Rainbow Electronics MAX1839 - Manual (Page 17)

MAX1739/MAX1839
†
Wide Brightness Range
CCFL Backlight Controllers
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Figure 8. Standard Application Circuit
increases the circuit’s demand for current but may not
be enough to blow the fuse. With the buck switch short-
ed, the center tap rises above its regulation point,
which causes the CCV amplifier’s output (V
CCV ) to go
low. To detect this, the MAX1739/MAX1839 check that
V
CCV is below 1V at the end of every DPWM period. If
this condition persists for over 250ms (or 64 DPWM
pulses), the inverter switch commutation is stopped
with either DL1 or DL2 on. With the buck switch short-
ed, this will cause a short circuit with enough current to
blow the fuse. If the buck switch is not shorted, then the
inverter latches off as in a lamp-out condition.
Both buck-switch short and lamp-out detection will
clear the STATUS1 bit in the SMBus interface. STA-
TUS1 does not clear immediately but will clear about 2
seconds after the inverter has been forced off (see
Digital Interface).
Note that once the inverter board fuse has blown,
SMBus communications with the part will cease since
the MAX1739 will then be without power.
Applications Information
As shown in the standard application circuit (Figure 8),
the MAX1739/MAX1839 regulate the current of a 4.5W
CCFL. The IC’s analog voltage interface sets the lamp
brightness with a minimum 20:1 power adjustment
range. This circuit operates from a wide supply-voltage
range of 7V to 24V. Typical applications include note-
book, desktop monitor, and car navigation displays.
CCFL Specifications
To select the correct component values for the
MAX1739/MAX1839 circuit, several CCFL parameters
(Table 4) and the minimum DC input voltage must be
specified.
Royer Oscillator
Components T1, C6, C7, N2A, and N2B form the Royer
oscillator. A Royer oscillator is a resonant tank circuit
that oscillates at a frequency dependent on C7, the pri-
mary magnetizing inductance of T1 (LP), and the
impedance seen by the T1 secondary. Figure 8 shows
BATT
V IN
(5V TO 28V)
C9
4.7 µF C3
C4
C6
C7
N2B
R13
N2A
D5
T1
N1 D2
D1
R4
R5
C5 L1
C2
C1
VL
VL
DHI
BST
LX
SYNC
CTFB
DL2
DL1
GND
CS
CSAV
CCV
CCI
REF
MINDAC
CRF/SDA
DIMMING
ON/OFF
CTL/SCL
SH/SUS
MAX1739
MAX1839
MODE