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L6258EX Arkusz danych(PDF) 16 Page - STMicroelectronics |
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L6258EX Arkusz danych(HTML) 16 Page - STMicroelectronics |
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16 / 32 page ![]() PWM current control loop L6258EX 16/32 3 PWM current control loop 3.1 Open loop transfer function analysis Block diagram: refer to Figure 5. Input parameters: ● VS = 24V ● LL = 12mH ● RL = 12Ω ● RS = 0.33Ω ● RC = to be calculated ● CC = to be calculated ● Gs transconductance gain = 1/Rb ● Gin transconductance gain = 1/Ra ● Ampl. of the Tria_0_180 ref. = 1.6V (peak to peak) ● Ra = 40KΩ ● Rb = 20KΩ ● Vr = Internal reference equal to VDD/2 (Typ. 2.5V) these data refer to a typical application, and will be used as an example during the analysis of the stability of the current control loop. The block diagram shows the schematics of the L6258EX internal current control loop working in PWM mode; the current into the load is a function of the input control voltage VDAC , and the relation between the two variables is given by the following formula: ILOAD · RS · GS = VDAC · Gin where: VDAC is the control voltage defining the load current value Gin is the gain of the input transconductance amplifier ( 1/Ra ) Gs is the gain of the sense transconductance amplifier ( 1/Rb ) Rs is the resistor connected in series to the output to sense the load current In this configuration the input voltage is compared with the feedback voltage coming from the sense resistor, then the difference between this two signals is amplified by the error amplifier in order to have an error signal controlling the duty cycle of the output stage keeping the load current under control. It is clear that to have a good performance of the current control loop, the error amplifier must have an high DC gain and a large bandwidth. I LOAD R S 1 R b ------- ⋅⋅ V DAC 1 R a ------- ⋅ = I LOAD V DAC R b R a R s ⋅ ---------------------- ⋅ 0,5 V DAC R S --------------- A () ⋅ == |
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