MAX753CPE Maxim Integrated Products, MAX753CPE Datasheet - Page 4

• Manufacturer Part Number: MAX753CPE
• Description: Display Drivers CCFL Backlight & LCD Neg Contrast Ctlr
• Manufacturer: Maxim Integrated Products

Specifications of MAX753CPE

• Lead Free Status / Rohs Status: No

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.
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
CCFL Backlight and
LCD Contrast Controllers
4
______________________________________________________________Pin Description
_______________Theory of Operation
PIN
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
_______________________________________________________________________________________
NAME
PGND
CDRV
CADJ
LDRV
LADJ
BATT
CON
GND
LON
REF
CFB
V
LFB
CC
CS
LX
DD
Drive Requirements for CCFL Tubes
5V Power-Supply Input
Digital Input for LCD Backplane Bias Adjustment. See Table 1.
Digital Input to Control LCD Bias Section. See Table 1.
Digital Input to Control CCFT Section. See Table 1.
Digital Input for CCFT Brightness Adjustment. See Table 1.
Analog Ground
Reference Voltage Output, 1.25V
Inverting Input for the CCFT Error Amplifier
Output of the CCFT Error Amplifier
Connect to V
Leave unconnected
Power Ground Connection for LDRV
Gate-Driver Output. Drives LCD backplane N-channel MOSFET.
LCD Backplane Inductor Voltage-Sense Pin. Used to sense inductor voltage for on time determination.
Battery Connection. Used to sense battery voltage for on time determination.
Voltage Feedback for the LCD Backplane Section
DD
CCFL Inverter
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.
FUNCTION