Figure 5 shows the frequency response of the resonant

tank’s voltage gain under different load conditions.

The primary series capacitor is 1µF, the secondary par-

allel capacitor is 15pF, the transformer turns ratio is

1:93, and the secondary leakage inductance is 260mH.

nant peak determined by the secondary leakage

inductance (L) and the series capacitor reflected to the

determined by the secondary leakage inductance (L),

The inverter is designed to operate between these two

resonant peaks. When the lamp is off, the operating

point of the resonant tank is close to the parallel resonant

peak due to the lamp’s infinite impedance. The circuit

displays the characteristics of a parallel-loaded resonant

converter. While in parallel-loaded resonant operation,

the inverter behaves like a voltage source to generate

the necessary striking voltage. Theoretically, the output

voltage of the resonant converter will increase until the

lamp is ionized or until it reaches the IC’s secondary volt-

age limit, without regard to the transformer turns ratio or

the input voltage level. Once the lamp is ionized, the

equivalent load resistance decreases rapidly and the

operating point moves toward the series resonant peak.

While in series resonant operation, the inverter behaves

like a current source.

Lamp-Current Regulation

The MAX8722 uses a lamp-current control loop to regu-

late the current delivered to the CCFL. The heart of the

control loop is a transconductance error amplifier. The

AC lamp current is sensed with a resistor connected in

series with the low-voltage terminal of the lamp. The

voltage across this resistor is fed to the IFB input and is

internally full-wave rectified. The transconductance

error amplifier compares the rectified IFB voltage with a

790mV (typ) internal threshold to generate an error cur-

rent. The error current charges and discharges a

capacitor connected between COMP and ground to

pared with an internal ramp signal to set the high-side

1

2π L

f

LC

S

S

'

=

1

2π

Low-Cost CCFL Backlight Controller

14

______________________________________________________________________________________

AC

SOURCE

CCFL

L

1:N

(a)

AC

SOURCE

L

(b)

N

2

Figure 4. Equivalent Resonant Tank Circuit

FREQUENCY (kHz)

80

60

40

20

1

2

3

4

0

0

100

Figure 5. Frequency Response of the Resonant Tank