MAX8727 데이터 시트보기 (PDF) - Maxim Integrated
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MAX8727 Datasheet PDF : 13 Pages
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TFT-LCD Step-Up DC-DC Converter
SHDN
COMP
FB
FREQ
SKIP
COMPARATOR
BIAS
SKIP
IN
4μA
SOFT-
START
SS
1.24V
OSCILLATOR
ERROR
AMPLIFIER
∞
SLOPE
COMPEN-
SATION
ERROR
COMPARATOR
CLOCK
Σ
CONTROL
AND DRIVER
LOGIC
CURRENT
SENSE
LX
N
GND
5μA
MAX8727
Figure 2. MAX8727 Functional Diagram
Detailed Description
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.
Output Current Capability
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
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) and D is the
duty cycle.
The output current capability depends on the current-
limit value and is governed by the following equation:
IOUT(MAX)
⎡
= ⎣⎢⎢ILIM
−
0.5 ×
D
×
VIN
⎤
⎥
×
fOSC × L ⎦⎥
VIN
VOUT
×
η
where ILIM 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:
D=
VOUT − VIN + VDIODE
VOUT − ILIM × RON + VDIODE
where VDIODE is the rectifier diode forward voltage and
RON is the on-resistance of the internal MOSFET.
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