specified in the Electrical Characteristics table. To set

the RMS lamp current to 6mA, the value of R1 should

be 148Ω. The closest standard 1% resistors are 147Ω

and 150Ω. The precise shape of the lamp-current

waveform, which is dependent on lamp parasitics, influ-

ences the actual RMS lamp current. Use a true RMS

current meter connected between the R1/IFB junction

and the low-voltage side of the lamp to make final

adjustments to R1.

Setting the Secondary Voltage Limit

The MAX8722 limits the transformer secondary voltage

during startup and lamp-out faults. The secondary volt-

age is sensed through the capacitive voltage-divider

formed by C3 and C4 (Figure 1). The voltage on VFB is

proportional to the CCFL voltage. The selection of

the parallel resonant capacitor C3 is described in

the Transformer Design and Resonant Component

Selection section. C3 is usually between 10pF and

22pF. After the value of C3 is determined, select C4

using the following equation to set the desired maxi-

where 2.3V is the typical value of the VFB overvoltage

threshold specified in the Electrical Characteristics table.

To set the maximum RMS secondary voltage to 1600V

using 18pF for C3, use approximately 15nF for C4.

Setting the Secondary Current Limit

The MAX8722 limits the secondary current even if the

IFB sense resistor (R1) is shorted or transformer sec-

ondary current finds its way to ground without passing

through R1. ISEC monitors the voltage across the

sense resistor R3, connected between the low-voltage

terminal of the transformer secondary winding and

ground. Determine the value of R3 using the following

equation:

transformer secondary current during fault conditions,

and 1.217V is the typical value of the ISEC regulation

point specified in the Electrical Characteristics table. To

set the maximum RMS secondary current in the circuit

of Figure 1 to 22mA, use approximately 40.2Ω for R3.

Transformer Design and Resonant

Component Selection

The transformer is the most important component of the

resonant tank circuit. The first step in designing the

transformer is to determine the turns ratio (N). The ratio

must be high enough to support the CCFL operating

voltage at the minimum supply voltage. N can be cal-

culated as follows:

input voltage. If the maximum RMS lamp voltage in nor-

mal operation is 650V and the minimum DC input volt-

age is 8V, the turns ratio should be greater than 90. The

turns ratio of the transformer used in the circuit of

Figure 1 is 93.

The next step in the design procedure is to determine

the desired operating frequency range. The MAX8722

is synchronized to the natural resonant frequency of the

resonant tank. The resonant frequency changes with

operating conditions, such as the input voltage, lamp

impedance, etc.; therefore, the switching frequency

varies over a certain range. To ensure reliable opera-

tion, the resonant frequency range must be within the

operating frequency range specified by the CCFL

transformer manufacturer. As discussed in the

Resonant Operation section, the resonant frequency

range is determined by the transformer secondary leak-

age inductance L, the primary series DC blocking

capacitor C2, and the secondary parallel resonant

capacitor C3. Since it is difficult to control the trans-

former leakage inductance, the resonant tank design

should be based on the existing secondary leakage

inductance of the selected CCFL transformer. Leakage-

inductance values can have large tolerance and signifi-

cant variations among different batches, so it is best to

work directly with transformer vendors on leakage-

inductance requirements. The MAX8722 works best

when the secondary leakage inductance is between

250mH and 350mH. The series capacitor C2 sets the

minimum operating frequency, which is approximately

two times the series resonant peak frequency. Choose:

C

N

fL

MIN

2

2

2

2

≤

××

π

N

V

V

LAMP RMS

IN MIN

.

()

()

≥

×

09

R

V

3

1 217

2

.

() _

=

×

C

V

V

C

LAMP RMS

MAX

4

2

23

3

.

() _

=

×

×

Low-Cost CCFL Backlight Controller

18

______________________________________________________________________________________