Rainbow Electronics MAX1839 - Manual (Page 18)

MAX1739/MAX1839
†
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;
select a logic-level N-channel MOSFET with low R
DSON
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-
channel MOSFET with low R
DSON to minimize conduc-
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:
where V
S
is the maximum RMS secondary transformer
voltage (above the strike voltage), and N is the turns
ratio of the transformer.
R R
V
N
S RMS
4 5
1 5
1 = −

 
 .
( )
SPECIFICATION SYMBOL UNITS DESCRIPTION
CCFL Minimum Strike
Voltage
(Kick-Off Voltage)
V S V RMS
Although CCFLs typically operate at <550VRMS, a higher voltage
(1000V
RMS and up) is required initially to start the tube. The strike
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
V
CTFB = 0.6V) and the transformer (T1) turns ratio.
CCFL Typical Operating
Voltage
(Lamp Voltage)
VL V RMS
Once a CCFL has been struck, the voltage required to maintain light
output falls to approximately 550V
RMS . Short tubes may operate on as
little as 250V RMS . The CCFL operating voltage stays relatively
constant, even as the tube’s brightness is varied.
CCFL Maximum Operating
Current (Lamp Current)
IL mA RMS
The maximum RMS AC current through a CCFL is typically 5mA RMS .
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