Rainbow Electronics MAX754 User Manual
Page 15
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
______________________________________________________________________________________
15
MAX753
6-BIT DAC
PULSE-SKIP
COMPARATOR
16
13
R4
PRESET
6-BIT COUNTER
CLK
Q3
V
DAC
LFB
7
REF
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
D8
1N5819
NEGATIVE
LCD-BIAS
OUTPUT
ON/OFF
CONTROL
ON-TIME
LOGIC
OFF-TIME
LOGIC
R3
C2
10
µ
F
C15
1
µ
F
R19
2.2
Ω
C6
10
µ
F
35V
ALTERNATE
D8 CONNECTION
(SEE TEXT)
C4
0.22
µ
F
D3
1N5819
V
DD
Figure 8. MAX753 Negative LCD-Bias Generator
0 1
* DAC OUTPUT VOLTAGE = REF - LFB
625
635
645
2
DAC CODE
ZERO SCALE
DAC OUTPUT VOLTAGE (mV)*
MID SCALE
FULL SCALE
30 31 32
61 62 63
918
928
937
1220
1230
1240
Figure 9. MAX753 LCD DAC Transfer Function
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.
V
REF 1
R3
R4
REF LFB
OUT
=
−
+
−
(
)