CCFL Backlight and

LCD Contrast Controllers

8

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MAX754

6-BIT DAC

PULSE-SKIP

COMPARATOR

FULL-SCALE OUTPUT = 1.250V

HALF-SCALE OUTPUT = 0.938V

ZERO-SCALE OUTPUT = 0.635V

16

13

R4

PRESET

6-BIT COUNTER

CLK

Q3

LFB

LDRV

14

LX

15

BATT

+5V INPUT

1

3

LON

6

GND

12

PGND

2

LADJ

C1

0.22

µF

L2

33

µH

BATTERY

INPUT

D3

1N5819

POSITIVE

LCD-BIAS

OUTPUT

ON/OFF

CONTROL

ON-TIME

LOGIC

OFF-TIME

LOGIC

R3

C2

10

µF

C6

10

µF

35V

Figure 6. MAX754 Positive LCD-Bias Generator

Table 1. CCFL Circuit Component Descriptions

ITEM

DESCRIPTION

C5

Integrating Capacitor. 1 / (C5 x R18) sets the dominant pole for the feedback loop, which regulates the lamp

current. Set the dominant pole at least two decades below the Royer frequency to eliminate the AC compo-

nent of the voltage on R8. For example, if your Royer is oscillating at 50kHz = 314159rad/s, you should set

1 / (C5 x R18)

≤ 3142rad/s.

R18

Integrating Resistor. The output source-current capability of the CC pin (50µA) limits how small R18 can be.

Do not make R18 smaller than 70k

Ω, otherwise CC will not be able to servo CFB to the DAC voltage (i.e., the

integrator will not be able to integrate) and the loop will not be able to regulate.

R8

R8 converts the half-wave rectified lamp current into a voltage. The average voltage on R8 is not equal to the

root mean square voltage on R8. The accuracy of R8 is important since it, along with the MAX754 reference,

sets the full-scale lamp current. Use a ±1%-accurate resistor.

D7A, D7B

D7A and D7B half-wave rectify the CCFL lamp current. Half-wave rectification of the lamp current and then

averaging is a simple way to perform AC-to-DC conversion. D7A and D7B’s forward voltage drop and speed

are unimportant; they do not need to pass currents larger than about 10mA, and their reverse breakdown

voltage can be as low as 10V.

CCFL

ating cold-cathode fluorescent lamp at 6W of power with a +12V input voltage. The lower the input voltage,

the less power the circuit can deliver.