Page 188 - The Art and Science of Analog Circuit Design
P. 188

Tripping the Light Fantastic




             Figure 11-29.
         Additional optimiza-
            tion of RC time
          constant and com-
         pensation capacitor
            reduces turn-on
             transient. Run
            voltage is large,
          indicating possible
            lossy layout and                  HORIZ = 2ms/DIV
                  display.
                          the improvements, more margin is possible if display losses can be con-
                          trolled. Figures 11-26-11-29 were taken with an exceptionally lossy
                          display. The metal enclosure was very close to the foil wrapped lamps,
                          causing large losses with subsequent high turn-on and running voltages.
                          If the display is selected for lower losses, performance can be greatly
                          improved.
                            Figure 11-30 shows a low loss display responding to turn-on with
                          a 2\if compensation capacitor and 10kH-l|nf RC values. Trace A
                          is the transformer's output while Traces B and C are the LT1172's
                          Vcompensation and feedback pins, respectively. The output overshoots
                          and rings badly, peaking to about 3000 volts. This activity is reflected by
                          overshoots at the Vcompensation pin (the LT1172's error amplifier out-
                          put) and the feedback pin. In Figure 11-31, the RC is reduced to lOkQ-
                          .l[if. This substantially reduces loop delay. Overshoot goes down to only
                          800 volts—a reduction of almost a factor of four. Duration is also much
                          shorter. The Vcompensation and feedback pins reflect this tighter con-
                          trol. Damping is much better, with slight overshoot induced at turn-on.
                          Further reduction of the RC to lOkQ-.Oljif (Figure 11-32) results in
                          even faster loop capture, but a new problem appears. In Trace A, lamp
                          turn on is so fast that the overshoot does not register in the photo. The






             Figure 11-30.
           WavefofMsfora





          fleeted at compen-
          sation node (Trace  c = IV/DIV
           B) and feedback                     HORIZ = 10ms/DIV
              pin (Trace C).




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