CCFL Backlight and

LCD Contrast Controllers

4

_______________________________________________________________________________________

______________________________________________________________Pin Description

Output of the CCFT Error Amplifier

CC

9

CS

10

Leave unconnected

CDRV

11

Power Ground Connection for LDRV

PGND

12

Gate-Driver Output. Drives LCD backplane N-channel MOSFET.

LDRV

13

Digital Input for CCFT Brightness Adjustment. See Table 1.

CADJ

5

Analog Ground

GND

6

Reference Voltage Output, 1.25V

REF

7

Inverting Input for the CCFT Error Amplifier

CFB

8

Digital Input to Control CCFT Section. See Table 1.

CON

4

Digital Input to Control LCD Bias Section. See Table 1.

LON

3

PIN

Digital Input for LCD Backplane Bias Adjustment. See Table 1.

LADJ

2

5V Power-Supply Input

1

FUNCTION

NAME

LCD Backplane Inductor Voltage-Sense Pin. Used to sense inductor voltage for on time determination.

LX

14

Battery Connection. Used to sense battery voltage for on time determination.

BATT

15

Voltage Feedback for the LCD Backplane Section

LFB

16

_______________Theory of Operation

CCFL Inverter

The MAX753/MAX754’s CCFL inverter is designed to

drive one or two cold-cathode fluorescent lamps

(CCFLs) with power levels from 100mW to 6W. These

lamps commonly provide backlighting for LCD panels

in portable computers.

Drive Requirements for CCFL Tubes

CCFL backlights require a high-voltage, adjustable AC

power source. The MAX753/MAX754 generate this AC

waveform with a self-oscillating, current-fed, parallel

resonant circuit, also known as a Royer-type oscillator.

Figure 1 shows one such circuit. The Royer oscillator is

comprised of T1, C9, the load at the secondary, Q4,

and Q5. The circuit self-oscillates at a frequency deter-

mined by the effective primary inductance and capaci-

tance. Q4 and Q5 are self-driven by the extra winding.

The current source feeding the Royer oscillator is com-

prised of L1, D5, and the MAX758A. When current from

the current source increases, so does the lamp current.

The lamp current is half-wave rectified by D7A and

D7B, and forms a voltage across resistor R8. The

MAX753’s error amplifier compares the average of this

voltage to the output of its internal DAC. Adjusting the

DAC output from zero scale to full scale (digital control)

causes the error amplifier to vary the tube current from

a minimum to a maximum. The DAC’s transfer function

is shown in Figure 2.

On power-up or after a reset, the counter sets the DAC

output to mid scale. Each rising edge of CADJ (with

CON high) decrements the DAC output. When decre-

mented beyond full scale, the counter rolls over and

sets the DAC to the maximum value. In this way, a sin-

gle pulse applied to CADJ decreases the DAC set-

point by one step, and 31 pulses increase the set-point

by one step.

The error amplifier’s output voltage controls the peak

current output of the MAX758A. The peak switch cur-

rent is therefore controlled by the output of the error

amplifier. The lower the error amplifier’s output, the

lower the peak current. Since the current through the

current source is related to the current through the

tube, the lower the error amplifier’s output, the lower the

tube current.