Wide Brightness Range

CCFL Backlight Controllers

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17

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,

low. To detect this, the MAX1739/MAX1839 check that

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

(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