CCFL Backlight and

LCD Contrast Controllers

6

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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:

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

CENTER-TAP

TRANSISTOR

EMITTERS

ROYER

OSCILLATOR

Figure 3. CCFL Tube Current-Regulation Loop