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19-3698; Rev 0; 5/05
KIT ATION EVALU E AILABL AV
TFT-LCD Step-Up DC-DC Converter
General Description Features
90% Efficiency Adjustable Output from VIN to 28V 2.6V to 5.5V Input Supply Range Input Supply Undervoltage Lockout Pin-Programmable 640kHz/1.2MHz Switching Frequency Programmable Soft-Start 0.1A Shutdown Current Small, 10-Pin Thin DFN Package
MAX8740
The MAX8740 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 MAX8740 incorporates currentmode, fixed-frequency, pulse-width modulation (PWM) circuitry with a built-in n-channel power MOSFET to achieve high efficiency and fast transient response. Users can select 640kHz or 1.2MHz operation using a logic input pin (FREQ). The high switching frequencies allow the use of ultra-small inductors and low-ESR ceramic capacitors. The current-mode architecture provides fast transient response to pulsed loads. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The 30V internal MOSFET can generate output voltages up to 28V from a 2.6V and 5.5V input voltage range. Soft-start slowly ramps the input current and is programmed with an external capacitor. The MAX8740 is available in a 10-pin thin DFN package.
Applications
Notebook Computer Displays LCD Monitor Panels
PART MAX8740ETB
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 10 TDFN 3mm x 3mm
Pin Configuration
VIN 2.6V TO 5.5V FREQ SS LX LX IN
Minimal Operating Circuit
VOUT
TOP VIEW
10
9
8
7
6 8 IN
6 LX
7 LX FB 2
MAX8740 MAX8740
9 3 FREQ SHDN GND 5 GND 4
1 COMP
2 FB
3 SHDN
4 GND
5 GND
10
SS
COMP 1
THIN DFN 3mm x 3mm
________________________________________________________________ Maxim Integrated Products
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 MAX8740
ABSOLUTE MAXIMUM RATINGS
LX to GND ..............................................................-0.3V to +30V IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V COMP, SS to GND .......................................-0.3V to (VIN + 0.3V) LX Switch Maximum Continuous RMS Current .....................2.4A Continuous Power Dissipation (TA = +70C) 10-Pin TDFN (derate 24.1mW/C above +70C) .......1481.5mW 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 = V SHDN = 3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Input Voltage Range Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Current ERROR AMPLIFIER FB Regulation Voltage FB Input Bias Current FB Line Regulation Transconductance Voltage Gain OSCILLATOR Frequency Maximum Duty Cycle n-CHANNEL MOSFET Current Limit On-Resistance Leakage Current Current-Sense Transresistance SOFT-START Reset Switch Resistance Charge Current VSS = 1.2V 2.5 4.5 100 7.5 A VFB = 1V, 71% duty cycle VIN = 3V (typ value at TA = +25C) VIN = 5V (typ value at TA = +25C) VLX = 28V 0.09 3.9 4.6 0.11 0.095 30 0.15 5.3 0.17 0.15 55 0.25 A A V/A FREQ = GND FREQ = IN 540 1000 88 640 1220 91 740 1500 94 kHz % Level to produce VCOMP = 1.24V VFB = 1.24V Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 100 1.22 50 1.24 125 0.05 200 2400 1.26 250 0.15 315 V nA %/V S V/V VIN rising, typical hysteresis is 50mV; LX remains off below this level VFB = 1.3V, not switching VFB = 1.0V, switching, FREQ = GND SHDN = GND 2.20 2.38 0.22 2 0.1 VOUT < 18V 18V < VOUT < 24V CONDITIONS MIN 2.6 4.0 TYP MAX 5.5 5.5 28 2.57 0.44 5 10.0 UNITS V V V mA A
2
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TFT-LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V SHDN = 3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER CONTROL INPUTS SHDN, FREQ Input Low Voltage SHDN, FREQ Input High Voltage SHDN, FREQ Input Hysteresis FREQ Pulldown Current SHDN Input Current SHDN = GND VIN = 2.6V to 5.5V VIN = 2.6V to 5.5V VIN = 2.6V to 5.5V 2.3 0.7 x VIN 0.1 x VIN 6.0 0.001 9.5 1 0.3 x VIN V V V A A CONDITIONS MIN TYP MAX UNITS
MAX8740
ELECTRICAL CHARACTERISTICS
(VIN = V SHDN = 3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Output Voltage Range IN Quiescent Current IN Shutdown Current ERROR AMPLIFIER FB Regulation Voltage FB Line Regulation Transconductance OSCILLATOR Frequency n-CHANNEL MOSFET Current Limit Current-Sense Transresistance CONTROL INPUTS SHDN, FREQ Input Low Voltage SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V VIN = 2.6V to 5.5V 0.7 x VIN 0.3 x VIN V V VFB = 1V, 71% duty cycle 3.9 0.09 5.3 0.25 A V/A FREQ = GND FREQ = IN 490 900 770 1600 kHz Level to produce VCOMP = 1.24V Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 100 1.215 1.260 0.15 330 V %/V S VFB = 1.3V, not switching VFB = 1.0V, switching, FREQ = GND SHDN = GND VOUT < 18V 18V < VOUT < 28V CONDITIONS MIN 2.6 4.0 TYP MAX 5.5 5.5 28 0.44 5 10 UNITS V V mA A
Note 1: -40C specifications are guaranteed by design, not production tested.
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TFT-LCD Step-Up DC-DC Converter MAX8740
Typical Operating Characteristics
(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (1.2MHz OPERATION)
MAX8740 toc01
EFFICIENCY vs. LOAD CURRENT
L = 5.6H fOSC = 640kHz
MAX8740 toc02
OUTPUT VOLTAGE vs. LOAD CURRENT
12.9 12.7 OUTPUT VOLTAGE (V) 12.5 12.3 12.1 11.9 11.7 11.5 fOSC = 1.2MHz L = 2.7H VIN = 3.3V VIN = 5.0V
MAX8740 toc03
100 90 EFFICIENCY (%) 80 70 60 50 40 1
L = 2.7H fOSC = 1.2MHz
100 90 EFFICIENCY (%) 80 70 60 50 40 1
VIN = 5.0V
VIN = 5.0V
VIN = 3.3V
VIN = 3.3V
10
100
1000
10
100
1000
1
10
LOAD CURRENT (mA)
LOAD CURRENT (mA)
100 1000 LOAD CURRENT (mA)
10,000
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX8740 toc04
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX8740 toc05
SUPPLY CURRENT vs. TEMPERATURE (SWITCHING)
MAX8740 toc06
1400
0.7 0.6 SUPPLY CURRENT (mA) 0.5 0.4 0.3 0.2 0.1 NONSWITCHING
0.60
SWITCHING FREQUENCY (kHz)
1200
SUPPLY CURRENT (mA)
FREQ = IN
SWITCHING
VIN = 5.0V 0.55 VIN = 3.3V 0.50
1000
800 FREQ = GND 600
400 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5
0.45 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 5.5 -40 -20 0 20 40 60 TEMPERATURE (C) 80 100
SOFT-START (RLOAD = 30)
MAX8740 toc07
SWITCHING WAVEFORMS (ILOAD = 800mA)
MAX8740 toc08
2ms/div
400ns/div
4
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TFT-LCD Step-Up DC-DC Converter MAX8740
Pin Description
PIN 1 NAME COMP FUNCTION Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop Compensation section for component selection guidelines. Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltage-divider between the step-up regulator's output (VOUT) 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 VOUT according to the Output Voltage Selection section. Shutdown Control Input. Drive SHDN low to turn off the MAX8740. Ground. Connect pins 4 and 5 directly together. Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and minimize the trace area for lower EMI. Connect pins 6 and 7 directly together. Supply Pin. Bypass IN with a minimum 1F ceramic capacitor directly to GND. Frequency-Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high, the frequency is 1.2MHz. This input has a 5A pulldown current. Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The softstart capacitor is charged with a constant current of 4.5A. Full current limit is reached after t = 2.5 x 105 CSS. The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start capacitor is charged to 0.4V, after which soft-start begins.
2
FB SHDN GND LX IN FREQ
3 4, 5 6, 7 8 9
10
SS
VIN 4.5V TO 5.5V C1 10F 6.3V R3 10 8 C3 1F 9 3
L1 2.7H
D1 R1 196k 1% FB 2 R2 20k 1% GND 5 GND 4 C2 10F 20V C7 10F 20V
VOUT 13.5V/800mA
6 LX IN
7 LX
MAX8740
FREQ SHDN
10 C6 33nF
SS
COMP 1 R4 47k 1% C4 560pF C5 68pF
Figure 1. Typical Operating Circuit
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5
TFT-LCD Step-Up DC-DC Converter MAX8740
SHDN BIAS
SKIP COMPARATOR SKIP SOFTSTART
4A
IN
SS
COMP ERROR AMPLIFIER FB ERROR COMPARATOR CONTROL AND DRIVER LOGIC CLOCK
LX N
1.24V
GND FREQ OSCILLATOR SLOPE COMPENSATION CURRENT SENSE
5A
MAX8740
Figure 2. Functional Diagram
Detailed Description
The MAX8740 is a highly efficient power supply that employs a current-mode, fixed-frequency, PWM architecture for fast transient response and low-noise operation. The device 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 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 command 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. At light loads, this architecture allows the MAX8740 to "skip" cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up to a peak value of approximately 150mA, discharges to the output, and waits until another pulse is needed again.
current limit depends on the duty cycle. The current limit is determined by the following equation: ILIM = (1.26 - 0.35 x D) x ILIM_EC where ILIM_EC is the current limit specified at 71% duty cycle (see the Electrical Characteristics table) 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 x VIN VIN IOUT(MAX) = ILIM - x x fOSC x L VOUT 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: D= VOUT - VIN + VDIODE VOUT - ILIM x RON + VDIODE
Output Current Capability
The output current capability of the MAX8740 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
where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET.
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TFT-LCD Step-Up DC-DC Converter
Soft-Start
The MAX8740 can be programmed for soft-start upon power-up with an external capacitor. When the shutdown pin is taken high, the soft-start capacitor (CSS) is immediately charged to 0.4V. Then the capacitor is charged at a constant current of 4.5A (typ). During this time, the SS voltage directly controls the peak inductor current, allowing 0A at VSS = 0.4V to the full current limit at VSS = 1.5V. The maximum load current is available after the soft-start is completed. When the SHDN pin is taken low, the softstart capacitor is discharged to ground.
Shutdown
The MAX8740 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. The step-up regulator's output is connected to IN by the external inductor and rectifier diode.
MAX8740
Applications Information
Step-up regulators using the MAX8740 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 L is known, choose the diode and capacitors.
Frequency Selection
The MAX8740's frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to GND for 640kHz operation. For a 1.2MHz switching frequency, connect FREQ to IN. This allows the use of small, minimum-height external components while maintaining low output noise. FREQ has an internal pulldown, allowing the user the option of leaving FREQ unconnected for 640kHz operation.
Table 1. Component List
DESIGNATION DESCRIPTION 10F 10%, 6.3V X5R ceramic capacitor (0805) Murata GRM21BR60J106K Taiyo Yuden JMK212BJ106KD 10F 20%, 25V X5R ceramic capacitors (1210) TDK C3225X5R1E106M, Taiyo Yuden TMK325BJ106MM 3A, 40V Schottky diode (SM8) Central Semiconductor CMSH3-40M 3.3H 30%, 4.0A power inductor Sumida CDRH8D28-3R3, 3.3H (alternate : Sumida CDRH103R-3R3, 3.3H)
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, transientresponse time, and output voltage ripple. Physical size and cost are also important factors to be considered. 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.
C1
C2, C7
D1
L1
Table 2. Component Suppliers
SUPPLIER Murata Sumida Taiyo Yuden TDK Toshiba PHONE 770-436-1300 847-545-6700 800-348-2496 847-803-6100 949-455-2000 FAX 770-436-3030 847-545-6720 847-925-0899 847-390-4405 949-859-3963 WEBSITE www.murata.com www.sumida.com www.t-yuden.com www.component.tdk.com www.toshiba.com/taec
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7
TFT-LCD Step-Up DC-DC Converter
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 the 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. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IOUT(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 VOUT - VIN TYP L = IN VOUT IOUT(MAX) x fOSC LIR
MAX8740
The inductor's saturation current rating and the MAX8740'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 operating circuit, the maximum load current (IOUT(MAX)) is 900mA with a 13.5V output and a 5V typical input voltage. Choosing an LIR of 0.35 and estimating efficiency of 85% at this operating point: 5V 13.5V - 5V 0.85 L= 2.7H 13.5V 0.9A x 1.2MHz 0.35 Using the circuit's minimum input voltage (4.5V) and estimating efficiency of 85% at that operating point: IIN(DC, MAX) = 0.9A x 3.5V 3.2A 4.5V x 0.85
2
The ripple current and the peak current are: IRIPPLE = 4.5V x (12.5V - 4.5V) 0.93A 2.7H x 13.5V x 1.2MHz 0.93A 3.7A 2
IPEAK = 3.2A +
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) IOUT COUT VOUT - VIN V , and OUT fOSC
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: IIN(DC, MAX) = IOUT(MAX) x VOUT VIN(MIN) x MIN
VRIPPLE(C)
Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) x (VOUT - VIN(MIN) ) L x VOUT x fOSC
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.
I IPEAK = IIN(DC, MAX) + RIPPLE 2
8
_______________________________________________________________________________________
TFT-LCD Step-Up DC-DC Converter
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 operating 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 operating circuit. Ensure a low noise supply at IN by using adequate CIN. 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 MAX8740'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 MAX8740 operates with an adjustable output from VIN to 28V. Connect a resistive voltage-divider from the output (VOUT) 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 OUT - 1 VFB where VFB, the step-up regulator's feedback set point, is 1.28V (typ). Place R1 and R2 close to the IC. sen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: RCOMP 315 x VIN x VOUT x COUT L x IOUT(MAX) VOUT x COUT 10 x IOUT(MAX) x RCOMP
MAX8740
CCOMP
CCOMP2
0.0036 x RESR x L x IOUT(MAX) VIN x VOUT
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 MAX8740. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance.
Soft-Start Capacitor
The soft-start capacitor should be large enough that it does not reach final value before the output has reached regulation. Calculate CSS to be: CSS > 21 x 10 -6 x COUT x 2 VOUT - VIN x VOUT V x I - IOUT x VOUT IN INRUSH where COUT is the total output capacitance including any bypass capacitor on the output bus, VOUT is the maximum output voltage, IINRUSH is the peak inrush current allowed, IOUT is the maximum output current during power-up, and VIN is the minimum input voltage. The load must wait for the soft-start cycle to finish before drawing a significant amount of load current. The duration after which the load can begin to draw maximum load current is: tMAX = 6.77 x 105 x CSS
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 cho-
_______________________________________________________________________________________
9
TFT-LCD Step-Up DC-DC Converter MAX8740
V3 -10V C10 0.22F
D2 V2 +28V C9 1F
C7 C8 0.1F 0.1F
D3
VIN 4.5V TO 5.5V C1 10F 6.3V R4 10 8 C5 1F 9 3
L1 2.7H
D1 R1 196k 1% FB 2 R2 20k 1% GND 5 GND 4 C2 10F 25V C7 10F 25V
VOUT 13.5V/800mA
6 LX IN
7 LX
MAX8740
FREQ SHDN
10 C4 33nF
SS
COMP 1 R3 47k 1% C3 560pF C6 68pF
Figure 3. Multiple-Output TFT-LCD Power Supply
Multiple-Output Power Supply for TFT LCD
Figure 3 shows a power supply for active-matrix TFTLCD flat-panel displays. Output-voltage transient performance is a function of the load characteristic. Add or remove output capacitance (and recalculate compensation-network component values) as necessary to meet the required transient performance. Regulation performance for secondary outputs (V2 and V3) depends on the load characteristics of all three outputs.
and to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Connect these loop components with short, wide connections. Avoid using vias in the high-current 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 pins. 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 ground connection, the COMP and SS capacitor ground connections, and the device's exposed backside pad. Connect the AGND and PGND islands by connecting the GND pins directly to the exposed backside pad. Make no other connections between these separate ground planes.
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 of GND, and to the input capacitor's negative terminal. The high-current output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode (D1),
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______________________________________________________________________________________
TFT-LCD Step-Up DC-DC Converter
3) Place the feedback voltage-divider-resistors as close to the FB pin 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 IN pin bypass capacitor as close to the device as possible. The ground connection of the IN bypass capacitor should be connected directly to GND pins with a wide trace. 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 analog ground. Use DC traces as a shield if necessary. Refer to the MAX8740 evaluation kit for an example of proper board layout.
MAX8740
Chip Information
TRANSISTOR COUNT: 2746 PROCESS: BiCMOS
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11
TFT-LCD Step-Up DC-DC Converter
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
MAX8740
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
2
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 12 (c) 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

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