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19-0197; Rev 1; 1/95
CCFL Backlight and LCD Contrast Controllers
_______________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 25A. 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.
____________________________Features
o Drives Backplane and Backlight o 4V to 30V Battery Voltage Range o Low 500A Supply Current o Digital or Potentiometer Control of CCFL Brightness and LCD Bias Voltage o Negative LCD Contrast (MAX753) o Positive LCD Contrast (MAX754) o Independent Shutdown of Backlight and Backplane Sections o 25A Shutdown Supply Current
MAX753/MAX754
______________Ordering Information
PART MAX753CPE MAX753CSE MAX753C/D MAX753EPE MAX753ESE MAX754CPE MAX754CSE MAX754C/D MAX754EPE MAX754ESE TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 16 Plastic DIP 16 Narrow SO Dice* 16 Plastic DIP 16 Narrow SO 16 Plastic DIP 16 Narrow SO Dice* 16 Plastic DIP 16 Narrow SO
________________________Applications
Notebook Computers Palmtop Computers Pen-Based Data Systems Personal Digital Assistants Portable Data-Collection Terminals
* Contact factory for dice specifications.
__________________Pin Configuration
TOP VIEW
VDD 1 LADJ 2 LON 3 CON 4 CADJ 5 GND 6 REF 7 CFB 8 16 LFB 15 BATT 14 LX
MAX753 MAX754
13 LDRV 12 PGND 11 CDRV 10 CS 9 CC
DIP/SO
Block Diagram located at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
ABSOLUTE MAXIMUM RATINGS
VDD to 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 = +70C) Plastic DIP (derate 10.53mW/C above +70C) ...........842mW Narrow SO (derate 8.70mW/C above +70C) .............696mW Operating Temperature Ranges MAX75_C_ _ ........................................................0C to +70C MAX75_E_ _......................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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.
ELECTRICAL CHARACTERISTICS
(V DD = 5V, BATT = 15V, CON = LON = 5V, LX = GND = PGND = 0V, I REF = 0mA, all digital input levels are 0V or 5V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SUPPLY AND REFERENCE BATT Input Range VDD Supply Range REF Output Voltage REF Line Regulation REF Load Regulation VDD Quiescent Current VDD Shutdown Current DIGITAL INPUTS AND DRIVER OUTPUTS Input Low Voltage Input High Voltage Input Leakage Current Driver Sink/Source Current Driver On-Resistance CCFT CONTROLLER Zero-Crossing-Comparator Threshold Voltage (CS) Overcurrent-Comparator Threshold Voltage (CS) CS Input Bias Current VCO Frequency DAC Resolution VCS = 0V Minimum, CFB = 5V Maximum, CFB = 0V Guaranteed monotonic 32 85 5 -10 1.2 20 1.3 -5 47 115 mV V A kHz Bits LON, CON, CADJ, LADJ; VDD = 4.5V LON, CON, CADJ, LADJ; VDD = 5.5V LON, CON, CADJ, LADJ; VIN = 0V or 5V LDRV = CDRV = 2V LDRV, CDRV; VDD = 4.5V Output high Output low 0.5 10 7 2.4 1 0.8 V V A A No external load 4V < VDD < 6V 0A < IL < 100A LON = CON = CS = LFB = CFB = LADJ = CADJ = 5V LON = CON = CS = LFB = CFB = LADJ = CADJ = LX = BATT = 0V (Note 1) 5 0.5 25 4 4.5 1.21 1.25 30 5.5 1.29 0.1 15 2 40 V V V %/V mV mA A CONDITIONS MIN TYP MAX UNITS
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CCFL Backlight and LCD Contrast Controllers
ELECTRICAL CHARACTERISTICS (continued)
(V DD = 5V, BATT = 15V, CON = LON = 5V, LX = GND = PGND = 0V, I REF = 0mA, all digital input levels are 0V or 5V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS At full scale (DAC code = 31) Feedback Voltage (CFB) At preset DAC, CON = 0V, CADJ = 5V (code = 15) At zero scale (code = 0) Feedback-Amplifier Input Bias Current Feedback-Amplifier Unity-Gain Bandwidth Feedback-Amplifier Slew Rate Feedback-Amplifier Output Current LCD CONTROLLER Switch On-Time Switching Period DAC Resolution BATT = 4V BATT = 16V BATT = 4V, LX = 0V Guaranteed monotonic At full scale (DAC code = 63) MAX753 Feedback Voltage (REF-LFB) At preset DAC, LON = 0V, LADJ = 5V (code = 31) At zero scale (code = 0) At full scale (DAC code = 63) MAX754 Feedback Voltage (LFB) At preset DAC, LON = 0V, LADJ = 5V (code = 31) At zero scale (code = 0) LFB Input Leakage Current BATT Input Current LX Input Current TIMING (Note 2) Reset Pulse Width (tR) Reset Setup Time (tRS) Reset Hold Time (tRH) CADJ, LADJ High Width (tSH) CADJ, LADJ Low Width (tSL) CADJ Low to CON Low or LADJ Low to LON Low (tSD) Note 1: Maximum shutdown current occurs at BATT = LX = 0V. Note 2: Timing specifications are guaranteed by design and not production tested. 110 0 0 100 100 50 ns ns ns ns ns ns LON = CON = CS = LFB = CFB = LADJ = CADJ = LX = 0V LON = CON = CS = LFB = CFB = LADJ = CADJ = 0V, LX = BATT = 15V 12 12 2 0.5 35 6 1200 893 595 1210 905 610 1240 928 625 1250 938 635 1280 963 655 1290 971 660 150 20 20 nA A A mV mV 5 1.5 70 s s Bits Source current, CFB = 0V, CC = 2.5V Sink current, CFB = 5V, CC = 2.5V 50 200 1 0.4 MIN 1210 745 320 TYP 1250 782 343 MAX 1290 820 365 100 nA MHz V/s A mV UNITS
MAX753/MAX754
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME VDD LADJ LON CON CADJ GND REF CFB CC CS CDRV PGND LDRV LX BATT LFB 5V Power-Supply Input Digital Input for LCD Backplane Bias Adjustment. See Table 1. Digital Input to Control LCD Bias Section. See Table 1. Digital Input to Control CCFT Section. See Table 1. Digital Input for CCFT Brightness Adjustment. See Table 1. Analog Ground Reference Voltage Output, 1.25V Inverting Input for the CCFT Error Amplifier Output of the CCFT Error Amplifier Connect to VDD Leave unconnected Power Ground Connection for LDRV Gate-Driver Output. Drives LCD backplane N-channel MOSFET. LCD Backplane Inductor Voltage-Sense Pin. Used to sense inductor voltage for on time determination. Battery Connection. Used to sense battery voltage for on time determination. Voltage Feedback for the LCD Backplane Section FUNCTION
_______________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
4
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.
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
+5V, 5% 10 1 15 UNREGULATED INPUT VOLTAGE 1, 15, 16 2 V+ SHDN
CS VDD
BATT
C1
MAX754CSE
CON CADJ LON LADJ
D1A 4 5 3 2
D1B
R16 3 C2 +5V CMOS LOGIC CONTROL SIGNALS R17 7 C3 D5 REF GND 10, 11
MAX758ACWE
D2A
D2B 8 R2 R1 Q2
SS CC
LX
12, 13, 14
11
CDRV
Q1 L1 POSITIVE CONTRAST VOLTAGE
L2 14 D3
7
LX REF
D4 8 T1 12
C4 LDRV PGND LFB
13 Q3 12 R3 16 R4 C6
R5
5
3,4 R10
2
6
1 C10 CCFL
R6
C9 Q5 C8 Q4 D7B
GND 8
6 D6A D6B
D7A
CFB
CC
9
C7 C5
R7 R18 R8
Figure 1. CCFL and Positive LCD Power Supply
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
1250 1221 1191 DAC OUTPUT VOLTAGE (mV) 811 782 753
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.
402 372 343
01 2 3
14 15 16 DAC CODE
29 30 31
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: VREF R8 = 2 ILAMP,RMS
where VREF = 1.25V and ILAMP,RMS is the desired fullscale root mean square lamp current.
ZERO SCALE
MID SCALE
FULL SCALE
Figure 2. CCFT DAC Transfer Function
CON
CADJ
MAX754
FULL-SCALE = 1.250V HALF-SCALE = 0.782V ZERO-SCALE = 0.343V
LOGIC AND 5-BIT COUNTER SS 5-BIT VOLTAGE OUTPUT DAC
MAX758A
SWITCH-MODE VOLTAGE CONTROLLED CURRENT SOURCE
IBUCK ERROR AMPLIFIER CC CENTER-TAP ROYER OSCILLATOR CFB TRANSISTOR EMITTERS C5 CCFL R8 C10
R18
Figure 3. CCFL Tube Current-Regulation Loop
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
VTAP(t) C10 VTAP, PK VSEC (t)
ILAMP(t)
VLAMP(t) T
t
Figure 4. Simple Model of the CCFL
Figure 5. Voltage at the Center Tap of T1
The minimum operating input voltage is determined by the transformer turns ratio (n), the lamp operating voltage (VLAMP), 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(t) and V LAMP (t) = 2V LAMP,RMS cos(t); then the peak voltage at the center tap will be as follows: 2 ILAMP,RMS VTAP,PK = - nC10 sin() where, -ILAMP,RMS = tan -1 C10VLAMP,RMS
pulse-frequency-modulation (PFM) switching regulator. The MAX753 adds a simple diode-capacitor voltage inverter to the switching regulator.
, n is the secondary-to-primary turns ratio of T1, and 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 VTAP 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 VTAP.
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
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).
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
BATTERY INPUT C2 10F +5V INPUT C1 0.22F 2 LADJ 3 LON 1 VDD 15 BATT ON-TIME LOGIC CONTROL ON/OFF PRESET 6-BIT COUNTER CLK VDAC PULSE-SKIP COMPARATOR LFB 16 14 LX OFF-TIME LOGIC LDRV 13 Q3 R3 D3 1N5819 L2 33H POSITIVE LCD-BIAS OUTPUT C6 10F 35V
R4
MAX754
PGND 12
6-BIT DAC GND 6
FULL-SCALE OUTPUT = 1.250V HALF-SCALE OUTPUT = 0.938V ZERO-SCALE OUTPUT = 0.635V
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 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. Integrating Resistor. The output source-current capability of the CC pin (50A) limits how small R18 can be. Do not make R18 smaller than 70k, 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 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. The circuit of Figure 1, with the components shown in the bill of materials (Table 4), will drive a 500VRMS 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.
C5
R18
R8
D7A, D7B
CCFL
8
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
Table 1. CCFL Circuit Component Descriptions (continued)
ITEM DESCRIPTION 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 1500VRMS maximum strike voltage will require C10 to withstand 1500 x 2 = 2121V. T1 must have high primary inductance (greater than 30H), 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. 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 0.22F
C10
T1
LMAG
C9
C9 , where LMAG is the magvalue for C9. The characteristic impedance of the resonant tank equals netizing 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.
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. 510 is a good "always works but may not be the most efficient" value for use with the FMMT619 transistors from ZETEX. 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, 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 100k so it does not excessively load the CC pin. 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 100k so it does not excessively load the CC pin. Inductor for the Switching-Current Source. Use a 47H to 150H inductor with a 1A to 1.5A saturation current. Schottky Catch Diode. Use a 1A to 1.5A Schottky diode with low forward-voltage power. Supply Bypass Capacitor. Use low-ESR capacitor.
R10
R5, R6
D6B, C7, R7 D6A, R17 L1 D5 C2
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
Table 2. CCFL Circuit Design Example (Note 1)
PARAMETER CCFL Specifications Strike Voltage (VS) Discharging Tube Current (IL) Discharging Tube Voltage (VL) LCD Contrast Voltage Specifications Bias Voltage Output Current Royer Specifications T1 Turns Ratio (Sec/Pri) (Note 2) T1 Resonating Inductance (Note 2) C9 Value (Note 3) C10 Value Royer Frequency MAX754 Specifications Reference Voltage Second Volts Constant CCFL Circuit Calculations R8 Current-Sensing Resistor Secondary Voltage Phase vs. Tube Voltage T1 Center-Tap Peak Voltage Secondary Limit Voltage T1 Center-Tap Limit Peak Voltage R5/R6 LCD Circuit Calculations VIN(min) Full-Load Switching Period L2 Inductance L2 Peak Currrent R4/R3 Application Circuit Operating Range Input Voltage Note 1: Note 2: Note 3: VIN 5.978103 18 V RLCD,RATIO 0.0398724 TFL L2 1.96E-05 1.639E-06 2.4E-05 1.22704 s H A / ROTP,RATIO R8 phi VTAP,PK VLIM 555.36037 -1.1776341 9.3903817 1350 11.164844 0.1341944 Radian VPEAK VRMS VPEAK / VREF sV 0.000008 1.25 2.4E-05 V sV n LMAG CRES CBAL w 171 0.000045 2.2E-07 1.5E-11 317820.86 H F F rad/s VLCD ILCD 16.3 0.0245 32.6 V A VS,RMS ILAMP,RMS VLAMP,RMS 1100 0.001376 435 1500 0.005 VRMS ARMS VRMS SYMBOL MIN TYP MAX UNITS
To perform your own calculations for the parameters given in Table 2 (Design Example), use the equations given in Table 3 (Design Equations). T1 = Sumida's EPS207 C9 = Wima's SMD 7.3 __/63
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
Table 3. Spreadsheet Design Equations
PARAMETER CCFL Specifications Strike Voltage (VS) Discharging Tube Current (IL) Discharging Tube Voltage (VL) LCD Contrast Voltage Specifications Bias Voltage Output Current Royer Specifications T1 Turns Ratio (Sec/Pri) T1 Resonating Inductance C9 Value C10 Value Royer Frequency MAX754 Specifications Reference Voltage Second Volts Constant CCFL Circuit Calculations R8 Current-Sensing Resistor Secondary Voltage Phase vs. Tube Voltage T1 Center-Tap Peak Voltage Secondary Limit Voltage T1 Center-Tap Limit Peak Voltage R5/R6 LCD Circuit Calculations VIN(min) Full-Load Switching Period L2 Inductance L2 Peak Currrent R4/R3 Application Circuit Operating Range Input Voltage VIN = (2 / PI()) * VTAP,PK 18 RLCD,RATIO = VREF / (VLCD(max) - VREF) TFL L2 = L2(max) * 0.8 = sV(min) / VIN(min) + sV(min) / (VLCD(max) - VIN(min)) = sV(min) ^ 2 / (2 * TFL * VLCD(max) * ILCD(min)) = sV(max) / L2(min) ROTP,RATIO R8 phi VTAP,PK VLIM = PI() * VREF * SQRT(2) / (2 * ILAMP,RMS(max)) = ATAN (-ILAMP,RMS(max) / (CBAL * w * + VLAMP,RMS) = -SQRT(2) * ILAMP,RMS(max) / (CBAL * w * SIN(phi)) / n = VS,RMS(max) * 0.9 = SQRT(2) * VLIM / n = VREF / (D25 - 0.6 - VREF) VREF sV 0.000008 1.25 2.4E-05 n LMAG CRES CBAL w 171 0.000045 2.2E-07 1.5E-11 = SQRT [1 / (LMAG * CRES)] VLCD ILCD = VLCD(max) / 2 0.0245 32.6 VS,RMS ILAMP,RMS VLAMP,RMS 1100 = 0.28 * ILAMP,RMS(max) 435 1500 0.005 SYMBOL MIN TYP MAX
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
Table 4. Bill of Materials
RESISTOR R1 R2 R3 R4 R5 R6 R7 R8 R10 R16 R17 R18 CAPACITOR C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 OTHER COMPONENTS Q1 Q2 Q3 Q4 Q5 D1A D1B D2A D2B D3 D4 D5 D6A D6B D7A D7B VALUE () 100,000 100,000 1,000,000 40,200 100,000 13,300 100,000 549 680 100,000 100,000 100,000 VALUE (F) 0.1 22 0.1 0.1 0.01 10 1 1 22 1.5E-5 SURFACEMOUNT PART NUMBER CMPTA06 CMPT2907A MMFT3055ELT1 FMMT619 FMMT619 CMPD4150 CMPD4150 CMPD4150 CMPD4150 EC10QS05 CMPD4150 EC10QS02L CMPD4150 CMPD4150 CMPD4150 CMPD4150 TOLERANCE (%) 10 10 1 1 1 1 10 1 5 10 10 5 WORKING VOLTAGE (V) 6 20 20 6 6 50 6 30 63 3000 CHARACTERISTICS
Low ESR
Non-polarized
High voltage BREAKDOWN VOLTAGE (V) 80 60 60 50 50 75 75 75 75 50 75 20 75 75 75 75 GENERIC PART NO. MPSA06 2N2907 3055EL
PACKAGE SOT-23 SOT-23 SOT-23 SOT-23 SOT-23 SOT-23 SOT-23 SOT-23 SOT-23 D-64 SOT-23 D-64 SOT-23 SOT-23 SOT-23 SOT-23
MANUFACTURER Central Semi. Central Semi. Motorola Zetex Zetex Central Semi. Central Semi. Central Semi. Central Semi. Nihon Central Semi. Nihon Central Semi. Central Semi. Central Semi. Central Semi.
1N4150 1N4150 1N4150 1N4150 1N5819 1N4150 1N5817 1N4150 1N4150 1N4150 1N4150
Note: For T1, Use Sumida EPS207. Request No. USC-145, Special No. 6358-JP5-010. 12 ______________________________________________________________________________________
CCFL Backlight and LCD Contrast Controllers
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:
R3 VOUT = VDAC 1 + R4 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.
MAX753/MAX754
1250 1240 1230 DAC OUTPUT VOLTAGE (mV) 947 938 928
655 645 635
01
2
30 31 32 DAC CODE
61 62 63
ZERO SCALE
MID SCALE
FULL SCALE
Figure 7. MAX754 LCD DAC Transfer Function
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 (VHIGH) that is higher than |VOUT|. 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 -VHIGH. 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 |VOUT(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.
13
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CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
Table 5. Logic-Signal Truth Table
CCF CONTROL LON X X X X LCD BIAS CONTROL LON 0 0 1 1 LADJ 0 1 0 01 CON X X X X CADJ X X X X LCD STATUS Off On On On LCD DAC Hold Reset Hold Dec LADJ X X X X CON 0 0 1 1 CADJ 0 1 0 01 CCFT STATUS Off On On On CCFT DAC Hold Reset Hold Dec
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
Table 6. Component Suppliers
MANUFACTURER Central Semiconductor Coiltronics Maxim ADDRESS 145 Adams Ave. Hauppauge, NY 11788 6000 Park of Commerce Blvd. Boca Raton, FL 33287 120 San Gabriel Dr. Sunnyvale, CA 94025 c/o Quantum Marketing 12900 Rolling Oaks Rd. Twin Oaks, CA 93518 5999 New Wilke Rd., Suite 110 Rolling Meadows, IL 60008 2269 Saw Mill River Rd., Suite 400 P.O. Box 217 Elmsford, NY 10523 87 Modular Ave. Commack, NY 11725 PHONE (516) 435-1110 (407) 241-7876 (408) 737-7600 FAX (516) 435-1824 (407) 241-9339 (408) 737-7194
Nihon (NIEC)*
(805) 867-2555
(805) 867-2698
Sumida
(708) 956-0666
(708) 956-0702
Wima
(914) 347-2474
(914) 347-7230
Zetex
(516) 543-7100
(516) 864-7630
* Contact John D. Deith, ask for "Maxim Discount" on orders less than 5k units.
14
______________________________________________________________________________________
CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
BATTERY INPUT C2 10F L2 33H C15 1F D8 1N5819 R3 PULSE-SKIP COMPARATOR LFB 16 R19 2.2 D3 1N5819 NEGATIVE LCD-BIAS OUTPUT C6 10F 35V VDD R4 VDAC ALTERNATE D8 CONNECTION (SEE TEXT)
+5V INPUT C1 0.22F 2 LADJ 3 LON 1 VDD 15 BATT ON-TIME LOGIC CONTROL ON/OFF PRESET 6-BIT COUNTER CLK 14 LX OFF-TIME LOGIC LDRV 13 Q3
MAX753
PGND 12
6-BIT DAC GND 6 REF 7 C4 0.22F
Figure 8. MAX753 Negative LCD-Bias Generator
1240 1230 1220 DAC OUTPUT VOLTAGE (mV)* 937 928 918
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): R3 VOUT = REF - 1 + REF - LFB R4
(
)
645 635 625
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.
01 2 30 31 32 DAC CODE 61 62 63
ZERO SCALE * DAC OUTPUT VOLTAGE = REF - LFB
MID SCALE
FULL SCALE
Figure 9. MAX753 LCD DAC Transfer Function
______________________________________________________________________________________
15
CCFL Backlight and LCD Contrast Controllers MAX753/MAX754
_____________________Block Diagram
2 LADJ 3 LON 15 BATT ON-TIME LOGIC 14 LX OFF-TIME LOGIC LDRV 13
___________________Chip Topography
V DD LFB BATT LX
LADJ LON CON
CONTROL
CLK
PRESET 6-BIT COUNTER
PULSE-SKIP COMPARATOR
LFB 16
CADJ
LDRV 0.112" PGND (2.845mm) CDRV
6-BIT D/A CONVERTER
VDD
1
MAX753/MAX754
5-BIT D/A CONVERTER ERROR AMPLIFIER
REF CDRV
7 11
GND
REF
5-BIT COUNTER CLK PRESET LOGIC CONTROL REF CON 4 CADJ 5 PGND 12 GND 6 CFB 8 CC 9 CS 10
CC CFB CS 0.076" (1.930mm)
TRANSISTOR COUNT: 321; SUBSTRATE CONNECTED TO VDD.
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 (408) 737-7600 (c) 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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