MAX8727 Maxim, MAX8727 Datasheet - Page 7

• Manufacturer Part Number: MAX8727
• Description: TFT-LCD Step-Up DC-DC Converter
• Manufacturer: Maxim

Figure 2. MAX8727 Functional Diagram
The MAX8727 is a highly efficient power supply that
employs a current-mode, fixed-frequency, pulse-width
modulation (PWM) architecture for fast transient
response and low-noise operation. The device regu-
lates 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 cur-
rent 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 MAX8727 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.
The output current capability of the MAX8727 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
COMP
SHDN
FREQ
FB
_______________________________________________________________________________________
Detailed Description
Output Current Capability
5µA
OSCILLATOR
1.24V
TFT-LCD Step-Up DC-DC Converter
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) 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 75% duty
I
OUT
LIM
= (1.26 - 0.35 x D) x I
V
OUT
−
−
MAX8727
SOFT-
START
I
0 5
LIM
.
−
f
OSC
×
V
×
4µA
IN
D
R
N
×
+
ON
×
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L
V
V
DIODE
IN
+
LIM_EC
⎤
⎥
⎥
⎦
IN
SS
LX
V
GND
DIODE
×
V
V
OUT
IN
×
η
7