Page 176 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

Figure 11-18.
An 88% efficiency
cold cathode fluo-
rescent lamp
(CCFL) power 2000
TEST ONLY
supply. (SEE TEXT)

4.5V TO +20V

CONNECT LT1172 TO
LOWEST VOLTAGE
AVAILABLE (M Mm = 3V)
Is
V SOkii
"sw INTENSITY
ADJUST
LT1172
E2 FB
GND
1MF
HHMIMMI

C1 = MUST BE A LOW LOSS CAPACITOR.
METALIZED POLYCARB
WIMA FKP2 OR MKP-20 (GERMAN) RECOMMENDED
L1 = SUMIDA 6345-020 OR COILTRONICS CTX110092-1
PIN NUMBERS SHOWN FOR COILTRONICS UNIT
L2 = COILTRONICS CTX300-4
Q1, 02 = ZETEX ZTX849 OR ROHM 2SC5001
*=1% FILM RESISTOR
00 NOT SUBSTITUTE COMPONENTS
COILTRONICS (305) 781-8900, SUMIDA (708) 956-0666

applied the LTl 172 switching regulator's feedback pin is below the de-
vice's internal 1.2V reference, causing full duty cycle modulation al the
V sw pin (Trace A, Figure 11-19). L2 conducts current (Trace B) which
flows from Li's center tap, through the transistors, into L2; L2*s current
is deposited in switched fashion to ground by the regulator's action.
LI and the transistors comprise a current driven Royer class converter
which oscillates at a frequency primarily set by LI's characteristics (in-
cluding its load) and the .033uF capacitor. LTl 172 driven L2 sets the mag-
nitude of the Q1-Q2 tail current, and hence Li's drive level. The 1N5818
diode maintains L2's current flow when the LTl 172 is off. The LTl 172's
100kHz clock rate is asynchronous with respect to the push-pull con-
verter's (60kHz) rate, accounting for Trace B's waveform thickening.

158