MAX753 Maxim, MAX753 Datasheet - Page 6

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

CCFL Backlight and
LCD Contrast Controllers
Figure 2. CCFT DAC Transfer Function
Figure 3. CCFL Tube Current-Regulation Loop
6
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1250
1221
1191
811
782
753
402
372
343
ZERO SCALE
0 1
2
3
FULL-SCALE = 1.250V
HALF-SCALE = 0.782V
ZERO-SCALE = 0.343V
MAX754
MID SCALE
DAC CODE
14 15 16
CFB
5-BIT VOLTAGE
5-BIT COUNTER
OUTPUT DAC
ERROR
AMPLIFIER
LOGIC AND
29 30 31
CON
FULL SCALE
CADJ
CC
In Figure 1, the MAX758A, L1, and D5 form a voltage-
controlled switch-mode current source. The current out
of L1 is proportional to the voltage applied to the SS
pin. The MAX758A contains a current-mode pulse-
width-modulating buck regulator that switches at
170kHz. The voltage on the SS pin sets the switch cur-
rent limit and thus sets the current out of L1.
Figure 3 shows a block diagram of the regulation loop,
which maintains a fixed CCFL average lamp current
despite changes in input voltage and lamp impedance.
This loop regulates the average value of the half-wave
rectified lamp current. The root mean square lamp cur-
rent is related to, but not equal to, the average lamp
current. Assuming a sinusoidal lamp current, select R8
as follows:
where V
scale root mean square lamp current.
C5
R18
REF
SS
R
8
= 1.25V and I
CENTER-TAP
OSCILLATOR
TRANSISTOR
EMITTERS
ROYER
MAX758A
2
I
BUCK
VOLTAGE CONTROLLED
I
LAMP RMS
CURRENT SOURCE
V
SWITCH-MODE
CCFL Current-Regulation Loop
REF
,
LAMP,RMS
C10
R8
is the desired full-