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19-1042; Rev 0; 10/07
KIT ATION EVALU BLE AVAILA
TFT-LCD Step-Up DC-DC Converter
General Description Features
o 90% Efficiency o Adjustable Output from VIN to 20V o 2.6V to 5.5V Input Supply Range o Input Supply Undervoltage Lockout o Pin-Programmable 640kHz/1.2MHz Switching Frequency o Programmable Soft-Start o Improved EMI o FB Regulation Voltage Tolerance < 1% o Small 10-Pin TDFN Package o Thermal-Overload Protection
MAX17062
The MAX17062 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 MAX17062 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 22V internal MOSFET can generate output voltages up to 20V from an input voltage between 2.6V and 5.5V. Soft-start slowly ramps the input current and is programmed with an external capacitor. The MAX17062 is available in a 10-pin TDFN package.
Ordering Information
PART TEMP RANGE -40C to +85C PINPACKAGE PKG CODE
Applications
Notebook Computer Displays LCD Monitor Panels LCD TV Panels
MAX17062ETB+T
10 TDFN-EP* T1033-2 (3mm x 3mm)
+Denotes a lead-free package. *EP = Exposed pad. T = Tape and reel.
Pin Configuration
FREQ VIN 2.6V TO 5.5V LX LX IN
Minimal Operating Circuit
VOUT
TOP VIEW
10
SS
9
8
7
6 8
6 LX IN
7 LX FB 2
MAX17062
9 *EP 1 COMP 2 FB 3 SHDN 4 PGND 5 PGND 10 SS 3 FREQ SHDN
MAX17062
PGND 5 PGND 4
AGND EP
COMP 1
TDFN (3mm x 3mm) *EP = EXPOSED PAD.
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
TFT-LCD Step-Up DC-DC Converter MAX17062
ABSOLUTE MAXIMUM RATINGS
LX to AGND ............................................................-0.3V to +22V IN, SHDN, FREQ, FB to AGND..............................-0.3V to +7.5V COMP, SS to AGND ....................................-0.3V to (VIN + 0.3V) PGND to AGND .....................................................-0.3V to +0.3V LX Switch Maximum Continuous RMS Current .....................3.2A Continuous Power Dissipation (TA = +70C) 10-Pin TDFN (derate 24.4mW/C above +70C) ..........1951mW 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, FREQ = 3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Current Thermal Shutdown ERROR AMPLIFIER FB Regulation Voltage FB Input Bias Current FB Line Regulation Transconductance Voltage Gain Shutdown FB Input Voltage 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 4 100 6 A VFB = 1V, 75% duty cycle, IN = 5V IN = 5V IN = 3V VLX = 20V IN = 5V 0.09 3.9 4.6 100 125 11 0.15 5.3 170 210 20 0.25 A m A V/A FREQ = AGND FREQ = IN 500 1000 88 640 1200 91 780 1400 94 kHz % SHDN = AGND 0.05 Level to produce VCOMP = 1.24V VFB = 1.24V Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 110 1.23 75 1.24 150 0.01 250 2400 0.10 0.15 1.25 225 0.15 450 V nA %/V S V/V V VIN rising, typical hysteresis is 50mV VFB = 1.3V, not switching VFB = 1.0V, switching SHDN = AGND, TA = +25C SHDN = AGND, TA = +85C Temperature rising Hysteresis 2.30 2.45 0.3 1.5 0.01 0.01 160 20 VOUT < 18V 18V < V OUT < 20V CONDITIONS MIN 2.6 4.0 TYP MAX 5.5 5.5 20 2.57 0.6 2.5 10.0 UNITS V V V mA A C
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TFT-LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V SHDN = 3V, FREQ = 3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER CONTROL INPUTS SHDN, FREQ Input Low Voltage SHDN, FREQ Input High Voltage SHDN, FREQ Input Hysteresis FREQ Pulldown Current SHDN Input Current SHDN = AGND, TA = +25C SHDN = AGND, TA = +85C VIN = 2.6V to 5.5V VIN = 2.6V to 5.5V VIN = 2.6V to 5.5V 3 -1 0 0.7 VIN 0.1 VIN 6 9 +1 0.3 VIN V V V A A CONDITIONS MIN TYP MAX UNITS
MAX17062
ELECTRICAL CHARACTERISTICS
(VIN = V SHDN = 3V, FREQ = 3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current ERROR AMPLIFIER FB Regulation Voltage FB Input Bias Current FB Line Regulation Transconductance Shutdown FB Input Voltage OSCILLATOR Frequency Maximum Duty Cycle n-CHANNEL MOSFET Current Limit On-Resistance Current-Sense Transresistance SOFT-START Reset Switch Resistance Charge Current VSS = 1.2V 2 100 6 A VFB = 1V, 75% duty cycle, IN = 5V IN = 5V IN = 3V IN = 5V 0.09 3.9 5.3 170 210 0.25 A m V/A FREQ = AGND FREQ = IN 450 950 87 830 1500 95 kHz % SHDN = AGND Level to produce VCOMP = 1.24V VFB = 1.24V Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 110 0.05 1.227 1.253 225 0.15 450 0.15 V nA %/V S V VIN rising, typical hysteresis is 50mV VFB = 1.3V, not switching VFB = 1.0V, switching 2.30 VOUT < 18V 18V < V OUT < 20V CONDITIONS MIN 2.6 4.0 TYP MAX 5.5 5.5 20 2.57 0.6 2.5 UNITS V V V mA
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TFT-LCD Step-Up DC-DC Converter MAX17062
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V SHDN = 3V, FREQ = 3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER 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 VIN 0.3 VIN V V CONDITIONS MIN TYP MAX UNITS
Note 1: Limits are 100% tested at TA = +25C. Maximum and minimum limits over temperature are guaranteed by design.
Typical Operating Characteristics
(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VIN = 5V, VOUT = 15V)
MAX17062 toc01
EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V, VOUT = 9V)
fOSC = 640kHz L = 4.7H 90 EFFICIENCY (%)
MAX17062 toc02
LOAD REGULATION (VOUT = 15V)
MAX17062 toc03
100
100
1.0
90 EFFICIENCY (%)
80 fOSC = 1.2MHz L = 2.7H
80
fOSC = 1.2MHz L = 2.7H
LOAD REGULATION (%)
fOSC = 640kHz L = 4.7H
0.5
VIN = 5.0V
0 VIN = 3.3V -0.5
70
70
60
60
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 LOAD CURRENT (mA)
-1.0 1 10 100 1000 LOAD CURRENT (mA)
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX17062 toc04
SUPPLY CURRENT vs. SUPPLY VOLTAGE
3.5 SUPPLY CURRENT (mA) 3.0 2.5 2.0 1.5 1.0 0.5 0 NONSWITCHING 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SWITCHING
MAX17062 toc05
1400 1300 SWITCHING FREQUENCY (kHz) 1200 1100 1000 900 800 700 600 500 2.5 3.0 3.5 4.0 4.5 5.0 FREQ = GND FREQ = IN
4.0
5.5
INPUT VOLTAGE (V)
SUPPLY VOLTAGE (V)
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TFT-LCD Step-Up DC-DC Converter MAX17062
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25C, unless otherwise noted.)
SOFT-START (RLOAD = 30)
MAX17062 toc06
LOAD-TRANSIENT RESPONSE (ILOAD = 50mA TO 550mA)
MAX17062 toc07
VOUT 5V/div
15V VOUT 500mV/div AC-COUPLED 0V
OV INDUCTOR CURRENT 1A/div OA 2ms/div L = 2.7H RCOMP = 47k CCOMP1 = 560pF 100s/div
IOUT 500mA/div 50mA INDUCTOR CURRENT 2A/div 0A
PULSED LOAD-TRANSIENT RESPONSE (ILOAD = 100mA TO 1.1A)
MAX17062 toc08
SWITCHING WAVEFORMS (ILOAD = 600mA)
MAX17062 toc09
15V VOUT 200mV/div AC-COUPLED IOUT 1A/div 0.1A INDUCTOR CURRENT 1A/div 0A
LX 10V/div 0V INDUCTOR CURRENT 1A/div
0A 1s/div
10s/div L = 2.7H RCOMP = 47k CCOMP1 = 560pF
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TFT-LCD Step-Up DC-DC Converter MAX17062
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 AGND, 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 MAX17062. Power 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 together. Supply Pin. Bypass IN with a minimum 1F ceramic capacitor directly to AGND. 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 6A 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 4A. Full current limit is reached when the voltage of SS pin is charged to 1.5V, which is the current-limit time, t = 2.4 10 5 C SS. 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. Exposed Pad. Connect to AGND.
2
FB SHDN PGND LX IN FREQ
3 4, 5 6, 7 8 9
10
SS
EP
AGND
VIN 4.5V TO 5.5V
L1 2.7H
D1 C7 10F 25V
VOUT +15V/600mA C8 10F 25V
C1 4.7F 10V
C2 4.7F 10V
R1 10 8 C3 1F 3 9 SHDN FREQ LX IN LX
6 7 5 4 R4 221k
MAX17062
PGND PGND FB AGND
2 EP R3 20k
10 C4 33nF
SS
COMP 1 R2 47k C5 560pF
C6 OPEN
Figure 1. Typical Operating Circuit
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TFT-LCD Step-Up DC-DC Converter MAX17062
SHDN BIAS
SKIP COMPARATOR SKIP SOFTSTART
4A
IN
SS
COMP ERROR AMPLIFIER FB ERROR COMPARATOR CONTROL AND DRIVER LOGIC CLOCK
LX N
1.24V
PGND FREQ OSCILLATOR SLOPE COMPENSATION CURRENT SENSE
6A
MAX17062
Figure 2. MAX17062 Functional Diagram
Detailed Description
The MAX17062 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 MAX17062 to "skip" cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up to a peak value of approximately 50mA, discharges to the output, and waits until another pulse is needed again.
inductor 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 75% 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 MAX17062 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
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 MAX17062
Soft-Start
The MAX17062 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 4A (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. 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.
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 I2R 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
Frequency Selection
The MAX17062's frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to AGND 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.
Shutdown
The MAX17062 shuts down to reduce the supply current to 0.01A 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.
Thermal-Overload Protection
Thermal-overload protection prevents excessive power dissipation from overheating the MAX17062. When the junction temperature exceeds TJ = +160C, a thermal sensor immediately activates the fault protection, which shuts down the MAX17062, allowing the device to cool down. Once the device cools down by approximately 20C, the MAX17062 starts up automatically.
Table 1. Component List
DESIGNATION C1, C2 DESCRIPTION 4.7F 10%, 10V X5R ceramic capacitors (0603) TDK C1608X5RIA475K 10F10%, 25V X5R ceramic capacitors (1210) TDK C3225X5RIE106K 3A, 30V Schottky diode (M-Flat) Toshiba CMS03 2.7H 20% power inductor TOKO FDV0630-2R7M
Applications Information
Step-up regulators using the MAX17062 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 (Figure 1). Table 2 lists component suppliers.
C7, C8
D1 L1
Table 2. Component Suppliers
SUPPLIER TDK TOKO Toshiba PHONE 847-803-6100 847-297-0070 949-455-2000 FAX 847-390-4405 847-699-7864 949-859-3963 WEBSITE www.component.tdk.com www.tokoam.com www.toshiba.com/taec
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TFT-LCD Step-Up DC-DC Converter
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 (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: V L = IN VMAIN
2
The inductor's saturation current rating and the MAX17062's LX current limit (I LIM ) should exceed I PEAK , 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 (Figure 1), the maximum load current (IMAIN(MAX)) is 600mA with a 15V output and a typical input voltage of 5V. Choosing an LIR of 0.5 and estimating efficiency of 85% at this operating point: 5V 15V - 5V 0.85 L= 2.7H 15V 0.6 A x 1.2MHz 0.50 Using the circuit's minimum input voltage (4.5V) and estimating efficiency of 85% at that operating point: IIN(DC, MAX) = 0.6A x 15V 4.5V x 0.85 2.35A
2
MAX17062
VMAIN - VIN TYP I MAIN(MAX) x fOSC LIR
The ripple current and the peak current are: IRIPPLE = 4.5V x (15V - 4.5V) 0.97A 2.7H x 15V x 1.2MHz 0.97A 2.84A 2
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) = IMAIN(MAX) x VMAIN VIN(MIN) x MIN
IPEAK = 2.35A +
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) V I - VIN VRIPPLE(C) MAIN MAIN COUT VMAIN fOSC and: VRIPPLE(ESR) IPEAK RESR(COUT)
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
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.
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9
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. Two 4.7F ceramic capacitors are 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 R1 and C3 in Figure 1). Rectifier Diode Selection The MAX17062'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 MAX17062 operates with an adjustable output from VIN to 20V. Connect a resistive voltage-divider from the output (VMAIN) to AGND 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.
MAX17062
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 outputcapacitance ESR. For optimal performance, choose the components using the following equations: RCOMP 315 x VIN x VOUT x COUT L x IMAIN(MAX) VOUT x COUT 10 x IMAIN(MAX) x RCOMP
CCOMP
CCOMP2
0.0036 x RESR x L x IMAIN(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 MAX17062. 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 IN INRUSH - IOUT x VOUT 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 = 2.4 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 AGND, and another capacitor (CCOMP2) from COMP to AGND. R COMP is chosen to set the highfrequency integrator gain for fast transient response,
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TFT-LCD Step-Up DC-DC Converter MAX17062
D2 D3
VGOFF -15V C11 0.22F
1
C12 0.1F 3
C14 0.1F 3
2 C15 0.22F 1
VGON +29V
2 L1 2.7H
VIN 4.5V TO 5.5V
D1 C7 10F 25V C8 10F 25V
C1 4.7F 10V
C2 4.7F 10V
VOUT +15V/600mA
R5 10 8 C3 1F 3 R1 100k IN
U1
6 LX 7 LX
MAX17062 PGND 5
SHDN PGND 4 R4 221k 2 EP R3 20k
9 10
FREQ SS COMP 1
FB AGND
C4 33nF
R2 47k C5 560pF
C6 OPEN
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 (VGON and VGOFF) depends on the load characteristics of all three outputs.
reconnecting negative terminals of output capacitors to PGND of the IC. This loop has very high di/dt, and it is critical to minimize the area of this loop. 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 PGND pins. Connect all 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 PGND pins directly to the exposed backside pad. Make no other connections between these separate ground planes.
11
PCB Layout and Grounding
Careful PCB layout is important for proper operation. Use the following guidelines for good PCB 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 PGND pins. The high-current output 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 LX switch node to the rectifier diode (D1) to the output capacitors, and
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TFT-LCD Step-Up DC-DC Converter MAX17062
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 AGND 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 MAX17062 Evaluation Kit for an example of proper board layout.
Chip Information
TRANSISTOR COUNT: 3612 PROCESS: BiCMOS
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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
MAX17062
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13
TFT-LCD Step-Up DC-DC Converter MAX17062
Package Information (continued)
(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
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

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