MAXIM MAX753, MAX754 User Manual

_______________General Description

The 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 note­book-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 sep­arate digital inputs or using external potentiometers.
LCD contrast and backlight brightness settings are pre­served 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 architec­ture. 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.

________________________Applications

Notebook Computers

Palmtop Computers

Pen-Based Data Systems

Personal Digital Assistants

Portable Data-Collection Terminals

____________________________Features

♦ Drives Backplane and Backlight

♦ 4V to 30V Battery Voltage Range

♦ Low 500µA Supply Current

♦ Digital or Potentiometer Control of CCFL

Brightness and LCD Bias Voltage

♦ Negative LCD Contrast (MAX753)

♦ Positive LCD Contrast (MAX754)

♦ Independent Shutdown of Backlight and

Backplane Sections

♦ 25µA Shutdown Supply Current

______________Ordering Information

* Contact factory for dice specifications.

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

________________________________________________________________ Maxim Integrated Products 1

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

LFB

BATT

LX

LDRV

CON

LON

LADJ

V

DD

MAX753
MAX754

PGND

CDRV

CS

CC

CFB

REF

GND

CADJ

DIP/SO

TOP VIEW

__________________Pin Configuration

19-0197; Rev 1; 1/95

PART TEMP. RANGE

MAX753CPE

0°C to +70°C

MAX753CSE 0°C to +70°C

MAX753C/D 0°C to +70°C Dice*

16 Narrow SO

16 Plastic DIP

PIN-PACKAGE

MAX753EPE -40°C to +85°C

MAX753ESE -40°C to +85°C 16 Narrow SO

16 Plastic DIP

MAX754CPE

0°C to +70°C

MAX754CSE 0°C to +70°C

MAX754C/D 0°C to +70°C Dice*

16 Narrow SO

16 Plastic DIP

MAX754EPE -40°C to +85°C

MAX754ESE -40°C to +85°C 16 Narrow SO

16 Plastic DIP

Block Diagram located at end of data sheet.

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD= 5V, BATT = 15V, CON = LON = 5V, LX = GND = PGND = 0V, I

REF

= 0mA, all digital input levels are 0V or 5V,

T

A

= T

MIN

to T

MAX

, 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, (V

DD

+ 0.3V)
Inputs/Outputs to GND (LADJ, CADJ, LON,

CON, REF, CFB, CC, CDRV, LDRV, LFB) .....-0.3V, (V

DD

+ 0.3V)
Continuous Power Dissipation (T

A

= +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

Guaranteed monotonic

Maximum, CFB = 0V

Minimum, CFB = 5V

VCS= 0V

LON, CON, CADJ, LADJ; VDD= 5.5V

LON, CON, CADJ, LADJ; VDD= 4.5V

LDRV, CDRV;
VDD= 4.5V

No external load

4V < VDD< 6V

0µA < IL< 100µA

LDRV = CDRV = 2V

LON, CON, CADJ, LADJ; VIN= 0V or 5V

CONDITIONS

Bits5DAC Resolution

85 115

kHz

32 47

VCO Frequency

µA-5CS Input Bias Current

V1.2 1.3Overcurrent-Comparator Threshold Voltage (CS)

mV-10 20Zero-Crossing-Comparator Threshold Voltage (CS)

7

Ω

10

Driver On-Resistance

A0.5Driver Sink/Source Current

µA±1Input Leakage Current

V4.5 5.5VDDSupply Range

V430BATT Input Range

V2.4Input High Voltage

V0.8Input Low Voltage

µA25 40VDDShutdown Current

V1.21 1.25 1.29REF Output Voltage

%/V0.1REF Line Regulation

mV515REF Load Regulation

mA0.5 2VDDQuiescent Current

UNITSMIN TYP MAXPARAMETER

LON = CON = CS = LFB = CFB =
LADJ = CADJ = 5V

Output high

Output low

LON = CON = CS = LFB = CFB = LADJ
= CADJ = LX = BATT = 0V (Note 1)

SUPPLY AND REFERENCE

DIGITAL INPUTS AND DRIVER OUTPUTS

CCFT CONTROLLER

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, I

REF

= 0mA, all digital input levels are 0V or 5V,

T

A

= T

MIN

to T

MAX

, unless otherwise noted.)

At zero scale (code = 0)

At full scale (DAC code = 31)

At full scale (DAC code = 63)

Guaranteed monotonic

BATT = 4V, LX = 0V

BATT = 16V

Sink current, CFB = 5V, CC = 2.5V

Source current, CFB = 0V, CC = 2.5V

At zero scale (code = 0)

BATT = 4V

CONDITIONS

µA12 20LX Input Current

µA12 20BATT Input Current

nA±150LFB Input Leakage Current

595 625 655

893 928 963

mV

1200 1240 1280

MAX753 Feedback Voltage (REF-LFB)

Bits6DAC Resolution

µs35 70Switching Period

0.5 1.5

µs

25

Switch On-Time

745 782 820

mV

1210 1250 1290

Feedback Voltage (CFB)

200

µA

50

Feedback-Amplifier Output Current

V/µs0.4Feedback-Amplifier Slew Rate

320 343 365

nA±100Feedback-Amplifier Input Bias Current

MHz1Feedback-Amplifier Unity-Gain Bandwidth

UNITSMIN TYP MAXPARAMETER

At preset DAC, CON = 0V, CADJ = 5V
(code = 15)

ns100CADJ, LADJ High Width (tSH)

ns0Reset Hold Time (tRH)

ns0Reset Setup Time (tRS)

ns110Reset Pulse Width (tR)

At preset DAC, LON = 0V, LADJ = 5V
(code = 31)

LON = CON = CS = LFB = CFB = LADJ =
CADJ = LX = 0V

LON = CON = CS = LFB = CFB = LADJ =
CADJ = 0V, LX = BATT = 15V

ns100CADJ, LADJ Low Width (tSL)

ns50

CADJ Low to CON Low or
LADJ Low to LON Low (t

SD

)

At zero scale (code = 0)

At preset DAC, LON = 0V, LADJ = 5V
(code = 31)

At full scale (DAC code = 63)

610 635 660

905 938 971

mV

1210 1250 1290

MAX754 Feedback Voltage (LFB)

LCD CONTROLLER

TIMING (Note 2)

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

4 _______________________________________________________________________________________

______________________________________________________________Pin Description

Output of the CCFT Error AmplifierCC9

Connect to V

DD

CS10

Leave unconnectedCDRV11

Power Ground Connection for LDRVPGND12

Gate-Driver Output. Drives LCD backplane N-channel MOSFET.LDRV13

Digital Input for CCFT Brightness Adjustment. See Table 1.CADJ5

Analog GroundGND6

Reference Voltage Output, 1.25VREF7

Inverting Input for the CCFT Error AmplifierCFB8

Digital Input to Control CCFT Section. See Table 1.CON4

Digital Input to Control LCD Bias Section. See Table 1.LON3

PIN

Digital Input for LCD Backplane Bias Adjustment. See Table 1.LADJ2

5V Power-Supply InputV

DD

1

FUNCTIONNAME

LCD Backplane Inductor Voltage-Sense Pin. Used to sense inductor voltage for on time determination.LX14

Battery Connection. Used to sense battery voltage for on time determination.BATT15

Voltage Feedback for the LCD Backplane SectionLFB16

_______________Theory of Operation

CCFL Inverter

The 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 deter­mined by the effective primary inductance and capaci­tance. Q4 and Q5 are self-driven by the extra winding.
The current source feeding the Royer oscillator is com­prised 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 decre­mented beyond full scale, the counter rolls over and
sets the DAC to the maximum value. In this way, a sin­gle pulse applied to CADJ decreases the DAC set­point 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 cur­rent 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

_______________________________________________________________________________________ 5

MAX754CSE

MAX758ACWE

3,45

Q5

Q2

Q3

C4

C6

C8

C5

C7

Q4

C9

POSITIVE

CONTRAST

VOLTAGE

R10

2

61

812

T1

Q1

14

R1

C3C2

R16

LX

13

LDRV

12

PGND

16

LFB

6

GND

9

CC

2

LADJ

3

LON

5

D1B

CADJ

D1A

D2B

D2A

4

CON

10, 11

SS

GND

12, 13, 14

LX

7

L1

CS

10

V

DD

C1

1

CDRV

11

REF

7

CFB

8

SHDN

2

V+

1, 15, 16

REF

D5

3

CC

8

15

+5V, ±5%

UNREGULATED INPUT VOLTAGE

BATT

R2

R17

L2

D4

D3

D7B

D6B

D6A

D7A

+5V CMOS
LOGIC
CONTROL
SIGNALS

C10

R8

R4

R5

R6

R7

R18

R3

CCFL

Figure 1. CCFL and Positive LCD Power Supply

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

6 _______________________________________________________________________________________

In Figure 1, the MAX758A, L1, and D5 form a voltage­controlled 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 pulse­width-modulating buck regulator that switches at
170kHz. The voltage on the SS pin sets the switch cur­rent limit and thus sets the current out of L1.

CCFL Current-Regulation Loop

Figure 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 cur­rent is related to, but not equal to, the average lamp
current. Assuming a sinusoidal lamp current, select R8
as follows:

where V

REF

= 1.25V and I

LAMP,RMS

is the desired full-

scale root mean square lamp current.

R

V

I

REF

LAMP RMS

82=

π

,

01

343

372

402

2

DAC CODE

ZERO SCALE

DAC OUTPUT VOLTAGE (mV)

MID SCALE FULL SCALE

3

14 15 16 29 30 31

753

782

811

1191

1221

1250

Figure 2. CCFT DAC Transfer Function

MAX754

MAX758A

CC

LOGIC AND

5-BIT COUNTER

5-BIT VOLTAGE

OUTPUT DAC

SWITCH-MODE

VOLTAGE CONTROLLED

CURRENT SOURCE

FULL-SCALE = 1.250V
HALF-SCALE = 0.782V
ZERO-SCALE = 0.343V

CFB

ERROR
AMPLIFIER

CON CADJ

SS

C10

C5

R8

R18

I

BUCK

CENTER-TAP

TRANSISTOR

EMITTERS

ROYER

OSCILLATOR

CCFL

Figure 3. CCFL Tube Current-Regulation Loop

The minimum operating input voltage is determined by
the transformer turns ratio (n), the lamp operating volt­age (V

LAMP

), and the ballast capactor (C10). Using a
simple model of the CCFL (see Figure 4) we can calcu­late 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(ωt) and V

LAMP

(t) = √2V

LAMP,RMS

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 sec­ond. 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 con­stant-peak-current PFM control scheme. Figure 6, which
shows the MAX754’s boost switching regulator, illus­trates this control method. When Q3 closes (Q3 “on”) a
voltage equal to BATT is applied to the inductor, caus­ing current to flow from the battery, through the inductor
and switch, and to ground. This current ramps up linear­ly, 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 volt­age, but also maintain a constant peak inductor cur­rent, 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 MOS­FET 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 discon­tinuous-conduction mode).

φω=

−

⎛
⎝

⎜

⎞
⎠

⎟

−

tan

,

,

1

10

I

CV

LAMP RMS

LAMP RMS

V

I

nC

TAP PK

LAMP RMS

,

,

sin

= −

2

10ωφ()

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)

(t)

V

TAP

V

TAP, PK

T

t

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

8 _______________________________________________________________________________________

MAX754

6-BIT DAC

PULSE-SKIP

COMPARATOR

FULL-SCALE OUTPUT = 1.250V
HALF-SCALE OUTPUT = 0.938V
ZERO-SCALE OUTPUT = 0.635V

16

13

R4

PRESET
6-BIT COUNTER

CLK

Q3

V

DAC

LFB

LDRV

14

LX

15

BATT

+5V INPUT

1

V

DD

3

LON

6

GND

12

PGND

2

LADJ

C1

0.22µF

L2

33µH

BATTERY

INPUT

D3

1N5819

POSITIVE
LCD-BIAS

OUTPUT

ON/OFF

CONTROL

ON-TIME

LOGIC

OFF-TIME

LOGIC

R3

C2
10µF

C6
10µF
35V

Figure 6. MAX754 Positive LCD-Bias Generator

Table 1. CCFL Circuit Component Descriptions

ITEM DESCRIPTION

C5

Integrating Capacitor. 1 / (C5 x R18) sets the dominant pole for the feedback loop, which regulates the lamp
current. Set the dominant pole at least two decades below the Royer frequency to eliminate the AC compo­nent of the voltage on R8. For example, if your Royer is oscillating at 50kHz = 314159rad/s, you should set
1 / (C5 x R18) ≤ 3142rad/s.

R18

Integrating Resistor. The output source-current capability of the CC pin (50µA) limits how small R18 can be.
Do not make R18 smaller than 70kΩ, otherwise CC will not be able to servo CFB to the DAC voltage (i.e., the
integrator will not be able to integrate) and the loop will not be able to regulate.

R8

R8 converts the half-wave rectified lamp current into a voltage. The average voltage on R8 is not equal to the
root mean square voltage on R8. The accuracy of R8 is important since it, along with the MAX754 reference,
sets the full-scale lamp current. Use a ±1%-accurate resistor.

D7A, D7B

D7A and D7B half-wave rectify the CCFL lamp current. Half-wave rectification of the lamp current and then
averaging is a simple way to perform AC-to-DC conversion. D7A and D7B’s forward voltage drop and speed
are unimportant; they do not need to pass currents larger than about 10mA, and their reverse breakdown
voltage can be as low as 10V.

CCFL

The circuit of Figure 1, with the components shown in the bill of materials (Table 4), will drive a 500V

RMS

oper­ating cold-cathode fluorescent lamp at 6W of power with a +12V input voltage. The lower the input voltage,
the less power the circuit can deliver.

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

_______________________________________________________________________________________ 9

Table 1. CCFL Circuit Component Descriptions (continued)

ITEM DESCRIPTION

C10

The ballast capacitor linearizes the CCFL impedance and guarantees no DC current through the lamp. 15pF
will work with just about any lamp. Depending on the lamp, you can try higher values, but this may cause the
regulation loop to become unstable. Larger values of C10 allow the circuit to operate with lower input volt­ages. Don’t forget that C10 must be a high-voltage capacitor and cannot be polarized. A lamp with a
1500V

RMS

maximum strike voltage will require C10 to withstand 1500 x √2 = 2121V.

T1

T1 must have high primary inductance (greater than 30µH), otherwise an inflated value of C9 will be required
in order to keep the Royer frequency below 60kHz (the maximum allowed by most lamps). A higher T1 sec­ondary-to-primary turns ratio allows lower-voltage operation, but increases the size of the transformer.

C9

You must select a value for C9 high enough to keep the lamp current reasonably sinusoidal and yet low
enough that T1’s core does not saturate. For the Sumida EPS207 with a 171:1 turns ratio, choose a 0.22µF

value for C9. The characteristic impedance of the resonant tank equals , where L

MAG

is the mag­netizing inductance of T1. The characteristic impedance is defined as the ratio of the voltage across the par­allel LC circuit divided by the current flowing between the inductor and capacitor. This circulating current is
not delivered to the load. If C9 has too large a value, it will cause excessive circulating currents, which will in
turn saturate the core of T1. It’s easy to tell when you have excess circulating current in the resonant tank,
because when you touch T1 you burn your finger. However, reducing the value of C9 decreases tank Q,
which increases the harmonic content of the lamp-current waveform. If the lamp-current waveform does not
look sinusoidal, then the circuit may not regulate to the right root mean square current.

L

C

MAG

9

R10

R10 sets the base current for Q4 and Q5. If you choose too large a value for R10, Q4 and Q5 will overheat.
Too small a value will waste base current and slightly degrade efficiency. The optimal value will depend on
how much power you are trying to deliver to the lamp. 510Ω is a good “always works but may not be the most
efficient” value for use with the FMMT619 transistors from ZETEX.

R5, R6

This resistive divider senses the voltage at the center tap of T1. When the CC pin on the MAX758A rises
above 1.25V, the internal switch turns off, interrupting power to the Royer oscillator and limiting the open-lamp
transformer center-tap voltage.

D6B, C7, R7

D6B, C7, and R7 form a soft-start clamp, which limits the rate-of-rise of the peak current in the MAX758A.
Make sure R7 is at least 100kΩ so it does not excessively load the CC pin.

D6A, R17

D6A and R17 are also part of the soft-start clamp. The voltage on the SS pin controls the peak current in the
MAX758A’s switch. Make sure R17 is at least 100kΩ so it does not excessively load the CC pin.

L1 Inductor for the Switching-Current Source. Use a 47µH to 150µH inductor with a 1A to 1.5A saturation current.

D5 Schottky Catch Diode. Use a 1A to 1.5A Schottky diode with low forward-voltage power.

C2 Supply Bypass Capacitor. Use low-ESR capacitor.

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

10 ______________________________________________________________________________________

PARAMETER SYMBOL MIN TYP MAX UNITS

Strike Voltage (VS) V

S,RMS

1100 1500 V

RMS

Discharging Tube Current (IL) I

LAMP,RMS

0.001376 0.005 A

RMS

Discharging Tube Voltage (VL) V

LAMP,RMS

435 V

RMS

Bias Voltage V

LCD

16.3 32.6 V

Output Current I

LCD

0.0245 A

T1 Turns Ratio (Sec/Pri) (Note 2) n 171

T1 Resonating Inductance (Note 2) L

MAG

0.000045 H

C9 Value (Note 3) C

RES

2.2E-07 F

C10 Value C

BAL

1.5E-11 F

Royer Frequency w 317820.86 rad/s

Reference Voltage V

REF

1.25 V

Second Volts Constant sV 0.000008 2.4E-05 sV

R8 Current-Sensing Resistor R8 555.36037 Ω

Secondary Voltage Phase vs. Tube Voltage phi -1.1776341 Radian

Secondary Limit Voltage V

LIM

1350 V

RMS

T1 Center-Tap Limit Peak Voltage 11.164844 V

PEAK

R5/R6 R

OTP,RATIO

0.1341944 Ω/Ω

V

IN(min)

Full-Load Switching Period T

FL

1.639E-06 s

L2 Inductance L2 1.96E-05 2.4E-05 H

L2 Peak Currrent 1.22704 A

R4/R3 R

LCD,RATIO

0.0398724 Ω/Ω

Input Voltage V

IN

5.978103 18 V

Table 2. CCFL Circuit Design Example (Note 1)

T1 Center-Tap Peak Voltage V

TAP,PK

9.3903817 V

PEAK

Note 1: To perform your own calculations for the parameters given in Table 2 (Design Example), use the equations given in Table

3 (Design Equations).

Note 2: T1 = Sumida’s EPS207
Note 3: C9 = Wima’s SMD 7.3 __/63

CCFL Specifications

LCD Contrast Voltage Specifications

Royer Specifications

MAX754 Specifications

CCFL Circuit Calculations

LCD Circuit Calculations

Application Circuit Operating Range

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

______________________________________________________________________________________ 11

Table 3. Spreadsheet Design Equations

T1 Center-Tap Peak Voltage

PARAMETER SYMBOL MIN TYP MAX

V

TAP,PK

Strike Voltage (VS) V

S,RMS

1100 1500

= -SQRT(2) * I

LAMP,RMS(max)

/

(C

BAL

* w * SIN(phi)) / n

Discharging Tube Voltage (VL) V

LAMP,RMS

435

Bias Voltage V

LCD

= V

LCD(max)

/ 2 32.6

Output Current I

LCD

0.0245

T1 Turns Ratio (Sec/Pri) n 171

T1 Resonating Inductance L

MAG

0.000045

C9 Value C

RES

2.2E-07

C10 Value C

BAL

1.5E-11

Royer Frequency w = SQRT [1 / (L

MAG

* C

RES

)]

Reference Voltage V

REF

1.25

Second Volts Constant sV 0.000008 2.4E-05

R8 Current-Sensing Resistor R8

= PI() * V

REF

* SQRT(2) /

(2 * I

LAMP,RMS(max)

)

Secondary Voltage Phase vs. Tube
Voltage

phi

= ATAN (-I

LAMP,RMS(max)

/

(C

BAL

* w * + V

LAMP,RMS

)

Secondary Limit Voltage V

LIM

= V

S,RMS(max)

* 0.9

T1 Center-Tap Limit Peak Voltage = SQRT(2) * V

LIM

/ n

R5/R6 R

OTP,RATIO

= V

REF

/ (D25 - 0.6 - V

REF

)

V

IN(min)

Full-Load Switching Period T

FL

= sV(min) / V

IN(min)

+ sV(min) /

(V

LCD(max)

- V

IN(min)

)

L2 Inductance L2 = L2(max) * 0.8

= sV(min) ^ 2 / (2 * TFL*

V

LCD(max)

* I

LCD(min)

)

L2 Peak Currrent = sV(max) / L2(min)

R4/R3 R

LCD,RATIO

= V

REF

/ (V

LCD(max)

- V

REF

)

Input Voltage V

IN

= (2 / PI()) * V

TAP,PK

18

Discharging Tube Current (IL) I

LAMP,RMS

= 0.28 *

I

LAMP,RMS(max)

0.005

CCFL Specifications

LCD Contrast Voltage Specifications

Royer Specifications

MAX754 Specifications

CCFL Circuit Calculations

LCD Circuit Calculations

Application Circuit Operating Range

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

12 ______________________________________________________________________________________

Table 4. Bill of Materials

RESISTOR VALUE (Ω) TOLERANCE (%)

R1 100,000 ±10
R2 100,000 ±10
R3 1,000,000 ±1
R4 40,200 ±1
R5 100,000 ±1
R6 13,300 ±1
R7 100,000 ±10

R8 549 ±1
R10 680 ±5
R16 100,000 ±10
R17 100,000 ±10
R18 100,000 ±5

CAPACITOR

WORKING

VOLTAGE (V)

CHARACTERISTICS

C1 0.1 6

C2 22 20 Low ESR

C3 0.1 20

C4 0.1 6

C5 0.01 6 Non-polarized

C6 10 50

C7 1 6

C8 1 30

C9 22 63
C10 1.5E-5 3000 High voltage

OTHER

COMPONENTS

SURFACE-

MOUNT PART

NUMBER

PACKAGE

BREAKDOWN

VOLTAGE (V)

GENERIC
PART NO.

MANUFACTURER

Q1 CMPTA06 SOT-23 80 MPSA06 Central Semi.
Q2 CMPT2907A SOT-23 60 2N2907 Central Semi.
Q3 SOT-23 60 3055EL Motorola
Q4

MMFT3055ELT1

SOT-23 50 Zetex

Q5 FMMT619 SOT-23 50 Zetex
D1A CMPD4150 SOT-23 75 1N4150 Central Semi.
D1B CMPD4150 SOT-23 75 1N4150 Central Semi.
D2A CMPD4150 SOT-23 75 1N4150 Central Semi.
D2B CMPD4150 SOT-23 75 1N4150 Central Semi.

D3 EC10QS05 D-64 50 1N5819 Nihon

D4 CMPD4150 SOT-23 75 1N4150 Central Semi.

D5 EC10QS02L D-64 20 1N5817 Nihon
D6A CMPD4150 SOT-23 75 1N4150 Central Semi.
D6B CMPD4150 SOT-23 75 1N4150 Central Semi.
D7A CMPD4150 SOT-23 75 1N4150 Central Semi.
D7B CMPD4150 SOT-23 75 1N4150 Central Semi.

VALUE (µF)

FMMT619

Note: For T1, Use Sumida EPS207. Request No. USC-145, Special No. 6358-JP5-010.

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

______________________________________________________________________________________ 13

Positive LCD Bias: MAX754

The voltage-regulation loop is comprised of resistors R3
and R4, the pulse-skip comparator, the internal DAC,
the on-time and off-time logic, and the external power
components. The comparator compares a fraction of
the output voltage to the voltage generated by an on­chip 6-bit DAC. The part regulates by keeping the volt­age at LFB equal to the DAC’s output voltage. Thus,
you can set the output to different voltages by varying
the DAC’s output.

Varying the DAC output voltage (digital control) adjusts
the external voltage from 50% to 100% of full scale. On
power-up or after a reset, the counter sets the DAC out­put to mid scale. Each rising edge of LADJ (with LON
high) decrements the DAC output. When decremented
beyond zero scale, the counter rolls over and sets the
DAC to the maximum value. In this way, a single pulse
applied to LADJ decreases the DAC set point by one
step, and 63 pulses increase the set point by one step.

The MAX754’s DAC transfer function is shown in Figure 7.
The following equation relates the switching regulator’s
regulated output voltage to the DAC’s voltage:

Table 5 is the logic table for the LADJ and LON inputs,
which control the internal DAC and counter. As long as the
timing specifications for LADJ and LON are observed, any
sequence of operations can be implemented.

Negative LCD Bias: MAX753

The LCD bias generator of the MAX753 (Figure 8) gen­erates its negative output by combining the switching
regulator of the MAX754 with a simple diode-capacitor
voltage inverter. To best understand the circuit, look at
the part in a steady-state condition. Assume, for
instance, that the output is being regulated to -30V, and
that the battery voltage is +10V. When Q3 turns on, two
things occur: current ramps up in the inductor, just like
with the boost converter; and the charge on C15 (trans­ferred from the inductor on the previous cycle) is trans­ferred to C6, boosting the negative output. At the end of
the cycle, the voltage on C15 is 30V + Vd, where Vd is
the forward voltage drop of Schottky diode D3, and 30V
is the magnitude of the output.

When the MOSFET turns off, the inductor’s energy is
transferred to capacitor C15, charging the capacitor to
a positive voltage (V

HIGH

) that is higher than

|V

OUT

|

. In
this instance, diode D8 allows current to flow from the
right-hand side of the flying capacitor (C15) to ground.

When the MOSFET turns on, the left-hand side of
capacitor C15 is clamped to ground, forcing the right-

hand side to -V

HIGH

. This voltage is more negative than
the output, forcing D3 to conduct, and transferring
charge from the flying capacitor C15 to the output
capacitor C6. This charge transfer happens quickly,
resulting in a voltage spike at the output due to the
product of the output capacitor’s equivalent series
resistance (ESR) and the current that flows from C15 to
C6. To limit this drop, resistor R19 has been placed in
series with D3. R19 limits the rate of current flow. At the
end of this cycle, the flying capacitor has been dis­charged to 30V + Vd.

If BATT(MAX) (i.e., either the fully charged battery volt­age, or the wall-cube voltage) is greater than

|V

OUT

(MIN)|, tie the cathode of D8 to BATT instead of

GND, as shown by the dashed lines in Figure 8.
Efficiency is lower with this method, so tie the cathode
of D8 to GND whenever possible.

The MAX753’s regulation loop is similar to that of the
MAX754. The MAX753, however, uses different power
components, and its feedback resistors are returned to
the reference (1.25V) rather than ground.

The MAX753’s PFM comparator compares a fraction of
the output voltage to the voltage generated by the on­chip 6-bit DAC. The part regulates by keeping the volt­age at LFB equal to the DAC’s output voltage. Thus,
you can set the LCD bias voltage to different voltages
by varying the DAC’s output.

VV1

R3

R4

OUT DAC

=+

⎛
⎝

⎜

⎞
⎠

⎟

01

635

645

655

2

DAC CODE

ZERO SCALE

DAC OUTPUT VOLTAGE (mV)

MID SCALE FULL SCALE

30 31 32 61 62 63

928

938

947

1230

1240

1250

Figure 7. MAX754 LCD DAC Transfer Function

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

14 ______________________________________________________________________________________

Table 5. Logic-Signal Truth Table

Table 6. Component Suppliers

Hold = maintain last DAC value in counter
Reset = set DAC counter to half scale
Dec = decrement DAC counter one step
Off = section turned off, sleep state
On = section turned on
X = don’t care

LON LADJ CON CADJ CCFT STATUS

X X 0 0 Off

X X 0 1 On

X X 1 0 On

X X 1

0→1

On

LON LADJ CON CADJ LCD STATUS

0 0 X X Off

0 1 X X On

1 0 X X On

1

0→1

X X On

CCFT DAC

Hold

Reset

Hold

Dec

LCD DAC

Hold

Reset

Hold

Dec

CCF CONTROL

LCD BIAS CONTROL

* Contact John D. Deith, ask for “Maxim Discount” on orders less than 5k units.

MANUFACTURER

ADDRESS PHONE FAX

Central Semiconductor

145 Adams Ave.
Hauppauge, NY 11788

[phone] [phone]

Coiltronics

6000 Park of Commerce Blvd.
Boca Raton, FL 33287

[phone] [phone]

Maxim

120 San Gabriel Dr.
Sunnyvale, CA 94025

[phone] [phone]

Nihon (NIEC)*

c/o Quantum Marketing
12900 Rolling Oaks Rd.
Twin Oaks, CA 93518

[phone] [phone]

Sumida

5999 New Wilke Rd., Suite 110
Rolling Meadows, IL 60008

[phone] [phone]

Wima

2269 Saw Mill River Rd., Suite 400
P.O. Box 217
Elmsford, NY 10523

[phone] [phone]

Zetex [phone] [phone]

87 Modular Ave.
Commack, NY 11725

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

______________________________________________________________________________________ 15

MAX753

6-BIT DAC

PULSE-SKIP

COMPARATOR

16

13

R4

PRESET
6-BIT COUNTER

CLK

Q3

V

DAC

LFB

7

REF

LDRV

14

LX

15

BATT

+5V INPUT

1

V

DD

3

LON

6

GND

12

PGND

2

LADJ

C1

0.22µF

L2

33µH

BATTERY

INPUT

D8

1N5819

NEGATIVE
LCD-BIAS

OUTPUT

ON/OFF

CONTROL

ON-TIME

LOGIC

OFF-TIME

LOGIC

R3

C2
10µF

C15
1µF

R19

2.2Ω

C6
10µF
35V

ALTERNATE
D8 CONNECTION
(SEE TEXT)

C4

0.22µF

D3

1N5819

V

DD

Figure 8. MAX753 Negative LCD-Bias Generator

01

* DAC OUTPUT VOLTAGE = REF - LFB

625

635

645

2

DAC CODE

ZERO SCALE

DAC OUTPUT VOLTAGE (mV)*

MID SCALE FULL SCALE

30 31 32 61 62 63

918

928

937

1220

1230

1240

Figure 9. MAX753 LCD DAC Transfer Function

The MAX753’s DAC transfer function is shown in Figure 9.
The following equation relates the switching regulator’s
regulated output voltage to the DAC’s voltage (REF - LFB):

The value REF - LFB (and not LFB) is specified in the
Electrical Characteristics. The most negative output
voltage occurs for the largest value of REF - LFB.

The MAX753’s combination boost converter and
charge-pump inverter was chosen over a conventional
buck-boost inverter because it allows the use of low­cost N-channel MOSFETs instead of more expensive P­channel ones. Additionally, its efficiency is 5% to 10%
better than a standard buck-boost inverter.

V REF 1

R3

R4

REF LFB

OUT

= − +

⎛
⎝

⎜

⎞
⎠

⎟

−

()

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 [phone]

© 1995 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products.

MAX753/MAX754

CCFL Backlight and
LCD Contrast Controllers

MAX753/MAX754

LOGIC

CON CADJ

45

PGND GND

12 6

5-BIT

D/A CONVERTER

6-BIT

D/A CONVERTER

ERROR

AMPLIFIER

PULSE-SKIP

COMPARATOR

10

9

8

CFB

16

LFB

13

LDRV

LADJ LON BATT LX

1415

32

CC

CS

REF

5-BIT

COUNTER

PRESETCLK

CONTROL

CONTROL

6-BIT

COUNTER

PRESETCLK

ON-TIME

LOGIC

OFF-TIME

LOGIC

1

V

DD

7

REF

11

CDRV

_____________________Block Diagram

CDRV

PGND

LDRV

CADJ

GND

CON

LON

LADJ

V

DD

LFB

BATT

LX

CC

CS

REF

CFB

0.112"

(2.845mm)

0.076"

(1.930mm)

___________________Chip Topography

TRANSISTOR COUNT: 321;

SUBSTRATE CONNECTED TO V

DD

.