MAX1610 Datasheet, PDF (18/20 Pages) Maxim Integrated Products – Digitally Controlled CCFL Backlight Power Supplies

Digitally Controlled CCFL Backlight
Power Supplies
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
0.68Ω ±10% to set a peak current limit of about 735mA.
Use a 107Ω ±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.
BATT = 15V, MINDAC = VL, SS = OPEN, CC = 0.1µF,
C2 = 15pF, MID-SCALE SETTING, D/A VALUE = 10000
4V
SS
VOLTAGE
0V
15V
T1
CENTER-TAP
VOLTAGE
0V
500µs/div
Figure 14. Chopped Waveforms
VIN
16
+ ^BATT
C9
MAX1610
5
MAX1611
SS
C4
12
VL
BST ¹⁴
15
LX
6
CC
C3
OTP ¹⁰
R5
C8
R6
4
SYNC
9
REF
8
MINDAC
11
CS
7
CSAV
13
GND
D3
C6
R7
L1
C7
D2
R3
R4
C2
CCFL
10
6
T1
1
2
34
5
D1
R2
C1
Q1
Q2
C5
D40
D41
R1
R40
Figure 15. Directly Regulating the CCFL Current
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