The minimum operating input voltage is determined by

the transformer turns ratio (n), the lamp operating volt-

simple model of the CCFL (see Figure 4) we can calcu-

late what the T1 center-tap voltage will be at maximum

lamp current. The voltage on the CCFL is in phase with

cos(

ωt); then the peak voltage at the center tap will be

as follows:

where,

,

n is the secondary-to-primary turns ratio of T1, and

ω is

the frequency of Royer oscillation in radians per sec-

ond. The voltage on the center tap of T1 is a full-wave

rectified sine wave (see Figure 5). The average voltage

of the MAX758A, since there cannot be any DC voltage

on inductor L1; thus the minimum operating voltage

LCD Bias Generators

The MAX753/MAX754’s LCD bias generators provide

adjustable output voltages for powering LCD displays.

The MAX753’s LCD converter generates a negative

output, while the MAX754’s generates a positive output.

The MAX753/MAX754 employ a constant-peak-current

pulse-frequency-modulation (PFM) switching regulator.

The MAX753 adds a simple diode-capacitor voltage

inverter to the switching regulator.

Constant-Current PFM Control Scheme

The LCD bias generators in these devices use a con-

stant-peak-current PFM control scheme. Figure 6, which

shows the MAX754’s boost switching regulator, illus-

trates this control method. When Q3 closes (Q3 “on”) a

voltage equal to BATT is applied to the inductor, caus-

ing current to flow from the battery, through the inductor

and switch, and to ground. This current ramps up linear-

ly, storing energy in the inductor’s magnetic field. When

Q3 opens, the inductor voltage reverses, and current

flows from the battery, through the inductor and diode,

and into the output capacitor. The devices regulate the

output voltage by varying how frequently the switch is

opened and closed.

The MAX753/MAX754 not only regulate the output volt-

age, but also maintain a constant peak inductor cur-

rent, regardless of the battery voltage. The ICs vary the

switch on-time to produce the constant peak current,

and vary its off-time to ensure that the inductor current

reaches zero at the end of each cycle.

The internal circuitry senses both the output voltage

and the voltage at the LX node, and turns on the MOS-

FET only if: 1) The output voltage is out of regulation,

and 2) the voltage at LX is less than the battery voltage.

The first condition keeps the output in regulation, and

the second ensures that the inductor current always

resets to zero (i.e., the part always operates in discon-

tinuous-conduction mode).

φ

ω

=

−













−

tan

,

,

1

10

I

CV

LAMP RMS

LAMP RMS

V

I

nC

TAP PK

LAMP RMS

,

,

sin

=−

2

10

ωφ

()

CCFL Backlight and

LCD Contrast Controllers

_______________________________________________________________________________________

7

Figure 4. Simple Model of the CCFL

Figure 5. Voltage at the Center Tap of T1

C10

T

t