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LM3478MM 데이터시트(PDF) 11 Page - Texas Instruments |
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11 / 32 page 'In = Sf - Se Sn + Se 'In-1 VC VSEN Sn PWM Comparator Waveforms Time Sf Se 'I0 'I1 'I2 LM3478 LM3478-Q1 www.ti.com SNVS085V – JULY 2000 – REVISED FEBRUARY 2013 Figure 26. 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 Sn, Sf and Se as follows: (1) Hence, if the quantity (Sf - Se)/(Sn + Se) 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 VSL which is enough for many applications above 50% duty cycle to avoid subharmonic oscillation . For boost applications, the slopes Se, Sf and Sn can be calculated with the formulas below: Se = VSL x fs (2) Sf = Rsen x (VOUT - VIN)/L (3) Sn = VIN x Rsen/L (4) When Se 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, RSL, in the Isens path. Figure 27 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 Se becomes: Se = (VSL + (K x RSL)) x fs (5) A typical value for factor K is 40 µA. The factor changes with switching frequency. Figure 28 is used to determine the factor K for individual applications and the formula below gives the factor K. K = ΔVSL / RSL (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 © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3478 LM3478-Q1 |
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