Wide Brightness Range

CCFL Backlight Controllers

18

______________________________________________________________________________________

a proven application that is useful for a wide range of

CCFL tubes and power ranges. Table 5 shows the rec-

ommended components for a 4.5W application.

MOSFETs

The MAX1739/MAX1839 require three external switches

to operate: N1, N2A, and N2B. N1 is the buck switch;

to minimize conduction losses (100m

Ω, 30V typ). Also

select a comparable-power Schottky diode for D1.

N2A/N2B are the Royer oscillator switches that drive

the transformer primary; select a dual-logic-level N-

tion losses (100m

Ω, 30V typ).

Sense Resistors

R4 and R5 sense the transformer’s primary voltage.

Figure 9 shows the relationship between the primary

and secondary voltage. To set the maximum average

secondary transformer voltage, set R5 = 10k

Ω, and

select R5 according to the following formula:

voltage (above the strike voltage), and N is the turns

ratio of the transformer.

RR

V

N

S RMS

45

15

1

=−









.

()

SPECIFICATION

SYMBOL

UNITS

DESCRIPTION

CCFL Minimum Strike

Voltage

(Kick-Off Voltage)

Although CCFLs typically operate at <550VRMS, a higher voltage

voltage is typically higher at cold temperatures and at the tube’s end

of life. This voltage is set by the combination of the maximum primary

voltage (center-tap voltage limit corresponding to

CCFL Typical Operating

Voltage

(Lamp Voltage)

VL

Once a CCFL has been struck, the voltage required to maintain light

constant, even as the tube’s brightness is varied.

CCFL Maximum Operating

Current (Lamp Current)

IL

DC current is not allowed through CCFLs. The maximum lamp current

is set by the sense resistor (R13) at the maximum brightness setting.

CCFL Maximum Frequency

(Lamp Frequency)

fL

kHz

The maximum AC-lamp-current frequency. The MAX1739/ MAX1839

synchronize to the Royer oscillator frequency set by the external

components and are designed to operate between 32kHz and

100kHz.

DESIGNATION

DESCRIPTION

RECOMMENDED DEVICE

MANUFACTURER

L1

47µH, 1.1A inductor

CR104-470

Sumida

N1

30V, 0.1Ω N-channel MOSFET

FDN361AN

Fairchild

N2

30V, 95mΩ dual N-channel MOSFET

FDC6561AN

Fairchild

T1

8.7µH, 180:1 transformer

5371-T001 (CIUH842 style)

Sumida

D1

30V, 1A Schottky diode

CRS02

Toshiba

D2

0.1A Schottky diode

BAT54

Fairchild

D3

0.1A dual Schottky diode

MMBD4148SE

Fairchild

C6

22pF, 3.1kV capacitor

GHM1038-SL-220J-3K

Murata

C7

0.1µF, 63V, low-dissipation capacitor

SMD1812

WIMA

Table 4. CCFL Specifications

Table 5. Components for the Standard Application Circuit