CCFL Backlight and

LCD Contrast Controllers

______________________________________________________________________________________

15

MAX753

6-BIT DAC

PULSE-SKIP

COMPARATOR

16

13

R4

PRESET

6-BIT COUNTER

CLK

Q3

LFB

7

REF

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

D8

1N5819

NEGATIVE

LCD-BIAS

OUTPUT

ON/OFF

CONTROL

ON-TIME

LOGIC

OFF-TIME

LOGIC

R3

C2

10

µF

C15

1

µF

R19

2.2

Ω

C6

10

µF

35V

ALTERNATE

D8 CONNECTION

(SEE TEXT)

C4

0.22

µF

D3

1N5819

Figure 8. MAX753 Negative LCD-Bias Generator

01

* DAC OUTPUT VOLTAGE = REF - LFB

625

635

645

2

DAC CODE

ZERO SCALE

MID SCALE

FULL SCALE

30 31 32

61 62 63

918

928

937

1220

1230

1240

Figure 9. MAX753 LCD DAC Transfer Function

The MAX753’s DAC transfer function is shown in Figure 9.

The following equation relates the switching regulator’s

regulated output voltage to the DAC’s voltage (REF - LFB):

The value REF - LFB (and not LFB) is specified in the

Electrical Characteristics. The most negative output

voltage occurs for the largest value of REF - LFB.

The MAX753’s combination boost converter and

charge-pump inverter was chosen over a conventional

buck-boost inverter because it allows the use of low-

cost N-channel MOSFETs instead of more expensive P-

channel ones. Additionally, its efficiency is 5% to 10%

better than a standard buck-boost inverter.

V

REF

1

R3

R4

REF

LFB

+









−

()