Rainbow Electronics MAX754 User Manual
Page 7
The minimum operating input voltage is determined by
the transformer turns ratio (n), the lamp operating volt-
age (V
LAMP
), and the ballast capactor (C10). Using a
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
LAMP
(t) =
√
2I
LAMP,RMS
cos(
ω
t) and V
LAMP
(t) =
√
2V
LAMP,RMS
cos(
ω
t); 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
TAP
must equal the average voltage at the LX node
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
TAP
.
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
C V
LAMP RMS
LAMP RMS
V
I
n C
TAP PK
LAMP RMS
,
,
sin
= −
2
10
ω
φ
( )
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
_______________________________________________________________________________________
7
Figure 4. Simple Model of the CCFL
Figure 5. Voltage at the Center Tap of T1
C10
V
LAMP
(t)
V
SEC
(t)
I
LAMP
(t)
V
TAP, PK
T
V
TAP
(t)
t