Wide Brightness Range

CCFL Backlight Controllers

20

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open tube at the beginning of DPWM pulses.

can cause loss of regulation at low DPWM duty cycles

by increasing the delay to strike voltage. Smaller values

make the circuit more susceptible to ringing, and in

extreme cases may cause instability. Some ringing is

expected between the Royer oscillator and the buck

inductor. Some of the ringing can be suppressed by

adding a capacitor in parallel with R5. This capacitor

should be chosen such that:

When using high DPWM frequencies and low DPWM

duty cycles, the DPWM on-time is reduced. In some

cases, this causes the lamp current transient to exceed

the DPWM on-time. In this case, the MAX1739/

MAX1839 lose regulation and the lamp current never

reaches the lamp current set point. Supply rejection

while operating in this condition is degraded. If the

DPWM on-time is short enough, the lamp current does

not have enough time to reach the lamp-out threshold

and causes a lamp-out detection. To prevent this,

decrease the turn-on transient duration (by lowering

brightness code), or decrease the DPWM frequency

(see Synchronizing the DPWM Frequency).

DPWM or other “chopping” methods can cause audible

noise from some transformers. The transformer should

be carefully designed to avoid such behavior.

Dimming Range

The external components required to achieve a dim-

ming range are highly dependent on the CCFL used.

The standard application circuit uses a CCFL with strin-

gent requirements. To achieve a 20:1 dimming range,

the standard circuit drops slightly more voltage across

C6 as it does across the CCFL at the full lamp current

setting. This ensures good stability in that circuit with

range when using this CCFL, C6 must be increased,

which increases the maximum secondary transformer

voltage and requires a transformer with a higher volt-

age rating. Other components (such as the primary

adjusted to maintain good waveforms, Royer efficiency,

and the desired Royer frequency.

Other Components

The high-side MOSFET driver is powered by the exter-

nal boosting circuit formed by C5 and D2. Connect BST

through a signal-level Schottky diode to VL, and

bypass it to LX with a 0.1µF ceramic capacitor. This cir-

cuit delivers the necessary power to drive N1 as shown

in Figure 8. If a higher gate capacitance MOSFET is

used, the size of the bypass capacitor must be

increased. The current need at BST is as follows:

where d is the buck controller duty cycle (98% max),

Royer oscillator frequency.

D5A and D5B are used to generate the current-sense

voltage across R13. The current through these diodes is

the lamp current; use a dual-series signal-level diode.

Bypassing and Board Layout

Connect C4 from VL to GND as close as possible with

dedicated traces that are not shared with other signal

paths. The ground lines should terminate at the GND

end of C4: quiet ground, power ground, and lamp cur-

rent-sense ground. Quiet ground is used for REF, CCV,

R5, and MINDAC (if a resistor-divider is used). The

power ground goes from the ground of C4 directly to

the ground side of C9. Power ground should also sup-

ply the return path for D1, N2, and the buck current-

sense resistor (from CS to GND, if used). The ground

path for R13 should be separate to ensure that it does

not corrupt quiet ground and it is not affected by DC

drops in the power ground. Refer to the MAX1739 EV

kit for an example of good layout.

Digital Interface (MAX1739)

With MODE connected to VL, the CRF/SDA and

CTL/SCL pins no longer behave as analog inputs;

instead, they function as SMBus-compatible 2-wire dig-

ital interfaces. CRF/SDA is the bidirectional data line,

and CTL/SCL is the clock line of the 2-wire interfaces

corresponding, respectively, to the SMBDATA and

SMBCLK lines of the SMBus. The MAX1739 uses the

write-byte, read-byte, and receive-byte protocols

(Figure 11). The SMBus protocols are documented in

System Management Bus Specification v1.08 and are

available at www.sbs-forum.org.

The MAX1739 is a slave-only device and responds to

the 7-bit address 0b0101101 (i.e., with the RW bit clear

indicating a write, this corresponds to 0x5A). The

MAX1739 has three functional registers: a 5-bit bright-

ness register (BRIGHT4–BRIGHT0), a 3-bit shutdown

mode register (SHMD2–SHMD0), and a 2-bit status

register (STATUS1–STATUS0). In addition, the device