MAX1885EUP-T Maxim Integrated, MAX1885EUP-T Datasheet - Page 22

• 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

Quad-Output TFT LCD DC/DC
Converters with Buffer
MAX1778/MAX1880–MAX1885
22
During the first half-cycle, the p-channel MOSFET turns
on, and flying capacitor C
minus a diode drop (Figure 4). During the second
half-cycle, the p-channel MOSFET turns off, and the
n-channel MOSFET turns on, level shifting C
This connects C
tor C
a diode drop is greater than the voltage across
C
the diode (D5) turns off. The amount of charge trans-
ferred to the output is controlled by the variable
n-channel on-resistance.
The MAX1778/MAX1881/MAX1883/MAX1884 contain a
low-dropout linear regulator (Figure 5) that uses an
internal PNP pass transistor (Q
40mA. As illustrated in Figure 5, the 1.25V reference is
connected to the error amplifier, which compares this
reference with the feedback voltage and amplifies the
difference. If the feedback voltage is higher than the
reference voltage, the controller lowers the base
current of Q
the output. If the feedback voltage is too low, the
Figure 5. Low-Dropout Linear Regulator Block Diagram
X(NEG)
Low-Dropout Linear Regulator (MAX1778/
OUT(NEG)
, charge flows from C
MAX1881/MAX1883/MAX1884 Only)
P
, which reduces the amount of current to
. If the voltage across C
X(NEG)
in parallel with reservoir capaci-
X(NEG)
OUT(NEG)
Negative Charge Pump
P
) to supply loads up to
charges to V
OUT(NEG)
to C
X(NEG)
MAX1778
MAX1881
MAX1883
MAX1884
ERROR
AMPLIFIER
X(NEG)
THERMAL
SENSOR
minus
SUPN
CURRENT
until
LIMIT
.
device increases the pass transistor base current,
which allows more current to pass to the output and
increases the output voltage. However, the linear regu-
lator also includes an output current limit to protect the
internal pass transistor against short circuits.
The low-dropout linear regulator monitors and controls
the pass transistor’s base current, limiting the output
current to 130mA (typ). In conjunction with the thermal
overload protection, this current limit protects the
output, allowing it to be shorted to ground for an indefi-
nite period of time without damaging the part.
The MAX1778/MAX1880–MAX1885 include a VCOM
buffer, which uses an operational transconductance
amplifier (OTA) to provide a current output that is ideal
for driving capacitive loads, such as the backplane of a
TFT LCD panel. The unity-gain bandwidth of this
current-output buffer is:
where gm is the amplifier’s transconductance. The
bandwidth is inversely proportional to the output
capacitor, so large capacitive loads improve stability;
however, lower bandwidth decreases the buffer’s tran-
sient response time. To improve the transient response
V
1.25V
REF
LDOOUT
SUPL
GND
FBL
Q
P
R7
R8
C
SUPL
GBW = gm/C
C
V
V
LDOOUT
LDOOUT
REF
= 1.25V
V
1.25V TO (V
V
4.5V TO 15V
LDOOUT
SUPL
=
(
1 +
OUT
R7
R8
SUPL
)
V
REF
- 0.3V)
VCOM Buffer
Maxim Integrated