Page 191 - The Art and Science of Analog Circuit Design

Jim Williams

field imbalance. Figure 11-34's circuit does this. This circuit's most sig-
nificant aspect is that the lamp is fully floating—there is no galvanic con-
nection to ground as in the previous designs. This allows Tl to deliver
symmetric, differential drive to the lamp. Such balanced drive eliminates
field imbalance, reducing thermometering at low lamp currents. This ap-
proach precludes any feedback connection to the now floating output.
Maintaining closed loop control necessitates deriving a feedback signal
from some other point. In theory, lamp current proportions to Tl's or LI *s
drive level, arid some form of sensing this can be used to provide feed-
5
back. In practice, parasitics make a practical implementation difficult.
Figure 11-34 derives the feedback signal by measuring Royer con-
verter current and feeding this information back to the LT1172. The
Royer's drive requirement closely proportions to lamp current under all
conditions. Al senses this current across the .30 shunt and biases Q3,
closing a local feedback loop. Q3's drain voltage presents an amplified,
single ended version of the shunt voltage to the feedback point, closing
the main loop. The lamp current is not as tightly controlled as before, but
.5% regulation over wide supply ranges is possible. The dimming in this
circuit is controlled by a 1kHz PWM signal. Note the heavy filtering
(33k.O~2juf) outside the feedback loop. This allows a fast time constant,
minimizing turn-on overshoot. 6
In all other respects, operation is similar to the previous circuits. This
circuit typically permits the lamp to operate over a 40:1 intensity range
without "thermometering." The normal feedback connection is usually
limited to a 10:1 range.
The losses introduced by the current shunt and Al degrade overall
efficiency by about 2%. As such, circuit efficiency is limited to about
90%. Most of the loss can be recovered at moderate cost in complexity.
Figure 11-35's modifications reduce shunt and Al losses. Al, a precision
micropower type, cuts power drain and permits a smaller shunt value
without performance degradation. Unfortunately, Al does not function
when its inputs reside at the V+ rail. Because the circuit's operation re-
quires this, some accommodation must be made. 7
At circuit start-up, Al's input is pulled to its supply pin potential (actu-
ally, slightly above it). Under these conditions, Al's input stage is shut
off. Normally, Al's output state would be indeterminate but, for the am-
plifier specified, it will always be high. This turns ofTQ3, permitting the
LT1172 to drive the Royer stage. The Royer's operation causes Ql's col-
lector swing to exceed the supply rail. This turns on the 1N4148, the
BAT-85 goes off, and Al's supply pin rises above the supply rail. This
"bootstrapping" action results in Al's inputs being biased within the am-

5. See Appendix C, "A Lot of Cut-Off-Ears and No Van Goghs—Some Not-So-Great Ideas," for
details.
6. See section "Feedback Loop Stability Issues."
7. In other words, we need a hack.

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