Rainbow Electronics MAX1611 - Manual (Page 18)

MAX1610/MAX1611
minimum 9% duty cycle. DAC code 00000 shuts off the
lamp entirely (0% duty cycle). Figure 14 shows the
chopped waveforms with the DAC set to mid-scale.
__________ Applications Information
Directly Regulating the Lamp Current
The MAX1610/MAX1611 can directly regulate the CCFL
current by tapping into the secondary of T1 (Figure 15).
This allows more precise setting of the maximum lamp
current (I
L
). The disadvantage of this approach is that
the secondary-to-ground voltage is twice that shown in
Figure 4, increasing the likelihood of the thermometer
effect, where one end of the lamp is brighter than the
other. Figure 15 uses the same component values as
Figure 4, except for R1, R40, D40, and D41. D40 and
D41 are the same type of diode as D1. R1 should be
0.68
Ω
±10% to set a peak current limit of about 735mA.
Use a 107
Ω
±1% resistor for R40 to set a lamp current
of 5mA RMS . This circuit accepts a wide range of lamp
types without component adjustments.
Component Suppliers
Table 7 lists three different sources for C1. C1 requires
a low dissipation factor to prevent overheating as energy
is cycled between C1 and the T1 magnetizing induc-
tance in the Royer resonant tank. Table 8 lists suppliers
for the high-voltage ballast capacitor, C2.
Digitally Controlled CCFL Backlight
Power Supplies
18 ______________________________________________________________________________________
4V
0V
0V
500 µ s/div
15V
SS
VOLTAGE
T1
CENTER-TAP
VOLTAGE
BATT = 15V, MINDAC = VL, SS = OPEN, CC = 0.1 µ F,
C2 = 15pF, MID-SCALE SETTING, D/A VALUE = 10000
Figure 14. Chopped Waveforms
Figure 15. Directly Regulating the CCFL Current
C9
C4
C3
C8
R5
R6
BATT
SS
CC
SYNC
REF
MINDAC GND
CSAV
CS
OTP
LX
BST
VL
D2
R3
R4
D1
D40
R40
D41
C5
MAX1610
MAX1611
C2
CCFL
C1
R1
R2
6
5 3 4 2 1
10
Q2 Q1
VIN
12
14
15
10
11
7
13
4
6
5
16
+
9
8
C7
L1
T1
C6 D3
R7