MAX1778EUG+ Maxim Integrated Products, MAX1778EUG+ Datasheet - Page 22

• Manufacturer Part Number: MAX1778EUG+
• Description: IC DCDC CONV MULTI OUT 24TSSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1778EUG+

• Applications: Converter, TFT, LCD
• Voltage - Input: 2.7 ~ 5.5 V
• Number Of Outputs: 5
• Voltage - Output: 2.7 ~ 13 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Quad-Output TFT LCD DC-DC
Converters with Buffer
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
connects C
C
diode drop is greater than the voltage across C
charge flows from C
(D5) turns off. The amount of charge transferred to the
output is controlled by the variable N-channel on-resis-
tance.
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 cur-
rent of Q
output. If the feedback voltage is too low, the device
22
Figure 5. Low-Dropout Linear Regulator Block Diagram
OUT(NEG)
Low-Dropout Linear Regulator (MAX1778/
______________________________________________________________________________________
P
MAX1881/MAX1883/MAX1884 Only)
, which reduces the amount of current to the
. If the voltage across C
X(NEG)
in parallel with reservoir capacitor
OUT(NEG)
X(NEG)
to C
Negative Charge Pump
P
) to supply loads up to
X(NEG)
charges to V
OUT(NEG)
until the diode
X(NEG)
MAX1778
MAX1881
MAX1883
MAX1884
ERROR
AMPLIFIER
minus a
X(NEG)
THERMAL
SENSOR
. This
SUPN
CURRENT
LIMIT
,
increases the pass transistor base current, which
allows more current to pass to the output and increases
the output voltage. However, the linear regulator 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 out-
put, allowing it to be shorted to ground for an indefinite
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 cur-
rent-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