MAX754ESE Maxim Integrated Products, MAX754ESE Datasheet - Page 7

• Manufacturer Part Number: MAX754ESE
• Description: Display Drivers CCFL Backlight & LCD Neg Contrast Ctlr
• Manufacturer: Maxim Integrated Products

Specifications of MAX754ESE

• Lead Free Status / Rohs Status: No

Figure 4. Simple Model of the CCFL
The minimum operating input voltage is determined by
the transformer turns ratio (n), the lamp operating volt-
age (V
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
the current through it. Let us define I
√2I
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
at V
of the MAX758A, since there cannot be any DC voltage
on inductor L1; thus the minimum operating voltage
must be greater than the average voltage at V
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
LAMP,RMS
TAP
LAMP
I
must equal the average voltage at the LX node
LAMP
V
(t)
φ
), and the ballast capactor (C10). Using a
TAP PK
=
cos(ωt) and V
tan
,
_______________________________________________________________________________________
−
1
= −
⎛
⎜
⎝
ω
C V
−
V
10
I
SEC
LAMP RMS
2
n C
(t)
LAMP RMS
ω
LCD Bias Generators
LAMP
I
LAMP RMS
,
10
,
sin
(t) = √2V
,
φ ( )
⎞
⎟
⎠
C10
V
LAMP
LAMP
,
LAMP,RMS
(t)
TAP
.
(t) =
LCD Contrast Controllers
pulse-frequency-modulation (PFM) switching regulator.
The MAX753 adds a simple diode-capacitor voltage
inverter to the switching regulator.
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).
Figure 5. Voltage at the Center Tap of T1
V
TAP, PK
V
TAP
CCFL Backlight and
(t)
Constant-Current PFM Control Scheme
T
t
7