Rainbow Electronics MAX754 - Manual (Page 8)

MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
8 _______________________________________________________________________________________
MAX754 6-BIT DAC
PULSE-SKIP
COMPARATOR
FULL-SCALE OUTPUT = 1.250V
HALF-SCALE OUTPUT = 0.938V
ZERO-SCALE OUTPUT = 0.635V
16
13
R4
PRESET
6-BIT COUNTER
CLK
Q3
V DAC
LFB
LDRV
14
LX
15
BATT
+5V INPUT
1
V DD
3
LON
6
GND
12
PGND
2
LADJ
C1
0.22 µ F
L2
33 µ H
BATTERY
INPUT
D3
1N5819
POSITIVE
LCD-BIAS
OUTPUT
ON/OFF
CONTROL
ON-TIME
LOGIC
OFF-TIME
LOGIC
R3
C2
10
µ F
C6
10
µ F
35V
Figure 6. MAX754 Positive LCD-Bias Generator
Table 1. CCFL Circuit Component Descriptions
ITEM DESCRIPTION
C5
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.
R18
Integrating Resistor. The output source-current capability of the CC pin (50µA) limits how small R18 can be.
Do not make R18 smaller than 70k
Ω , 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
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.
D7A, D7B
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.
CCFL
The circuit of Figure 1, with the components shown in the bill of materials (Table 4), will drive a 500V RMS oper-
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.