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MAX8740ETB+T

P1-P3P4-P6P7-P9P10-P12
MAX8740
TFT-LCD Step-Up DC-DC Converter
4 _______________________________________________________________________________________
EFFICIENCY vs. LOAD CURRENT
(1.2MHz OPERATION)
MAX8740 toc01
LOAD CURRENT (mA)
EFFICIENCY (%)
10010
50
60
70
80
90
100
40
11000
L = 2.7
µ
H
f
OSC
= 1.2MHz
V
IN
= 5.0V
V
IN
= 3.3V
EFFICIENCY vs. LOAD CURRENT
MAX8740 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)
10010
50
60
70
80
90
100
40
1 1000
L = 5.6
µ
H
f
OSC
= 640kHz
V
IN
= 5.0V
V
IN
= 3.3V
MAX8740 toc03
OUTPUT VOLTAGE vs. LOAD CURRENT
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
1101001000 10,000
11.5
11.7
11.9
12.1
12.3
12.5
12.7
12.9
V
IN
= 5.0V
V
IN
= 3.3V
f
OSC
= 1.2MHz
L = 2.7
µ
H
MAX8740 toc04
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
400
600
800
1000
1200
1400
FREQ = IN
FREQ = GND
MAX8740 toc05
SUPPLY CURRENT vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
SWITCHING
NONSWITCHING
MAX8740 toc06
SUPPLY CURRENT vs. TEMPERATURE
(SWITCHING)
TEMPERATURE (
°
C)
SUPPLY CURRENT (mA)
-40 -20 0 20 40 60 80 100
0.45
0.50
0.55
0.60
V
IN
= 5.0V
V
IN
= 3.3V
SOFT-START
(R
LOAD
= 30Ω)
MAX8740 toc07
2ms/div
SWITCHING WAVEFORMS
(I
LOAD
= 800mA)
MAX8740 toc08
400ns/div
Typical Operating Characteristics
(Circuit of Figure 1. V
IN
= 5V, V
MAIN
= 15V, T
A
= +25°C, unless otherwise noted.)
MAX8740
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________ 5
PIN
NAME
FUNCTION
1
COMP
Compensation 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 (V
OUT
) 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 V
OUT
according to the
Output Voltage Selection section.
3
SHDN
Shutdown Control Input. Drive SHDN low to turn off the MAX8740.
4, 5
GND
Ground. Connect pins 4 and 5 directly together.
6, 7 LX
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.
8INSupply Pin. Bypass IN with a minimum 1µF ceramic capacitor directly to GND.
9
FREQ
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 5µA pulldown current.
10 SS
Soft-Start Control Pin. Connect a soft-start capacitor (C
SS
) 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 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.
Pin Description
LX LX
FB
GND
GND
FREQ
IN
COMP
SS
1
4
5
2
3
9
8
67
10
V
OUT
13.5V/800mA
V
IN
4.5V TO 5.5V
SHDN
MAX8740
C1
10µF
6.3V
R3
10Ω
C3
1µF
C6
33nF
C4
560pF
C5
68pF
L1
2.7µH
D1
R2
20kΩ
1%
R4
47kΩ
1%
R1
196kΩ
1%
C2
10µF
20V
C7
10µF
20V
Figure 1. Typical Operating Circuit
MAX8740
TFT-LCD Step-Up DC-DC Converter
6 _______________________________________________________________________________________
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:
I
LIM
= (1.26 - 0.35 x D) x I
LIM_EC
where I
LIM
_
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 current-
limit value and is governed by the following equation:
where I
LIM
is the current limit calculated above, η is the
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
where V
DIODE
is the rectifier diode forward voltage and
R
ON
is the on-resistance of the internal MOSFET.
D
VVV
VIRV
OUT IN DIODE
OUT LIM ON DIODE
=
−+
−× +
II
DV
fL
V
V
OUT MAX LIM
IN
OSC
IN
OUT
()
.
=−
××
×
××
05
η
GND
LX
IN
FREQ
FB
COMP
4µA
5µA
N
ERROR
COMPARATOR
ERROR
AMPLIFIER
SKIP
COMPARATOR
SS
CLOCK
SKIP
BIAS
SHDN
MAX8740
Σ
CURRENT
SENSE
CONTROL
AND DRIVER
LOGIC
SOFT-
START
SLOPE
COMPEN-
SATION
OSCILLATOR
∞
1.24V
Figure 2. Functional Diagram
P1-P3P4-P6P7-P9P10-P12

MAX8740ETB+T

Mfr. #:
Buy MAX8740ETB+T
Manufacturer:
Maxim Integrated
Description:
LCD Drivers TFT-LCD Step-Up DC/DC Converter
Lifecycle:
New from this manufacturer.
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