MAX8740ETB+T Fast Delivery |IC PHOENIX CO.,LIMITED

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MAX8740ETB+T
TFT-LCD Step-Up DC-DC Converter
General Description
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 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.
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 pro-
vides fast transient response to pulsed loads. A com-
pensation 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
Features90% EfficiencyAdjustable Output from VINto 28V2.6V to 5.5V Input Supply RangeInput Supply Undervoltage LockoutPin-Programmable 640kHz/1.2MHz Switching
FrequencyProgrammable Soft-Start0.1µA Shutdown CurrentSmall, 10-Pin Thin DFN Package
MAX8740
TFT-LCD Step-Up DC-DC Converter
Ordering Information
19-3698; Rev 0; 5/05
EVALUATION KIT
AVAILABLE
PARTTEMP RANGEPIN-PACKAGE
MAX8740ETB-40°C to +85°C10 TDFN 3mm x 3mm
COMP3
GND
GND9876
SHDNFREQLXLXIN
MAX8740
TOP VIEW
THIN DFN
3mm x 3mm
Pin ConfigurationLX
GND
GND
FREQ
COMPSS1
VOUTVIN
2.6V TO 5.5V
SHDN
MAX8740
Minimal Operating Circuit
MAX8740
TFT-LCD Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= VSHDN= 3V, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
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.
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= +70°C)
10-Pin TDFN (derate 24.1mW/°C above +70°C).......1481.5mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERCONDITIONSMINTYPMAXUNITS
VOUT < 18V2.65.5Input Voltage Range18V < VOUT < 24V4.05.5V
Output Voltage Range28V
IN Undervoltage-Lockout
Threshold
VIN rising, typical hysteresis is 50mV; LX remains off
below this level2.202.382.57V
VFB = 1.3V, not switching0.220.44IN Quiescent CurrentVFB = 1.0V, switching, FREQ = GND25mA
IN Shutdown CurrentSHDN = GND0.110.0µA
ERROR AMPLIFIER
FB Regulation VoltageLevel to produce VCOMP = 1.24V1.221.241.26V
FB Input Bias CurrentVFB = 1.24V50125250nA
FB Line RegulationLevel to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V0.050.15%/V
Transconductance100200315µS
Voltage Gain2400V/V
OSCILLATOR
FREQ = GND540640740FrequencyFREQ = IN100012201500kHz
Maximum Duty Cycle889194%
n-CHANNEL MOSFET
Current LimitVFB = 1V, 71% duty cycle3.94.65.3A
VIN = 3V (typ value at TA = +25°C)0.110.17On-ResistanceVIN = 5V (typ value at TA = +25°C)0.0950.15Ω
Leakage CurrentVLX = 28V3055µA
Current-Sense Transresistance0.090.150.25V/A
SOFT-START
Reset Switch Resistance100Ω
Charge CurrentVSS = 1.2V2.54.57.5µA
MAX8740
TFT-LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN= VSHDN= 3V, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
PARAMETERCONDITIONSMINTYPMAXUNITS
CONTROL INPUTS
SHDN, FREQ Input Low VoltageVIN = 2.6V to 5.5V0.3 x
VINV
SHDN, FREQ Input High VoltageVIN = 2.6V to 5.5V0.7 x
VINV
SHDN, FREQ Input HysteresisVIN = 2.6V to 5.5V0.1 x
VINV
FREQ Pulldown Current2.36.09.5µA
SHDN Input CurrentSHDN = GND0.0011µA
ELECTRICAL CHARACTERISTICS
(VIN= VSHDN= 3V, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETERCONDITIONSMINTYPMAXUNITS
VOUT < 18V2.65.5Input Voltage Range18V < VOUT < 28V4.05.5V
Output Voltage Range28V
VFB = 1.3V, not switching0.44IN Quiescent CurrentVFB = 1.0V, switching, FREQ = GND5mA
IN Shutdown CurrentSHDN = GND10µA
ERROR AMPLIFIER
FB Regulation VoltageLevel to produce VCOMP = 1.24V1.2151.260V
FB Line RegulationLevel to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V0.15%/V
Transconductance100330µS
OSCILLATOR
FREQ = GND490770FrequencyFREQ = IN9001600kHz
n-CHANNEL MOSFET
Current LimitVFB = 1V, 71% duty cycle3.95.3A
Current-Sense Transresistance0.090.25V/A
CONTROL INPUTS
SHDN, FREQ Input Low VoltageVIN = 2.6V to 5.5V0.3 x
VINV
SHDN, FREQ Input High VoltageVIN = 2.6V to 5.5V0.7 x
VINV
Note 1:-40°C specifications are guaranteed by design, not production tested.
MAX8740
TFT-LCD Step-Up DC-DC Converter
EFFICIENCY vs. LOAD CURRENT
(1.2MHz OPERATION)
MAX8740 toc01
LOAD CURRENT (mA)
EFFICIENCY (%)
L = 2.7μH
fOSC = 1.2MHzVIN = 5.0V
VIN = 3.3V
EFFICIENCY vs. LOAD CURRENT
MAX8740 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)
L = 5.6μH
fOSC = 640kHzVIN = 5.0V
VIN = 3.3V
MAX8740 toc03
OUTPUT VOLTAGE vs. LOAD CURRENT
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
110100100010,000
VIN = 5.0V
VIN = 3.3V
fOSC = 1.2MHz
L = 2.7μH
MAX8740 toc04
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
FREQ = IN
FREQ = GND
MAX8740 toc05
SUPPLY CURRENT vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
SWITCHING
NONSWITCHING
MAX8740 toc06
SUPPLY CURRENT vs. TEMPERATURE
(SWITCHING)
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
VIN = 5.0V
VIN = 3.3V
SOFT-START
(RLOAD = 30Ω)
MAX8740 toc07
2ms/div
SWITCHING WAVEFORMS
(ILOAD = 800mA)
MAX8740 toc08
400ns/div
Typical Operating Characteristics
(Circuit of Figure1. VIN= 5V, VMAIN= 15V, TA= +25°C, unless otherwise noted.)
MAX8740
TFT-LCD Step-Up DC-DC Converter
PINNAMEFUNCTIONCOMPCompensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
2FB
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.SHDNShutdown Control Input. Drive SHDN low to turn off the MAX8740.
4, 5GNDGround. Connect pins 4 and 5 directly together.
6, 7LXSwitch 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.
8INSupply Pin. Bypass IN with a minimum 1µF ceramic capacitor directly to GND.FREQFrequency-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 5µA pulldown current.SS
Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The soft-
start capacitor is charged with a constant current of 4.5µA. 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.
Pin DescriptionLX
GND
GND
FREQ
COMPSS1
VOUT
13.5V/800mA
VIN
4.5V TO 5.5V
SHDN
MAX8740
10μF
6.3V
10Ω
1μF
33nF
560pF
68pF
2.7μHD1
20kΩ
47kΩ
196kΩ
10μF
20V
10μF
20V
Figure 1. Typical Operating Circuit
MAX8740
TFT-LCD Step-Up DC-DC Converter
Detailed Description
The MAX8740 is a highly efficient power supply that
employs a current-mode, fixed-frequency, PWM archi-
tecture for fast transient response and low-noise opera-
tion. 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-compen-
sation 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.
Output Current Capability
The output current capability of the MAX8740 is a func-
tion of current limit, input voltage, operating frequency,
and inductor value. Because of the slope compensa-
tion 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= (1.26 - 0.35 x D) x ILIM_EC
where ILIM_ECis 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 current-
limit value and is governed by the following equation:
where ILIMis the current limit calculated above, ηis the
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
where VDIODEis the rectifier diode forward voltage and
RONis the on-resistance of the internal MOSFET.VVVRV
OUTINDIODE
OUTLIMONDIODE =−++DVOUTMAXLIMIN
OSC
OUT . =−××××05η
GND
FREQ
COMP
4μA
5μA
ERROR
COMPARATOR
ERROR
AMPLIFIER
SKIP
COMPARATOR
CLOCK
SKIP
BIAS
SHDN
MAX8740CURRENT
SENSE
CONTROL
AND DRIVER
LOGIC
SOFT-
START
SLOPE
COMPEN-
SATION
OSCILLATOR
1.24V
Figure 2. Functional Diagram

Partno	Mfg	Dc	Qty	Available	Descript
MAX8740ETB+T \|MAX8740ETBT	MAXIM	N/a	1894	avai	TFT-LCD Step-Up DC-DC Converter