MAX8740ETB+T Maxim Integrated Products, MAX8740ETB+T Datasheet - Page 6

• Manufacturer Part Number: MAX8740ETB+T
• Description: Display Drivers Step-Up DC-DC Conver p-Up DC-DC Converter
• Manufacturer: Maxim Integrated Products

Specifications of MAX8740ETB+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

TFT-LCD Step-Up DC-DC Converter
Figure 2. Functional Diagram
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.
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
6
_______________________________________________________________________________________
COMP
SHDN
FREQ
FB
Detailed Description
Output Current Capability
5µA
OSCILLATOR
1.24V
ERROR
AMPLIFIER
COMPEN-
SATION
SLOPE
∞
BIAS
Σ
ERROR
COMPARATOR
SKIP
COMPARATOR
current limit depends on the duty cycle. The current
limit is determined by the following equation:
where I
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
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
where V
R
I
OUT MAX
CLOCK
ON
(
is the on-resistance of the internal MOSFET.
LIM
LIM
D
DIODE
)
AND DRIVER
_
SKIP
CONTROL
CURRENT
=
is the current limit calculated above, η is the
I
EC
=
LOGIC
SENSE
LIM
V




is the rectifier diode forward voltage and
is the current limit specified at 71% duty
I
OUT
LIM
= (1.26 - 0.35 x D) x I
V
OUT
−
−
MAX8740
SOFT-
START
I
0 5
LIM
.
−
f
OSC
×
V
×
4µA
IN
D
R
N
×
+
ON
×
L
V
V
DIODE
IN
+
LIM_EC




IN
SS
LX
GND
V
DIODE
×
V
V
OUT
IN
×
η