MAX753 Maxim, MAX753 Datasheet - Page 8

• Manufacturer Part Number: MAX753
• Description: CCFL Backlight and LCD Contrast Controllers
• Manufacturer: Maxim

CCFL Backlight and
LCD Contrast Controllers
Figure 6. MAX754 Positive LCD-Bias Generator
Table 1. CCFL Circuit Component Descriptions
8
_______________________________________________________________________________________
D7A, D7B
CCFL
ITEM
R18
C5
R8
2
LADJ
MAX754
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.
Integrating Resistor. The output source-current capability of the CC pin (50µA) limits how small R18 can be.
Do not make R18 smaller than 70kΩ, 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 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.
The circuit of Figure 1, with the components shown in the bill of materials (Table 4), will drive a 500V
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.
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.
PGND
CONTROL
3
12
LON
0.22 F
C1
ON/OFF
GND
PRESET
6-BIT COUNTER
CLK
+5V INPUT
6-BIT DAC
6
V
DD
1
ON-TIME
LOGIC
15
BATT
V
DAC
COMPARATOR
FULL-SCALE OUTPUT = 1.250V
HALF-SCALE OUTPUT = 0.938V
ZERO-SCALE OUTPUT = 0.635V
PULSE-SKIP
OFF-TIME
LOGIC
14
LX
DESCRIPTION
LDRV
LFB
13
BATTERY
16
INPUT
33 H
L2
Q3
1N5819
C2
10 F
D3
R3
R4
LCD-BIAS
POSITIVE
OUTPUT
C6
10 F
35V
RMS
oper-