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LM3478 Scheda tecnica(PDF) 10 Page - National Semiconductor (TI) |
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LM3478 Scheda tecnica(HTML) 10 Page - National Semiconductor (TI) |
10 / 21 page Functional Description (Continued) SLOPE COMPENSATION RAMP The LM3478 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by- cycle current limit for the switch, and simpler control loop characteristics. It is also easy to parallel power stages using current mode control since current sharing is automatic. Current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 2.In Figure 2, a small increase in the load current causes the switch cur- rent to increase by ∆I O. The effect of this load change, ∆I 1,is : From the above equation, when D > 0.5, ∆I 1 will be greater than ∆I O. In other words, the disturbance is divergent. So a very small perturbation in the load will cause the disturbance to increase. To prevent the sub-harmonic oscillations, a compensation ramp is added to the control signal, as shown in Figure 3. With the compensation ramp, 10135507 FIGURE 1. Basic Operation of the PWM comparator 10135509 FIGURE 2. Sub-Harmonic Oscillation for D>0.5 www.national.com 10 |
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