Wide Brightness Range

CCFL Backlight Controllers

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19

The MAX1739/MAX1839 regulate the average current

through the CCFL. The current is sensed through the

sense resistor (R13) at CSAV. The voltage at CSAV is

the half-wave rectified representation of the current

through the lamp (Figure 10). The MAX1739/MAX1839

and are controlled by either the analog interface or the

SMBus interface. To set the maximum lamp current,

determine R13 as follows:

MINDAC and the wave shape influence the actual max-

imum RMS lamp current. Use an RMS current meter to

make final adjustments to R13.

Loop Compensation

used during startup, maintaining lamp current regula-

tion, and during transients caused by changing the

0.01µF. Larger values limit lamp current overshoot.

Smaller values speed up its response to changes in the

lamp current setting, but can lead to instability for

increase the delay to strike voltage in DPWM and can

cause loss of regulation in the extreme case. Note that

the waveform shape, overall efficiency, and the maxi-

mum necessary secondary transformer voltage. Low val-

using a CCFL with a large difference between its restrike

voltage and its operating voltage (characteristic of long

improves stability when the lamp’s operating voltage

drops with an increase in lamp current. However, low

Royer frequency, Royer Q value, and overall efficiency.

affects DPWM transients and operation in fault condi-

tions. If DPWM is not used, the voltage control loop

should only be active during fault conditions. The stan-

response and not cause excessive ringing at the begin-

ning of a DPWM pulse. Note that the worst-case fault

R4

TO

CTFB

R5

T1

L1

N2B

N2A

2

π

ω

N

2

-N

2

2

π

ω

2

D5A

D5B

CSAV

R13

T1

C6

CCFL

2

π

ω

2

π

ω

0.04304

R13

Figure 9. Transformer Primary/Secondary Voltage Relationship

Figure 10. Current-Sense Waveforms