MAX754CSE データシートの表示（PDF） - Maxim Integrated

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MAX754CSE

CCFL Backlight and LCD Contrast Controllers

Maxim Integrated 

CCFL Backlight and

LCD Contrast Controllers

1250

1221

1191

811

782

753

402

372

343

01 2 3

14 15 16

DAC CODE

ZERO SCALE

MID SCALE

Figure 2. CCFT DAC Transfer Function

29 30 31

FULL SCALE

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:

R8 =

πVREF

2 ILAMP,RMS

where VREF = 1.25V and ILAMP,RMS is the desired full-

scale root mean square lamp current.

MAX754

FULL-SCALE = 1.250V

HALF-SCALE = 0.782V

ZERO-SCALE = 0.343V

CON CADJ

LOGIC AND

5-BIT COUNTER

5-BIT VOLTAGE

OUTPUT DAC

ERROR

CC

AMPLIFIER

CFB

MAX758A

SWITCH-MODE

SS

VOLTAGE CONTROLLED

CURRENT SOURCE

IBUCK

CENTER-TAP

ROYER

OSCILLATOR

C10

TRANSISTOR

EMITTERS

C5

R18

R8

Figure 3. CCFL Tube Current-Regulation Loop

6 _______________________________________________________________________________________

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