MAX1885EUP-T Maxim Integrated, MAX1885EUP-T Datasheet - Page 23

• Manufacturer Part Number: MAX1885EUP-T
• Description: LCD Drivers Quad-Output TFT LCD DC-DC Converter with Buffer
• Manufacturer: Maxim Integrated

Specifications of MAX1885EUP-T

• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package / Case: TSSOP-20
• Mounting Style: SMD/SMT

times, the amplifier’s transconductance increases as
the output current increases (see the Typical Operating
Characteristics ).
The VCOM buffer’s positive feedback input features
dual mode operation. The buffer’s output voltage can
be internally set by a 50% resistive divider connected
to the buffer’s supply voltage (SUPB), or the output volt-
age can be externally adjusted for other voltages.
A logic-low level on SHDN shuts down all of the con-
verters and the reference. When shut down, the supply
current drops to 0.1µA to maximize battery life, and the
reference is pulled to ground. The output capacitance,
feedback resistors, and load current determine the rate
at which each output voltage decays. A logic-level high
on
MAX1880–MAX1885 (see the Power-Up Sequencing
section). Do not leave SHDN floating. If unused, con-
nect SHDN to IN. A logic-level transition on SHDN
clears the fault latch.
Upon power-up or exiting shutdown, the MAX1778/
MAX1880–MAX1885 start a power-up sequence. First,
the reference powers up. Then, the main DC-DC step-
up converter powers up with soft-start enabled. The lin-
ear regulator powers up at the same time as the main
step-up converter; however, the power sequence and
Figure 6. VCOM Buffer Block Diagram
Maxim Integrated
SHDN
power
activates
Power-Up Sequencing
Shutdown ( S S H H D D N N )
the
MAX1778/MAX1880–MAX1885
MAX1778
MAX1880
MAX1881
MAX1882
MAX1883
MAX1884
MAX1885
MAX1778/
Quad-Output TFT LCD DC/DC
gm
125mV
ready output signal are not affected by the regulation of
the linear regulator. While the main step-up converter
powers up, the output of the PWM comparator remains
low (Figure 2), and the step-up converter charges the
output capacitors, limited only by the maximum duty
cycle and current-limit comparator. When the step-up
converter approaches its nominal regulation value and
the PWM comparator’s output changes states for the
first time, the negative charge pump turns on. When the
negative output voltage reaches approximately 90% of
its nominal value (V
pump starts up. Finally, when the positive output volt-
age reaches 90% of its nominal value (V
the active-low ready signal (RDY) goes low (see the
Power Ready section ) , and the VCOM buffer powers
up. The MAX1883–MAX1885 do not contain the charge
pumps, but the power-up sequence still contains the
charge pumps’ startup logic, which appears as a delay
(2
ing regulation and when the ready signal and VCOM
buffer are activated.
For the main step-up regulator, soft-start allows a grad-
ual increase of the current-limit level during startup to
reduce input surge currents. The MAX1778/MAX1880–
MAX1885 divide the soft-start period into four phases.
During the first phase, the controller limits the current
limit to only 0.38A (see the Electrical Characteristics ),
approximately a quarter of the maximum current limit
Converters with Buffer

R
R
BUFOUT
4096/fOSC) between the step-up converter reach-
BUF+
SUPB
BUF-
GND
R11
R12
FBN
V
4.5V TO 13V
C
SUPB
V
BUF
BUFOUT
< 110mV), the positive charge
V
1.2V TO (V
=
BUFOUT
(
R11 + R12
R12
SUPB
- 1.2V)
)
V
SUPB
FBP
Soft-Start
> 1.125V),
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