Rainbow Electronics MAX754 User Manual
Page 6
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
6
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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.
CCFL Current-Regulation Loop
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
REF
= 1.25V and I
LAMP,RMS
is the desired full-
scale root mean square lamp current.
R
V
I
REF
LAMP RMS
8
2
=
π
,
0 1
343
372
402
2
DAC CODE
ZERO SCALE
DAC OUTPUT VOLTAGE (mV)
MID SCALE
FULL SCALE
3
14 15 16
29 30 31
753
782
811
1191
1221
1250
Figure 2. CCFT DAC Transfer Function
MAX754
MAX758A
CC
LOGIC AND
5-BIT COUNTER
5-BIT VOLTAGE
OUTPUT DAC
SWITCH-MODE
VOLTAGE CONTROLLED
CURRENT SOURCE
FULL-SCALE = 1.250V
HALF-SCALE = 0.782V
ZERO-SCALE = 0.343V
CFB
ERROR
AMPLIFIER
CON
CADJ
SS
C10
C5
R8
R18
I
BUCK
CENTER-TAP
TRANSISTOR
EMITTERS
ROYER
OSCILLATOR
CCFL
Figure 3. CCFL Tube Current-Regulation Loop