Wide Brightness Range

CCFL Backlight Controllers

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15

See Digital Interface for instructions on using the

SMBus interface.

Synchronizing the DPWM Frequency

MODE has two functions: one is to select the interface

mode as described in Interface Selection, and the other

is to synchronize the DPWM “chopping” frequency to

an external signal to prevent unwanted effects in the

display screen.

To synchronize the DPWM frequency, connect MODE

to VL, REF, or GND through a 10k

Ω resistor. Then con-

nect a 500pF capacitor from an AC signal source to

MODE as shown in Figure 6. The synchronization range

is from 32kHz to 100kHz, which corresponds to a

DPWM frequency range of 250Hz to 781Hz (128 MODE

pulses per DPWM cycle). High DPWM frequencies limit

the dimming range. See Loop Compensation for more

information concerning high DPWM frequencies.

Royer Oscillator MOSFET Drivers

The MAX1739/MAX1839 directly drive the two external

MOSFETs used in the Royer oscillator. This has many

advantages over the traditional method that uses bipo-

lar switching and an extra winding on the transformer.

Directly driving the MOSFET eliminates the need for an

extra winding on the transformer, which reduces cost

and minimizes the size of the transformer. Also, driving

the switches directly improves commutation efficiency

and commutation timing. Using MOSFETs for the

switches typically improves overall inverter efficiency

due to lower switch drops.

The Royer topology works as a zero voltage crossing

(ZVC) detector and switches currents between the two

sections of the transformer primary windings. The two

windings work alternately, each generating a half wave

that is transferred to the secondary to produce the full-

wave sinusoidal lamp voltage and current. The

MAX1739/MAX1839 detect the zero crossing through

the SYNC pin; the threshold is set at 500mV referred to

CS and has a typical delay of 50ns. The active switch-

ing forces commutation very close to the ZVC point and

has better performance than the traditional winding-

based ZVC switchover. Commutation can be further

BRIGHTNESS

POSITIVE-

SCALE ADC

NEGATIVE-

SCALE ADC

SMBus

DAC

OUTPUT

DPWM DUTY

CYCLE

(%)

COMBINED

POWER

LEVEL (%)

Maximum

Brightness

MODE = GND,

MODE = REF,

Bright [4:0] =

ob11111

Full-scale

DAC OUTPUT =

195.83mV

100

100

Minimum

Brightness

MODE = GND,

VREF/3

MODE = REF,

Bright [4:0] =

ob00000,

VREF/3

Zero-scale

DAC OUTPUT =

93

Table 3. Brightness Adjustment Ranges (for 33:1 Dimming)

Note: The current-level range is solely determined by the MINDAC-to-REF ratio and is externally set.

31

30

29

3

2

1

0

1

33

2

33

3

33

4

33

(MODE = GND)

(MODE = REF)

30

33

31

33

32

33

1

32

33

31

33

30

33

29

33

3

33

2

33

1

33

0

1

Figure 5. Brightness Code