MAX753 Maxim, MAX753 Datasheet - Page 15

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

Figure 8. MAX753 Negative LCD-Bias Generator
Figure 9. MAX753 LCD DAC Transfer Function
* DAC OUTPUT VOLTAGE = REF - LFB
1240
1230
1220
937
928
918
645
635
625
ZERO SCALE
2
LADJ
MAX753
0 1
PGND
CONTROL
3
12
LON
2
0.22 F
______________________________________________________________________________________
C1
ON/OFF
GND
PRESET
6-BIT COUNTER
CLK
+5V INPUT
6-BIT DAC
6
MID SCALE
DAC CODE
30 31 32
V
DD
1
ON-TIME
LOGIC
15
BATT
V
DAC
COMPARATOR
PULSE-SKIP
61 62 63
OFF-TIME
FULL SCALE
LOGIC
14
LX
LCD Contrast Controllers
LDRV
LFB
REF
13
BATTERY
16
7
The MAX753’s DAC transfer function is shown in Figure 9.
The following equation relates the switching regulator’s
regulated output voltage to the DAC’s voltage (REF - LFB):
The value REF - LFB (and not LFB) is specified in the
Electrical Characteristics . The most negative output
voltage occurs for the largest value of REF - LFB.
The MAX753’s combination boost converter and
charge-pump inverter was chosen over a conventional
buck-boost inverter because it allows the use of low-
cost N-channel MOSFETs instead of more expensive P-
channel ones. Additionally, its efficiency is 5% to 10%
better than a standard buck-boost inverter.
0.22 F
INPUT
33 H
C4
L2
CCFL Backlight and
Q3
C15
1 F
V
OUT
1N5819
C2
10 F
D8
REF
ALTERNATE
D8 CONNECTION
(SEE TEXT)
2.2
R19
1
1N5819
R3
R4
D3
R3
R4
REF
NEGATIVE
LCD-BIAS
OUTPUT
V
C6
10 F
35V
DD
LFB
15