LM3478MA_15 Datasheet, PDF (11/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

www.ti.com
LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
PWM Comparator
V_C
Waveforms
S_e
'I₀
'I₂
V_SEN
S_n
S_f
'I₁
Time
Figure 25. Sub-Harmonic Oscillation for D>0.5 and Compensation Ramp to Avoid Sub-Harmonic
Oscillation
Sub-harmonic Oscillation can be easily understood as a geometric 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 oscillation.
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:
'I_n =
S_f
S_n
-+SS_ee'In-1
(1)
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 subharmonic
oscillation .
For boost applications, the slopes S_e, S_f and S_n can be calculated with the formulas below:
S_e = V_SL x f_s
(2)
S_f = R_sen x (V_OUT - V_IN)/L
(3)
S_n = V_IN x R_sen/L
(4)
When S_e increases then the factor which determines if subharmonic 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 I_sens path.
Figure 26 shows the setup. The externally generated slope compensation is then added to the internal slope
compensation of the LM3478. When using external slope compensation, the formula for S_e becomes:
S_e = (V_SL + (K x R_SL)) x f_s
(5)
A typical value for factor K is 40 µA.
The factor changes with switching frequency. Figure 27 is used to determine the factor K for individual
applications and the formula below gives the factor K.
K = ΔV_SL / R_SL
(6)
It is a good design practice to only add as much slope compensation as needed to avoid subharmonic oscillation.
Additional slope compensation minimizes the influence of the sensed current in the control loop. With very large
slope compensation 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.
Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM3478MA
Submit Documentation Feedback
11