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MAX754 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX754 Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 16 page CCFL Backlight and LCD Contrast Controllers 6 _______________________________________________________________________________________ In Figure 1, the MAX758A, L1, and D5 form a voltage- controlled switch-mode current source. The current out of L1 is proportional to the voltage applied to the SS pin. The MAX758A contains a current-mode pulse- width-modulating buck regulator that switches at 170kHz. The voltage on the SS pin sets the switch cur- rent limit and thus sets the current out of L1. CCFL Current-Regulation Loop Figure 3 shows a block diagram of the regulation loop, which maintains a fixed CCFL average lamp current despite changes in input voltage and lamp impedance. This loop regulates the average value of the half-wave rectified lamp current. The root mean square lamp cur- rent is related to, but not equal to, the average lamp current. Assuming a sinusoidal lamp current, select R8 as follows: where VREF = 1.25V and ILAMP,RMS is the desired full- scale root mean square lamp current. R V I REF LAMP RMS 8 2 = π , 01 343 372 402 2 DAC CODE ZERO SCALE MID SCALE FULL SCALE 3 14 15 16 29 30 31 753 782 811 1191 1221 1250 Figure 2. CCFT DAC Transfer Function MAX754 MAX758A CC LOGIC AND 5-BIT COUNTER 5-BIT VOLTAGE OUTPUT DAC SWITCH-MODE VOLTAGE CONTROLLED CURRENT SOURCE FULL-SCALE = 1.250V HALF-SCALE = 0.782V ZERO-SCALE = 0.343V CFB ERROR AMPLIFIER CON CADJ SS C10 C5 R8 R18 IBUCK CENTER-TAP TRANSISTOR EMITTERS ROYER OSCILLATOR Figure 3. CCFL Tube Current-Regulation Loop |
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