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L6919CD 数据表(PDF) 19 Page - STMicroelectronics |
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L6919CD 数据表(HTML) 19 Page - STMicroelectronics |
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19 / 32 page  19/32 L6919C – · is the ACM PWM transfer function where ∆VOSC is the oscillator ramp amplitude and has a typical value of 2V Removing the dependence from the Error Amplifier gain, so assuming this gain high enough, the control loop gain results: With further simplifications, it results: Considering now that in the application of interest it can be assumed that Ro>>RL; ESR<<Ro and RDROOP<<Ro, it results: The ACM control loop gain is designed to obtain a high DC gain to minimize static error and cross the 0dB axes with a constant -20dB/dec slope with the desired crossover frequency ωT. Neglecting the effect of ZF(s), the transfer function has one zero and two poles. Both the poles are fixed once the output filter is designed and the zero is fixed by ESR and the Droop resistance. To obtain the desired shape an RF-CF series network is consid- ered for the ZF(s) implementation. A zero at ωF=1/RFCF is then introduced together with an integrator. This in- tegrator minimizes the static error while placing the zero in correspondence with the L-C resonance a simple - 20dB/dec shape of the gain is assured (See Figure 14). In fact, considering the usual value for the output filter, the LC resonance results to be at frequency lower than the above reported zero.Compensation network can be simply designed placing ωZ = ωLC and imposing the cross-over frequency ωT as desired obtaining: Figure 14. ACM Control Loop Gain Block Diagram (left) and Bode Diagram (right) PWM 4 5 --- V IN ∆ V OSC ∆ ------------------- ⋅ = G LOOP s () 4 5 --- V IN V OS C ∆ ------------------- Z F s () Z P s () Z L s () + ------------------------------------ Rs Rg -------- Z P s () R FB --------------- + ⋅⋅ ⋅ – = G LOO P s () 4 5 --- V IN V OSC ∆ ------------------- Z F s () R FB --------------- Ro R DROOP + Ro R L 2 ------- + -------------------------------------- 1s Co R DROOP//Ro ESR + () ⋅ ⋅ + s 2 Co L 2 --- s L 2R o ⋅ --------------- Co ESR Co R L 2 ------- ⋅ + ⋅ + 1 + ⋅ + ⋅ ⋅ ---------------------------------------------------------------------------------------------------------------------------------- ⋅⋅ ⋅ ⋅ – = G LOO P s () 4 5 --- V IN V OSC ∆ ------------------- Z F s () R FB --------------- 1s Co R DROOP ESR + () ⋅ ⋅ + s 2 Co L 2 --- s L 2Ro ⋅ --------------- Co ESR Co R L 2 ------- ⋅ + ⋅ + 1 + ⋅ + ⋅ ⋅ ---------------------------------------------------------------------------------------------------------------------------------- ⋅⋅ ⋅ – = R F R FB V OSC ∆ ⋅ V IN ---------------------------------- 5 4 --- ω T L 2R DROOP ESR + () ⋅ -------------------------------------------------------- ⋅⋅ ⋅ = C F Co L 2 --- ⋅ R F -------------------- = Rout Cout ESR L/2 RFB RF CF REF PWM IFB VCOMP VOUT d •VIN ZF dB ω ω T ω Z ω LC GLOOP ZF(s) K K 4 5 --- V IN V osc ∆ --------------- 1 R FB ---------- ⋅⋅ dB = |
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