Rainbow Electronics MAX1885 - Manual (Page 23)

MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
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times, the amplifier’s transconductance increases as
the output current increases (see 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.
Shutdown (SHDN)
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 will decay. A logic-level
high on SHDN power activates the MAX1778/
MAX1880–MAX1885 (see Power-Up Sequencing). Do
not leave SHDN floating. If unused, connect SHDN to
IN. A logic-level transition on SHDN clears the fault
latch.
Power-Up Sequencing
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
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
FBN < 110mV), the positive charge
pump starts up. Finally, when the positive output volt-
age reaches 90% of its nominal value (V
FBP > 1.125V),
the active-low ready signal (RDY) goes low (see Power
Ready), and the VCOM buffer powers up. The
MAX1883/MAX1884/MAX1885 do not contain the
charge pumps, but the power-up sequence still con-
tains the charge pumps’ startup logic, which appears
as a delay (2
✕ 4096/fOSC) between the step-up con-
verter reaching regulation and when the ready signal
and VCOM buffer are activated.
Soft-Start
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 Electrical Characteristics),
approximately a quarter of the maximum current limit
SUPB V SUPB
4.5V TO 13V
C BUF
BUFOUT
BUF-
V BUFOUT
1.2V TO (V SUPB - 1.2V)
BUF+
125mV
GND
R12
R
R
R11
V BUFOUT =
( )
V SUPB
R12
R11 + R12
MAX1778
MAX1880
MAX1881
MAX1882
MAX1883
MAX1884
MAX1885
gm
Figure 6. VCOM Buffer Block Diagram