty cycle is above 50%, any perturbation will not converge
but diverge from cycle to cycle and causes sub-harmonic os-
cillation.
It is apparent that the difference in the inductor current from
one cycle to the next is a function of S_n, S_f and S_e as follows:
It is a good design practice to only add as much slope com-
pensation as needed to avoid subharmonic oscillation. Addi-
tional slope compensation minimizes the influence of the
sensed current in the control loop. With very large slope com-
pensation the control loop characteristics are similar to a
voltage mode regulator which compares the error voltage to
a saw tooth waveform rather than the inductor current.
Hence, if the quantity (S_f - S_e)/(S_n + S_e) is greater than 1, the
inductor current diverges and subharmonic oscillation results.
This counts for all current mode topologies. The LM3478 has
some internal slope compensation V_SL which is enough for
many applications above 50% duty cycle to avoid subhar-
monic oscillation .
For boost applications, the slopes S_e, S_f and S_n can be cal-
culated with the formulas below:
S_e = V_SL x f_s
S_f = (V_OUT - V_IN)/L
S_n = V_IN/L
When S_e increases then the factor which determines if sub-
harmonic oscillation will occur decreases. When the duty
cycle is greater than 50%, and the inductance becomes less,
the factor increases.
For more flexibility slope compensation can be increased by
adding one external resistor, R_SL, in the Isens path. Figure 4
shows the setup. The externally generated slope compensa-
tion is then added to the internal slope compensation of the
LM3478. When using external slope compensation, the for-
mula for S_e becomes:
S_e = (V_SL + (K x R_SL)) x f_s
A typical value for factor K is 40 µA.
The factor changes with switching frequency. Figure 5 is used
to determine the factor K for individual applications and the
formula below gives the factor K.
K = ΔV_SL / R_SL
10135513
FIGURE 4. Adding External Slope Compensation
10135595
FIGURE 5. External Slope Compensation
ΔV_SL vs R_SL
FREQUENCY ADJUST/SHUTDOWN
The switching frequency of the LM3478 can be adjusted be-
tween 100kHz and 1MHz using a single external resistor. This
resistor must be connected between FA/SD pin and ground,
as shown in Figure 6. To determine the value of the resistor
required for a desired switching frequency refer to the typical
performance characteristics.
www.national.com
12