MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
4 _______________________________________________________________________________________
______________________________________________________________Pin Description
Output of the CCFT Error AmplifierCC9
Connect to V
DD
CS10
Leave unconnectedCDRV11
Power Ground Connection for LDRVPGND12
Gate-Driver Output. Drives LCD backplane N-channel MOSFET.LDRV13
Digital Input for CCFT Brightness Adjustment. See Table 1.CADJ5
Analog GroundGND6
Reference Voltage Output, 1.25VREF7
Inverting Input for the CCFT Error AmplifierCFB8
Digital Input to Control CCFT Section. See Table 1.CON4
Digital Input to Control LCD Bias Section. See Table 1.LON3
PIN
Digital Input for LCD Backplane Bias Adjustment. See Table 1.LADJ2
5V Power-Supply InputV
DD
1
FUNCTIONNAME
LCD Backplane Inductor Voltage-Sense Pin. Used to sense inductor voltage for on time determination.LX14
Battery Connection. Used to sense battery voltage for on time determination.BATT15
Voltage Feedback for the LCD Backplane SectionLFB16
_______________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.
