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19-3793; Rev 0; 8/05
KIT ATION EVALU E AILABL AV
TFT LCD Step-Up DC-DC Converter
Features
1.8V to 5.5V Input Supply Range Built-In 14V, 2.2A, 0.2 n-Channel MOSFET High Efficiency (> 85%) Fast Transient Response to Pulsed Load High-Accuracy Output Voltage (1.5%) Internal Digital Soft-Start Input Supply Undervoltage Lockout 1.2MHz Switching Frequency 0.1A Shutdown Current Small 8-Pin TDFN Package
General Description
The MAX8752 is a high-performance, step-up DC-DC converter that provides a regulated supply voltage for active-matrix thin-film transistor (TFT) liquid-crystal displays (LCDs). The MAX8752 incorporates current-mode, fixed-frequency, pulse-width modulation (PWM) circuitry with a built-in n-channel power MOSFET to achieve high efficiency and fast transient response. The input supply voltage of the MAX8752 is from 1.8V to 5.5V. The MAX8752 operates with a switching frequency of 1.2MHz, allowing the use of ultrasmall inductors and lowESR ceramic capacitors. The current-mode architecture provides fast transient response to the pulsed loads typical of LCD source-driver applications. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The 14V internal MOSFET can generate output voltages up to 13V. The internal digital soft-start and current limit effectively control inrush and fault currents. The MAX8752 is available in a 3mm x 3mm 8-pin TDFN package with a maximum height of 8mm.
MAX8752
Applications
Notebook Computer Displays LCD Monitor Panels Automotive Displays
MAX8752ETA PART
Ordering Information
TEMP RANGE -40C to +85C PINPACKAGE 8 TDFN 3mm x 3mm PKG CODE T833-2
Typical Operating Circuit
VIN +1.8V TO +5.5V VMAIN
Pin Configuration
SUP LDO
7
6 3 SHDN
IN
LX IN FB
MAX8752
COMP LDO GND
8
MAX8752
1
2
COMP
FB
TDFN 3mm x 3mm
________________________________________________________________ Maxim Integrated Products
GND
IN
4
SHDN
SUP
5
LX
TOP VIEW
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
TFT LCD Step-Up DC-DC Converter MAX8752
ABSOLUTE MAXIMUM RATINGS
LX, SUP to GND .....................................................-0.3V to +14V IN, SHDN, LDO to GND............................................-0.3V to +6V FB to GND ...................................................-0.3V to (VIN + 0.3V) COMP to GND ..........................................-0.3V to (VLDO + 0.3V) LX Switch Maximum Continuous RMS Current .....................1.6A Continuous Power Dissipation (TA = +70C) 10-Pin TDFN (derate 18.2mW/C above +70C) .......1454mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10s) .................................+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
(VIN = VSHDN = 2.5V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Input Supply Range Output Voltage Range IN Undervoltage Lockout Threshold IN Quiescent Current IN Shutdown Current LDO Output Voltage LDO Undervoltage Lockout LDO Output Current SUP Supply Voltage Range SUP Overvoltage-Lockout Threshold SUP Undervoltage-Lockout Threshold SUP Supply Current ERROR AMPLIFIER FB Regulation Voltage FB Input Bias Current FB Line Regulation Transconductance Voltage Gain OSCILLATOR Frequency Maximum Duty Cycle 1000 88 1220 92 1500 96 kHz % ILX = 200mA, T = 0C to +25C ILX = 200mA, T = +25C to +85C VFB = 1.24V VIN = 1.8V to 5.5V 70 1.218 1.223 1.240 1.240 0 0.05 180 700 1.262 1.257 40 0.15 280 V nA %/V S V/V VSUP rising, typical hysteresis is 200mV (Note 1) VSUP rising, typical hysteresis is 200mV (Note 2) LX not switching LX switching 1.5 4 VIN rising, typical hysteresis is 200mV VFB = 1.3V, not switching VFB = 1.0V, switching SHDN = GND 6V VSUP 13V, ILDO = 12.5mA VLDO rising, typical hysteresis is 200mV 4.6 2.4 15 4.5 13.2 13.6 13.0 14.0 1.4 2.0 8 0.90 1.30 0.18 2 0.1 5.0 2.7 CONDITIONS MIN 1.8 TYP MAX 5.5 13 1.75 0.35 5 10.0 5.4 3.0 UNITS V V V mA A V V mA V V V mA
2
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TFT LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSHDN = 2.5V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER n-CHANNEL MOSFET Current Limit On-Resistance Leakage Current Current-Sense Transresistance SOFT-START Soft-Start Period Soft-Start Step Size CONTROL INPUTS SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Current VIN = 1.8V to 5.5V VIN = 1.8V to 5.5V 0.7 x VIN 0.001 1.000 0.6 V V A 13 0.275 ms A VLX = 12V 0.2 VFB = 1V, 65% duty cycle 1.8 2.2 0.2 0.1 0.3 2.6 0.4 10 0.4 A A V/A CONDITIONS MIN TYP MAX UNITS
MAX8752
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 2.5V, TA = -40C to +85C. unless otherwise noted.)
PARAMETER Input Supply Range Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current LDO Output Voltage LDO Undervoltage Lockout LDO Output Current SUP Supply Voltage Range SUP Overvoltage-Lockout Threshold SUP Undervoltage-Lockout Threshold SUP Supply Current ERROR AMPLIFIER FB Regulation Voltage OSCILLATOR Frequency n-CHANNEL MOSFET Current Limit On-Resistance Current-Sense Transresistance 0.2 VFB = 1V, 65% duty cycle 1.7 2.7 0.4 0.4 A V/A 940 1560 kHz ILX = 200mA 1.210 1.270 V VSUP rising, typical hysteresis is 200mV (Note 1) VSUP rising, typical hysteresis is 200mV (Note 2) LX not switching LX switching VIN rising, typical hysteresis is 200mV VFB = 1.3V, not switching VFB = 1.0V, switching 6V VSUP 13V, ILDO = 12.5mA VLDO rising, typical hysteresis is 200mV 4.6 2.4 15 4.5 13.2 13.0 14.0 1.4 2.0 8 0.90 CONDITIONS MIN 1.8 TYP MAX 5.5 13 1.75 0.35 5 5.4 3.0 UNITS V V V mA V V mA V V V mA
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3
TFT LCD Step-Up DC-DC Converter MAX8752
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSHDN = 2.5V, TA = -40C to +85C. unless otherwise noted.)
PARAMETER CONTROL INPUTS SHDN Input Low Voltage SHDN Input High Voltage VIN = 1.8V to 5.5V VIN = 1.8V to 5.5V 0.7 x VIN 0.6 V V CONDITIONS MIN TYP MAX UNITS
Note 1: Step-up regulator inhibited when VSUP exceeds this threshold. Note 2: Step-up regulator inhibited until VSUP exceeds this threshold. Note 3: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 2.5V, VMAIN = 10V, TA = +25C, unless otherwise noted.)
OUTPUT VOLTAGE ERROR vs. LOAD CURRENT
MAX8752 toc02
EFFICIENCY vs. LOAD CURRENT
85 80 EFFICIENCY (%) 75 70 65 60 55 50 10 100 LOAD CURRENT (mA) 1000 VIN = 1.8V VIN = 3.3V EFFICIENCY (%) L1 = 2.6H VIN = 5V
MAX8752 toc01
EFFICIENCY vs. LOAD CURRENT
95 90 85 80 75 70 65 60 55 50 10 100 LOAD CURRENT (mA) 1000 VIN = 1.8V -3.0 1 VIN = 3.3V L1 = 3.3H VIN = 5V 0.5 0 OUTPUT VOLTAGE ERROR (%) -0.5 -1.0 -1.5 -2.0 -2.5
VIN = 5V
VIN = 1.8V VIN = 3.3V
10
100 1000 LOAD CURRENT (mA)
10,000
SWITCHING FREQUENCY ERROR vs. INPUT VOLTAGE
MAX8752 toc04
IN SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX8752 toc05
IN SUPPLY CURRENT vs. TEMPERATURE
NO LOAD IN SUPPLY CURRENT (A) 40 VIN = 1.8V
MAX8752 toc06
0.2 SWITCHING FREQUENCY ERROR (%) 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 1.8 2.8 3.8 4.8 INPUT VOLTAGE (V) 5.8
50 NORMAL FB 40 IN SUPPLY CURRENT (A)
50
30
30 VIN = 3.3V 20
20 VFB = 1.3V 10
10
VIN = 5V
0 1.5 2.5 3.5 4.5 SUPPLY VOLTAGE (V) 5.5
0 -40 -20 0 20 40 60 80 TEMPERATURE (C)
4
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MAX8752 toc03
90
TFT LCD Step-Up DC-DC Converter MAX8752
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 2.5V, VMAIN = 10V, TA = +25C, unless otherwise noted.)
SOFT-START (HEAVY LOAD)
MAX8752 toc07
LOAD TRANSIENT RESPONSE
MAX8752 toc08
PULSED-LOAD TRANSIENT RESPONSE
MAX8752 toc09
IMAIN 200mA/div INDUCTOR CURRENT 1A/div 0A 100mA INDUCTOR CURRENT 1A/div 0A
IMAIN 1A/div
0A
0A
INDUCTOR CURRENT 1A/div
VMAIN 5V/div 0V
10V
10V VMAIN 500mA/div 10V OFFSET 100s/div 10s/div
VMAIN 200mV/div 10V OFFSET
2ms/div
SWITCHING WAVEFORMS
MAX8752 toc10
SUP SUPPLY CURRENT vs. SUP VOLTAGE
MAX8752 toc11
SUP SUPPLY CURRENT vs. TEMPERATURE
ILOAD = 140mA SUP SUPPLY CURRENT (mA) 4.2 VIN = 3.3V 3.8 VIN = 1.8V VIN = 5V 3.4
MAX8752 toc12
4.5 4.0 SUP SUPPLY CURRENT (mA) LX 5V/div 0V 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0A 0 4 6 8 10 SUP VOLTAGE (V) 12 VIN = 5V VIN = 3.3V VIN = 1.8V NO LOAD
INDUCTOR CURRENT 500mA/div ILOAD = 300mA
1s/div
14
3.0 -40 -20 0 20 40 TEMPERATURE (C) 60 80
LDO OUTPUT VOLTAGE vs. TEMPERATURE
MAX8752 toc13
LDO OUTPUT VOLTAGE vs. LDO CURRENT
MAX8752 toc14
5.08
5.08 5.06 5.04
5.06 OUTPUT VOLTAGE (V)
5.04
LDO VOLTAGE (V)
5.02 5.00 4.98
5.02
5.00 4.96 4.98 -40 -20 0 20 40 TEMPERATURE (C) 60 80 4.94 0 10 20 30 LDO CURRENT (mA) 40 50
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5
TFT LCD Step-Up DC-DC Converter MAX8752
Pin Description
PIN 1 NAME COMP FUNCTION Compensation Pin for Error Amplifier. Connect a series resistance and capacitor from COMP to GND. See the Loop Compensation section for component selection guidelines. Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltagedivider between the step-up regulator's output (VMAIN) and GND, with the center tap connected to FB. Place the divider close to the IC and minimize the trace area to reduce noise coupling. Set VMAIN according to the Output Voltage Selection section. Shutdown Control Input. Drive SHDN low to turn off the MAX8752. Ground Switching Node. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and minimize the trace area for lower EMI. Supply Pin. Connect IN to the input supply through a series 100 resistor and bypass it to GND with 0.1F or greater ceramic capacitor. Internal 5V Linear-Regulator Output. This regulator powers all internal circuitry. Bypass LDO to GND with a 0.22F or greater ceramic capacitor. Linear-Regulator Supply Input. SUP is the supply input of the internal 5V linear regulator. Connect SUP to the step-up regulator output and bypass SUP to GND with a 0.1F capacitor. Backside Paddle. Connect the backside paddle to analog ground.
2
FB
3 4 5 6 7 8 BP
SHDN GND LX IN LDO SUP --
C11 0.1F
D4
VGON 28V/10mA C13 0.1F
VGOFF -9V/20mA C8 0.1F
D2
C9 0.1F
C10 0.1F
D3 C12 0.1F
VIN
+1.8V TO +5.5V C1 10F 6.3V
L1 2.6H
D1 C2 10F 16V
VMAIN +10V/240mA
R4 100 IN C3 0.1F R3 40.2k COMP C6 20pF
LX
R1 90.9k 1% FB R2 13k 1% GND
MAX8752
C4 1.2nF
SUP C14 0.22F LDO SHDN C7 0.1F
Figure 1. Typical Applications Circuit 6 _______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
current trip point each time the internal MOSFET turns on. As the load changes, the error amplifier sources or sinks current to the COMP output to set the inductor peak current necessary to service the load. To maintain stability at high duty cycles, a slope-compensation signal is summed with the current-sense signal. On the rising edge of the internal clock, the controller sets a flip-flop, turning on the n-channel MOSFET and applying the input voltage across the inductor. The current through the inductor ramps up linearly, storing energy in its magnetic field. Once the sum of the current-feedback signal and the slope compensation exceed the COMP voltage, the controller resets the flipflop and turns off the MOSFET. Since the inductor current is continuous, a transverse potential develops across the inductor that turns on the diode (D1). The voltage across the inductor then becomes the difference between the output voltage and the input voltage. This discharge condition forces the current through the inductor to ramp back down, transferring the energy stored in the magnetic field to the output capacitor and the load. The MOSFET remains off for the rest of the clock cycle. At light loads, this architecture allows the MAX8752 to "skip" cycles to prevent overcharging the output capacitor voltage. In this region of operation, the inductor ramps up to a peak value of approximately 250mA, discharges to the output, and waits until another pulse is needed.
MAX8752
CLOCK LOGIC AND DRIVER
LX
GND
IN
STARTUP OSC
CURRENT LIMIT SOFTSTART SLOPE COMP
ILIMIT
OSCILLATOR SHDN SUP LDO LINEAR REGULATOR AND BOOTSTRAP PWM COMPARATOR
CURRENT SENSE
ERROR AMP FB 1.24V COMP
MAX8752
Figure 2. MAX8752 Functional Diagram
Detailed Description
The MAX8752 is a highly efficient, step-up power supply designed for TFT-LCD panels. The typical circuit shown in Figure 1 operates from an input voltage as low as 1.8V, and produces a MAIN output of 10V at 220mA from 2.5V input while supporting discrete diode-capacitor charge pumps that produce -9V at 20mA and +28V at 10mA. If the charge-pump outputs are not required, the diodes and capacitors associated with them may be eliminated and the main output increased to 270mA. The MAX8752 employs a current-mode, fixed-frequency, pulse-width modulation (PWM) architecture for fast transient response and low-noise operation. The high switching frequency (1.2MHz) allows the use of lowprofile inductors and ceramic capacitors to minimize the thickness of LCD panel designs. The integrated high-efficiency MOSFET and the IC's built-in digital soft-start function reduce the number of external components required. The output voltage can be set from VIN to 13V with an external resistive voltage-divider. The MAX8752 regulates the output voltage through a combination of an error amplifier, two comparators, and several signal generators (Figure 2). The error amplifier compares the signal at FB to 1.24V and varies the COMP output. The voltage at COMP determines the
Output-Current Capability
The output-current capability of the MAX8752 is a function of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensation used to stabilize the feedback loop, the inductor current limit depends on the duty cycle. The current limit is determined by the following equation: ILIM = (1.162 - 0.361 x D) x ILIM_EC where ILIM_EC is the current limit specified at 65% duty cycle (see the Electrical Characteristics) and D is the duty cycle. The output current capability depends on the currentlimit value and is governed by the following equation: 0.5 x D VIN VIN IOUT(MAX) = ILIM - x x fOSC x L VOUT
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7
TFT LCD Step-Up DC-DC Converter MAX8752
where ILIM is the current limit calculated above, is the regulator efficiency (85% nominal), and D is the duty cycle. The duty cycle when operating at the current limit is: VOUT - VIN + VDIODE VOUT - ILIM x RON + VDIODE
Table 1. Component List
DESIGNATION DESCRIPTION 10F 10%, 4V X5R ceramic capacitor (0603) TDK C1608X5R0G106K Murata GRM188R60G106M 10F 10%, 16V X5R ceramic capacitor (1206) TDK C3216X5R1C106K Murata GRM319R61A106K 3A, 30V Schottky diode (M-flat) Toshiba CRS02 2.6H, 2.1A power inductor 3.3H, 1.7A power inductor Sumida CDRH6D12-3R3
C1
D=
C2
where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET.
Bootstrapping and Soft-Start
The MAX8752 features bootstrapping operation. In normal operation, the internal linear regulator supplies power to the internal circuitry. The input of the linear regulator (SUP) should be directly connected to the output of the step-up regulator. After the input voltage at SUP is above 1.75V, the regulator starts open-loop switching to generate the supply voltage for the linear regulator. The internal reference block turns on when the LDO voltage exceeds 2.7V (typ). When the reference voltage reaches regulation, the PWM controller and the current-limit circuit are enabled and the step-up regulator enters soft-start. During the soft-start, the main step-up regulator directly limits the peak inductor current, allowing from zero up to the full current limit in eight equal current steps. The maximum load current is available after the output voltage reaches regulation (which terminates soft-start), or after the soft-start timer expires (13ms typ). The soft-start routine minimizes the inrush current and voltage overshoot and ensures a well-defined startup behavior.
D1
L1
Applications Information
Step-up regulators using the MAX8752 can be designed by performing simple calculations for a first iteration. All designs should be prototyped and tested prior to production. Table 1 provides a list of power components for the typical applications circuit. Table 2 lists component suppliers. External component value choice is primarily dictated by the output voltage and the maximum load current, as well as maximum and minimum input voltages. Begin by selecting an inductor value. Once the inductor value and peak current are known, choose the diode and capacitors.
Shutdown
The MAX8752 shuts down to reduce the supply current to 0.1A when SHDN is low. In this mode, the internal reference, error amplifier, comparators, and biasing circuitry turn off and the n-channel MOSFET is turned off. In shutdown, the step-up regulator's output is connected to IN through the external inductor and rectifier diode.
Inductor Selection
The minimum inductance value, peak current rating, and series resistance are factors to consider when selecting the inductor. These factors influence the converter's efficiency, maximum output load capability, transient response time, and output voltage ripple. Physical size and cost are also important factors to consider.
Table 2. Component Suppliers
SUPPLIER Murata Sumida TDK Toshiba PHONE 770-436-1300 847-545-6700 847-803-6100 949-455-2000 FAX 770-436-3030 847-545-6720 847-803-6296 949-859-3963 WEBSITE www.murata.com www.sumida.com www.component.tdk.com www.toshiba.com/taec
8
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TFT LCD Step-Up DC-DC Converter
The maximum output current, input voltage, output voltage, and switching frequency determine the inductor value. Very high inductance values minimize the current ripple and therefore reduce the peak current, which decreases core losses in the inductor and I 2R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increase physical size and can increase I2R losses in the inductor. Low inductance values decrease the physical size but increase the current ripple and peak current. Finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. The equations used here include a constant, LIR, which is the ratio of the inductor peak-to-peak ripple current to the average DC inductor current at the full-load current. The best trade-off between inductor size and circuit efficiency for step-up regulators generally has an LIR between 0.3 and 0.5. However, depending on the AC characteristics of the inductor core material and ratio of inductor resistance to other power path resistances, the best LIR can shift up or down. If the inductor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. If extremely thin high-resistance inductors are used, as is common for LCD panel applications, the best LIR can increase to between 0.5 and 1.0. Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. In Figure 1, the LCD's gate-on and gate-off voltages are generated from two unregulated charge pumps driven by the step-up regulator's LX node. The additional load on LX must therefore be considered in the inductance calculation. The effective maximum output current IMAIN(EFF) becomes the sum of the maximum load current on the step-up regulator's output plus the contributions from the positive and negative charge pumps: IMAIN(EFF) = IMAIN(MAX) + NEG x INEG + (POS + 1) x IPOS where IMAIN(MAX) is the maximum main output current, nNEG is the number of negative charge-pump stages, nPOS is the number of positive charge-pump stages, INEG is the negative charge-pump output current, and
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I POS is the positive charge-pump output current, assuming the pump source for IPOS is VMAIN. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IMAIN(MAX)), the expected efficiency (TYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion:
2 V VMAIN - VIN TYP L = IN VMAIN IMAIN(MAX) x fOSC LIR
MAX8752
Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input current at the minimum input voltage VIN(MIN) using conservation of energy and the expected efficiency at that operating point (MIN) taken from an appropriate curve in the Typical Operating Characteristics: IMAIN(MAX) x VMAIN VIN(MIN) x MIN
IIN(DC, MAX) =
Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) x (VMAIN - VIN(MIN) )
L x VMAIN x fOSC I IPEAK = IIN(DC, MAX) + RIPPLE 2
The inductor's saturation current rating and the MAX8752's LX current limit (ILIM) should exceed IPEAK and the inductor's DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1 series resistance. Considering the Typical Applications Circuit (Figure 1), the maximum load current (IMAIN(MAX)) is 180mA with a 10V output and a typical input voltage of 2.5V: IMAIN(EFF) = 180mA + 1 x 20mA + 3 x 10mA = 230mA
TFT LCD Step-Up DC-DC Converter MAX8752
Choosing an LIR of 0.5 and estimating efficiency of 80% at this operating point: 2.5V 10V - 2.5V 0.80 L= 2.6H 10V 0.23A x 1.2MHz 0.50 Using the circuit's minimum input voltage (2.2V) and estimating efficiency of 75% at that operating point: IIN(DC, MAX) = 0.23A x 10V 2.2V x 0.75 1.4 A
2
The ripple current and the peak current are: IRIPPLE = 2.2V x (10V - 2.2V) 2.6H x 10V x 1.2MHz 0.55A 2 0.55A
Input Capacitor Selection The input capacitor (CIN) reduces the current peaks drawn from the input supply and reduces noise injection into the IC. A 10F ceramic capacitor is used in the Typical Applications Circuit (Figure 1) because of the high source impedance seen in typical lab setups. Actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. Typically, CIN can be reduced below the values used in the Typical Applications Circuit. Ensure a low noise supply at IN by using adequate C IN . Alternatively, greater voltage variation can be tolerated on CIN if IN is decoupled from CIN using an RC lowpass filter (see R3 and C3 in Figure 1). Rectifier Diode Selection The MAX8752's high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. The diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode's peak current rating is at least IPEAK calculated in the Inductor Selection section and that its breakdown voltage exceeds the output voltage. Output Voltage Selection The MAX8752 operates with an adjustable output from VIN to 13V. Connect a resistive voltage-divider from the output (VMAIN) to GND with the center tap connected to FB (see Figure 1). Select R2 in the 10k to 50k range. Calculate R1 with the following equation: V R1 = R2 x MAIN - 1 VFB where VFB, the step-up regulator's feedback set point, is 1.24V (typ). Place R1 and R2 close to the IC.
IPEAK = 1.4 A +
1.7A
Output Capacitor Selection
The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging of the output capacitance, and the ohmic ripple due to the capacitor's equivalent series resistance (ESR): VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) IMAIN COUT VMAIN - VIN V , and MAIN fOSC
VRIPPLE(C)
VRIPPLE(ESR) IPEAK RESR(COUT) where I PEAK is the peak inductor current (see the Inductor Selection section). For ceramic capacitors, the output voltage ripple is typically dominated by VRIPPLE(C). The voltage rating and temperature characteristics of the output capacitor must also be considered.
10
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TFT LCD Step-Up DC-DC Converter
Loop Compensation
The voltage-feedback loop needs proper compensation to prevent excessive output ripple and poor efficiency caused by instability. This is done by connecting a resistor (RCOMP) and capacitor (CCOMP) in series from COMP to GND, and another capacitor (CCOMP2) from COMP to GND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response, while CCOMP is chosen to set the integrator zero to maintain loop stability. The second capacitor, CCOMP2, is chosen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: RCOMP 264 x VIN x VOUT x COUT L x IMAIN(EFF) VOUT x COUT 10 x IMAIN(MAX) x RCOMP 0.02 x RESR x L x IMAIN(EFF) VIN x VOUT capacitor and input-capacitor ground terminals. Connect these loop components with short, wide connections. Avoid using vias in the high-current paths, especially the ground paths. If vias are unavoidable, use many vias in parallel to reduce resistance and inductance. 2) Create a power ground island (PGND) consisting of the input and output capacitor grounds and GND. Connect all of these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output voltage ripple and noise spikes. Create an analog ground plane (AGND) consisting of the feedback divider's ground, the COMP capacitor's ground, and the IC's exposed backside pad near pin 1. Connect the AGND and PGND islands by connecting the GND pin directly to the exposed backside pad. Make no other connections between these separate ground planes. 3) Place the feedback voltage-divider resistors as close to FB as possible. The divider's center trace should be kept short. Placing the resistors far away causes the FB trace to become an antenna that can pick up switching noise. Avoid running the feedback trace near LX. 4) Place the SUP and LDO bypass capacitors and the IN bypass capacitors (C3 in Figure 1) if within 5mm of their respective pins. Connect their ground terminals to GND through the IC's exposed back paddle near GND (pin4). 5) Minimize the length and maximize the width of the traces between the output capacitors and the load for best transient responses. 6) Minimize the size of the LX node while keeping it wide and short. Keep the LX node away from the feedback node and other sensitive nodes. Use DC traces as shield if necessary. Refer to the MAX8752 evaluation kit for an example of proper board layout.
MAX8752
CCOMP
CCOMP2
For the ceramic output capacitor, where ESR is small, CCOMP2 is optional. The best gauge of correct loop compensation is by inspecting the transient response of the MAX8752. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance.
PC Board Layout and Grounding
Careful PC board layout is important for proper operation. Use the following guidelines for good PC board layout: 1) Minimize the area of high-current loops by placing the inductor, rectifier diode, and output capacitors near the input capacitors and near the LX and GND pins. The high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the IC's LX pin, out the IC's GND pin, and to the input capacitor's negative terminal. The highcurrent output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode (D1), to the positive terminal of the output capacitors, reconnecting between the output-
Chip Information
TRANSISTOR COUNT: 3091 PROCESS: BiCMOS
______________________________________________________________________________________
11
TFT LCD Step-Up DC-DC Converter MAX8752
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
6, 8, &10L, DFN THIN.EPS
2
D2 D A2
N
PIN 1 ID
0.35x0.35 b
PIN 1 INDEX AREA
E DETAIL A
E2 e
[(N/2)-1] x e REF.
A1
k
C L
C L
A
L e e
L
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
1
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 MAX. 0.80 3.10 3.10 0.05
0.20 0.40 0.25 MIN. 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T633-1 T633-2 T833-1 T833-2 T833-3 T1033-1 T1433-1 T1433-2 N 6 6 8 8 8 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.95 BSC 0.65 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEEC MO229 / WEED-3 ------b 0.400.05 0.400.05 0.300.05 0.300.05 0.300.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.90 REF 1.95 REF 1.95 REF 1.95 REF 2.00 REF 2.40 REF 2.40 REF
DOWNBONDS ALLOWED
NO NO NO NO YES NO YES NO
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
2 2
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

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