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MAX8722C

Low-Cost CCFL Backlight Controller

Maxim Integrated 

Low-Cost CCFL Backlight Controller

Transformer Secondary Voltage Limiting

The MAX8722C reduces the voltage stress on the trans-

former’s secondary winding by limiting the secondary

voltage during startup and open-lamp faults. The AC

voltage across the transformer secondary winding is

sensed through a capacitive voltage-divider. The small

voltage across the larger capacitor of the divider is fed

to the VFB input and is internally half-wave rectified. An

overvoltage comparator compares the VFB voltage with

a 2.3V (typ) internal threshold. Once the sense voltage

exceeds the overvoltage threshold, the MAX8722C

turns on a 1100μA current source that discharges the

COMP capacitor. The high-side MOSFET on-time short-

ens as the COMP voltage decreases, reducing the

transformer secondary’s peak voltage below the thresh-

old set by the capacitive voltage-divider.

Lamp Startup

A CCFL is a gas discharge lamp that is normally driven

in the avalanche mode. To start ionization in a nonion-

ized lamp, the applied voltage (striking voltage) must

be increased to the level required for the start of

avalanche. At low temperatures, the striking voltage

can be several times the typical operating voltage.

Because of the MAX8722C’s resonant topology, the strik-

ing voltage is guaranteed. Before the lamp is ionized, the

lamp impedance is infinite. The transformer secondary

leakage inductance and the high-voltage parallel capaci-

tor determine the unloaded resonant frequency. Since

the unloaded resonant circuit has a high Q, it can gener-

ate very high voltages across the lamp.

Upon power-up, VCOMP slowly rises, increasing the

duty cycle of the high-side MOSFET switches and pro-

viding a measure of soft-start.

Feed-Forward Control and

Dropout Operation

The MAX8722C is designed to maintain tight control of

the lamp current under all transient conditions. The

feed-forward control instantaneously adjusts the on-

time for changes in input voltage (VBATT). This feature

provides immunity to input-voltage variations and sim-

plifies loop compensation over wide input voltage

ranges. The feed-forward control also improves the line

regulation for short on-times and makes startup tran-

sients less dependent on the input voltage.

Feed-forward control is implemented by increasing

the internal voltage ramp rate for higher VBATT. This

has the effect of varying tON as a function of the in-

put voltage while maintaining approximately the same

signal levels at VCOMP. Since the required voltage

change across the compensation capacitor is minimal,

the controller’s response to input voltage changes is

essentially instantaneous.

DPWM Dimming Control

The MAX8722C controls the brightness of the CCFL by

chopping the lamp current on and off using a low-fre-

quency (between 100Hz and 350Hz) DPWM signal

either from the internal oscillator or from an external sig-

nal source. The CCFL brightness is proportional to the

DPWM duty cycle, which can be adjusted from 9.766%

to 100% by the CNTL pin. CNTL is an analog input with

a usable input voltage range between 0 and 2000mV,

which is digitized to select one of 128 brightness levels.

As shown in Figure 6, the MAX8722C ignores the first

25 steps, so the first 25 steps all represent the same

brightness. When VCNTL is between 0 and 195.3mV,

the DPWM duty cycle is always 9.766%. When VCNTL is

above 195.3mV, a 7.8125mV change on CNTL results

in a 0.3906% change in the DPWM duty cycle. When

VCNTL is equal to or above 2000mV, the DPWM duty

cycle is always 100%.

100

90

80

60

70

50

40

30

20

10

0

0

400 800 1200 1600 2000

CONTROL VOLTAGE (mV)

Figure 6. Theoretical Brightness vs. Control Voltage

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