Analog ripple subtraction (

Authors	Stephen Woodward
Main Document	Article «PWM-programmed LM317 constant current source»
Description	Figure 7
File Format / Size	PDF / 19 Kb
Document Language	English

Download PDF

Other Materials from the Main Document

• Article «PWM-programmed LM317 constant current source»
• Figure 1. Classic LM317 constant current source, I_OUT = V_ADJ/R_S + I_ADJ ≈ ≈ V_ADJ/R_S = 1.25/R_S
• Figure 2. A novel LM317 topology enables control of amps of I_OUT with just milliamps of I_C, I_OUT = (V_ADJ - (I_C - I_ADJ)R_C)/ R_S - I_C + I_ADJ ≈ (V_ADJ
• Figure 3. The 1-A, 15-V, PWM-programmed grounded-load current source with a novel LM317 topology. The asterisked resistors are 1% or better and R_S = 1.25 Ω
• Figure 4. I_OUT versus D where D (x-axis) is the PWM duty factor and I_OUT (y-axis) is V_ADJ/1.25 = 1 A full-scale = 1 - 2/(1 + 1/D)
• Figure 5. D versus I_OUT where I_OUT (x-axis) is 1 A full-scale and D (y-axis) = 1/(2/(1 - I_OUT) - 1)
• Figure 6. C₃ adds 2nd order PWM ripple filtering for ~500% faster settling time. Thanks go to frequent contributor Christopher R. Paul for the comment that suggested it
• Figure 7. Analog ripple subtraction (
• Figure 8. A contribution from frequent contributor Christopher R. Paul suggests improving PSRR with capacitor C_C as well as making programming linear in D. For applications in which settling time isn't important, this is a useful modification. Thanks, Ch