MAX753CSE+T Datasheet MAX753CSE+, MAX754ESE+, MAX753CSE+T | P7-P9

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(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 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).

−

⎛

⎝

⎜

⎞

⎠

⎟

−

tan

LAMP RMS

TAP PK

LAMP RMS

sin

= −

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

LAMP

(t)

SEC

(t)

LAMP

(t)

TAP, PK

TAP

(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

PRESET

6-BIT COUNTER

CLK

DAC

LFB

LDRV

BATT

+5V INPUT

LON

GND

PGND

LADJ

0.22µF

33µH

BATTERY

INPUT

1N5819

POSITIVE

LCD-BIAS

OUTPUT

ON/OFF

CONTROL

ON-TIME

LOGIC

OFF-TIME

LOGIC

10µF

35V

Figure 6. MAX754 Positive LCD-Bias Generator

Table 1. CCFL Circuit Component Descriptions

ITEM DESCRIPTION

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 compo-

nent 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 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

oper-

ating 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 volt-

ages. 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 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 sec-

ondary-to-primary turns ratio allows lower-voltage operation, but increases the size of the transformer.

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 mag-

netizing inductance of T1. The characteristic impedance is defined as the ratio of the voltage across the par-

allel 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.

MAG

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.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

MAX753CSE+T

Mfr. #:

Buy MAX753CSE+T

Manufacturer:

Maxim Integrated

Description:

Display Drivers & Controllers CCFL Backlight & LCD Neg Contrast Ctlr

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MAX753CSE+T

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