MAX754ESE ,CCFL Backlight and LCD Contrast ControllersGeneral Description ________
MAX755CPA ,-5V/Adjustable, Negative-Output,Inverting, Current-mode PWM RegulatorsApplications
Board-Level DC-DC Conversion
Battery-Powered Equipment
Computer Peripherals
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MAX755CSA ,-5V/Adjustable, Negative-Output,Inverting, Current-mode PWM Regulators19-0017; Rev 1; 9/92
-5 V/Adjustable,
Inverting, Curren
MAX755CSA ,-5V/Adjustable, Negative-Output,Inverting, Current-mode PWM RegulatorsGeneral Description
The MAX735 and MAX755 are CMOS, inverting switch-
mode regulators with inte ..
MAX755CSA ,-5V/Adjustable, Negative-Output,Inverting, Current-mode PWM RegulatorsFeatures
. Converts +2.7V to +9V In
Negative Output (MAX75
t" to Adjustable
. Converts +4 ..
MAX755CSA+ ,-5V, Inverting, PWM DC-DC Converterfeatures, along with high efficiency and appli-
cations circuits that lend themselves to miniaturi ..
MB6M ,MINIATURE GLASS PASSIVATED SINGLE-PHASE BRIDGE RECTIFIERThermal Characteristics (TA = 25°C unless otherwise noted)Parameter Symbol MB2M MB4M MB6M UnitDevic ..
MB6S ,Bridge RectifiersThermal Characteristics (T = 25°C unless otherwise noted)AParameter Symbol MB2S MB4S MB6S UnitDevic ..
MB7117E , Schottky TTL 2048-Bit Bipolar Programmable Read-Only Memory
MB71A38-25 , PROGRAMMABLE SCHOTTKY 16384-BIT READ ONLY MEMORY
MAX753CSE-MAX753ESE-MAX754CPE-MAX754CSE-MAX754ESE
CCFL Backlight and LCD Contrast Controllers
_______________General DescriptionThe MAX753/MAX754 drive cold-cathode fluorescent
lamps (CCFLs) and provide the LCD backplane bias
(contrast) power for color or monochrome LCD panels.
These ICs are designed specifically for backlit note-
book-computer applications.
Both the backplane bias and the CCFL supply can be
shut down independently. When both sections are shut
down, supply current drops to 25µA. The LCD contrast
and CCFL brightness can be adjusted by clocking sep-
arate digital inputs or using external potentiometers.
LCD contrast and backlight brightness settings are pre-
served in their respective counters while in shutdown.
On power-up, the LCD contrast counter and CCFL
brightness counter are set to one-half scale.
The ICs are powered from a regulated 5V supply. The
magnetics are connected directly to the battery, for
maximum power efficiency.
The CCFL driver uses a Royer-type resonant architec-
ture. It can provide from 100mW to 6W of power to one
or two tubes. The MAX753 provides a negative LCD
bias voltage; the MAX754 provides a positive LCD bias
voltage.
________________________ApplicationsNotebook Computers
Palmtop Computers
Pen-Based Data Systems
Personal Digital Assistants
Portable Data-Collection Terminals
____________________________FeaturesDrives Backplane and Backlight4V to 30V Battery Voltage RangeLow 500µA Supply CurrentDigital or Potentiometer Control of CCFL
Brightness and LCD Bias VoltageNegative LCD Contrast (MAX753)Positive LCD Contrast (MAX754)Independent Shutdown of Backlight and
Backplane Sections25µA Shutdown Supply Current
______________Ordering Information* Contact factory for dice specifications.
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
________________________________________________________________Maxim Integrated Products1
__________________Pin Configuration
Call toll free 1-800-998-8800 for free samples or literature.19-0197; Rev 1; 1/95
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VDD= 5V, BATT = 15V, CON = LON = 5V, LX = GND = PGND = 0V, IREF= 0mA, all digital input levels are 0V or 5V, = TMINto TMAX, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VDDto GND.................................................................-0.3V, +7V
PGND to GND.....................................................................±0.3V
BATT to GND.............................................................-0.3V, +36V
LX to GND............................................................................±50V
CS to GND.....................................................-0.6V, (VDD+ 0.3V)
Inputs/Outputs to GND (LADJ, CADJ, LON,
CON, REF, CFB, CC, CDRV, LDRV, LFB).....-0.3V, (VDD+ 0.3V)
Continuous Power Dissipation (TA= +70°C)
Plastic DIP (derate 10.53mW/°C above +70°C)...........842mW
Narrow SO (derate 8.70mW/°C above +70°C).............696mW
Operating Temperature Ranges
MAX75_C_ _........................................................0°C to +70°C
MAX75_E_ _......................................................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
_______________________________________________________________________________________3
Note 1:Maximum shutdown current occurs at BATT = LX = 0V.
Note 2:Timing specifications are guaranteed by design and not production tested.
ELECTRICAL CHARACTERISTICS (continued)(VDD= 5V, BATT = 15V, CON = LON = 5V, LX = GND = PGND = 0V, IREF= 0mA, all digital input levels are 0V or 5V, = TMINto TMAX, unless otherwise noted.)
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers_______________________________________________________________________________________
______________________________________________________________Pin Description
_______________Theory of Operation
CCFL InverterThe MAX753/MAX754’s CCFL inverter is designed to
drive one or two cold-cathode fluorescent lamps
(CCFLs) with power levels from 100mW to 6W. These
lamps commonly provide backlighting for LCD panels
in portable computers.
Drive Requirements for CCFL Tubes CCFL backlights require a high-voltage, adjustable AC
power source. The MAX753/MAX754 generate this AC
waveform with a self-oscillating, current-fed, parallel
resonant circuit, also known as a Royer-type oscillator.
Figure 1 shows one such circuit. The Royer oscillator is
comprised of T1, C9, the load at the secondary, Q4,
and Q5. The circuit self-oscillates at a frequency deter-
mined by the effective primary inductance and capaci-
tance. Q4 and Q5 are self-driven by the extra winding.
The current source feeding the Royer oscillator is com-
prised of L1, D5, and the MAX758A. When current from
the current source increases, so does the lamp current.
The lamp current is half-wave rectified by D7A and
D7B, and forms a voltage across resistor R8. The
MAX753’s error amplifier compares the average of this
voltage to the output of its internal DAC. Adjusting the
DAC output from zero scale to full scale (digital control)
causes the error amplifier to vary the tube current from
a minimum to a maximum. The DAC’s transfer function
is shown in Figure 2.
On power-up or after a reset, the counter sets the DAC
output to mid scale. Each rising edge of CADJ (with
CON high) decrements the DAC output. When decre-
mented beyond full scale, the counter rolls over and
sets the DAC to the maximum value. In this way, a sin-
gle pulse applied to CADJ decreases the DAC set-
point by one step, and 31 pulses increase the set-point
by one step.
The error amplifier’s output voltage controls the peak
current output of the MAX758A. The peak switch cur-
rent is therefore controlled by the output of the error
amplifier. The lower the error amplifier’s output, the
lower the peak current. Since the current through the
current source is related to the current through the
tube, the lower the error amplifier’s output, the lower the
tube current.
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
_______________________________________________________________________________________5Figure 1. CCFL and Positive LCD Power Supply
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers_______________________________________________________________________________________In Figure 1, the MAX758A, L1, and D5 form a voltage-
controlled switch-mode current source. The current out
of L1 is proportional to the voltage applied to the SS
pin. The MAX758A contains a current-mode pulse-
width-modulating buck regulator that switches at
170kHz. The voltage on the SS pin sets the switch cur-
rent limit and thus sets the current out of L1.
CCFL Current-Regulation LoopFigure 3 shows a block diagram of the regulation loop,
which maintains a fixed CCFL average lamp current
despite changes in input voltage and lamp impedance.
This loop regulates the average value of the half-wave
rectified lamp current. The root mean square lamp cur-
rent is related to, but not equal to, the average lamp
current. Assuming a sinusoidal lamp current, select R8
as follows:
where VREF= 1.25V and ILAMP,RMSis the desired full-
scale root mean square lamp current.
Figure 2. CCFT DAC Transfer Function
Figure 3. CCFL Tube Current-Regulation Loop
The minimum operating input voltage is determined by
the transformer turns ratio (n), the lamp operating volt-
age (VLAMP), 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 ILAMP(t) =
√2ILAMP,RMScos(wt) and VLAMP(t) = √2VLAMP,RMS
cos(wt); then the peak voltage at the center tap will be
as follows:
where,
n is the secondary-to-primary turns ratio of T1, and w 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 VTAPmust 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 VTAP.
LCD Bias GeneratorsThe 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 SchemeThe 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).
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
_______________________________________________________________________________________7Figure 4. Simple Model of the CCFLFigure 5. Voltage at the Center Tap of T1
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers_______________________________________________________________________________________Figure 6. MAX754 Positive LCD-Bias Generator
Table 1. CCFL Circuit Component Descriptions
