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LM3481QMMX Datasheet(PDF) 10 Page - Texas Instruments |
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LM3481QMMX Datasheet(HTML) 10 Page - Texas Instruments |
10 / 30 page <1 M2 - MC M1 + MC - M2 - MC 'Vsamp1 = - M1 + MC 'Vsamp0 + _ PWM Comparator Control Signal Compensation Ramp -M2 -MC DTS (1-D)TS M1 'Vsamp1 'Vsamp0 Steady State Signal Vsamp Perturbed Signal VSL Control Signal Vsamp + _ PWM Comparator Control Signal -M2 DTS (1-D)TS M1 MC = 0 Steady State Signal Vsamp Perturbed Signal 'Vsamp1 'Vsamp0 < 1 M2 M1 - LM3481 SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012 www.ti.com From the above equation, when D > 0.5, ΔVsamp1 will be greater than ΔVsamp0. In other words, the disturbance is divergent. So a very small perturbation in the load will cause the disturbance to increase. To ensure that the perturbed signal converges we must maintain: Figure 19. Sub-Harmonic Oscillation for D>0.5 Figure 20. 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 20. With the compensation ramp, ΔVsamp1 and the convergence criteria are expressed by, The compensation ramp has been added internally in the LM3481. The slope of this compensation ramp has been selected to satisfy most applications, and it's value depends on the switching frequency. This slope can be calculated using the formula: MC = VSL x fS In the above equation, VSL is the amplitude of the internal compensation ramp and fS is the controller's switching frequency. Limits for VSL 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, RSL(as shown in Figure 22) increases the amplitude of the compensation ramp as shown in Figure 21. 10 Submit Documentation Feedback Copyright © 2007–2012, Texas Instruments Incorporated Product Folder Links: LM3481 |
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