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MAX8722C

Low-Cost CCFL Backlight Controller

Maxim Integrated 

Low-Cost CCFL Backlight Controller

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:

C2 ≤

N2

4 × π2

×

2

fMIN

×

L

where fMIN is the minimum operating frequency range.

In the circuit of Figure 1, the transformer’s turns ratio is

93 and its secondary leakage inductance is approxi-

mately 300mH. To set the minimum operating frequen-

cy to 45kHz, use 1μF for C2.

The parallel capacitor C3 sets the maximum operating

frequency, which is also the parallel resonant peak fre-

quency. Choose C3 with the following equation:

C3

≥

(4π2 ×

C2

fMAX2 × L ×

C2) − N2

In the circuit of Figure 1, to set the maximum operating

frequency to 65kHz, use 18pF for C3.

The transformer core saturation should also be consid-

ered when selecting the operating frequency. The pri-

mary winding should have enough turns to prevent

transformer saturation under all operating conditions.

Use the following expression to calculate the minimum

number of turns N1 of the primary winding:

N1 > DMAX × VIN(MAX)

BS × S × fMIN

where DMAX is the maximum duty cycle (approximately

0.4) of the high-side switches, VIN(MAX) is the maximum

DC input voltage, BS is the saturation flux density of the

core, and S is the minimal cross-section area of the core.

COMP Capacitor Selection

The COMP capacitor sets the speed of the current loop

that is used during startup, while maintaining lamp cur-

rent regulation, and during transients caused by chang-

ing the input voltage. The typical COMP capacitor value

is 0.01μF. Larger values increase the transient-response

delays. Smaller values speed up transient response, but

extremely small values can cause loop instability.

Other Components

The external bootstrap circuits formed by capacitors C5

and C6 in Figure 1 power the high-side MOSFET drivers.

Connect VDD to BST1/BST2 and couple BST1/BST2 to

LX1/LX2 through C5 and C6. C5 = C6 = 0.1μF or greater.

Layout Guidelines

Careful PC board (PCB) layout is important to achieve

stable operation. The high-voltage section and the

switching section of the circuit require particular atten-

tion. The high-voltage sections of the layout need to be

well separated from the control circuit. Most layouts for

single-lamp notebook displays are constrained to long

and narrow form factors, so this separation occurs natu-

rally. Follow these guidelines for good PCB layout:

1) Keep the high-current paths short and wide, espe-

cially at the ground terminals. This is essential for

stable, jitter-free operation and high efficiency.

2) Use a star-ground configuration for power and ana-

log grounds. The power and analog grounds should

be completely isolated—meeting only at the center

of the star. The center should be placed at the ana-

log ground pin (GND). Using separate copper

islands for these grounds may simplify this task.

Quiet analog ground is used for VCC, COMP, FREQ,

TFLT, and ILIM (if a resistive voltage-divider is

used).

3) Route high-speed switching nodes away from sen-

sitive analog areas (VCC, COMP, FREQ, TFLT, and

ILIM). Make all pin-strap control input connections

(ILIM, etc.) to analog ground or VCC rather than

power ground or VDD.

4) Mount the decoupling capacitor from VCC to GND

as close as possible to the IC with dedicated traces

that are not shared with other signal paths.

5) The current-sense paths for LX1 and LX2 to GND

must be made using Kelvin sense connections to

guarantee the current-limit accuracy.

6) Ensure the feedback connections are short and

direct. To the extent possible, IFB, VFB, and ISEC

connections should be far away from the high-volt-

age traces and the transformer.

7) To the extent possible, high-voltage trace clearance

on the transformer’s secondary should be widely

separated. The high-voltage traces should also be

separated from adjacent ground planes to prevent

lossy capacitive coupling.

8) The traces to the capacitive voltage-divider on the

transformer’s secondary need to be widely separated

to prevent arcing. Moving these traces to opposite

sides of the board can be beneficial in some cases.

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