Rainbow Electronics MAX1885 User Manual

Page 22

MAX1778/MAX1880–MAX1885

Quad-Output TFT LCD DC-DC
Converters with Buffer

______________________________________________________________________________________

Negative Charge Pump

During the first half-cycle, the P-channel MOSFET turns
on, and flying capacitor C

X(NEG)

charges to V

SUPN

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

X(NEG)

. This

connects C

X(NEG)

in parallel with reservoir capacitor

OUT(NEG)

. If the voltage across C

OUT(NEG)

minus a

diode drop is greater than the voltage across C

X(NEG)

charge flows from C

OUT(NEG)

to C

X(NEG)

until the diode

(D5) turns off. The amount of charge transferred to the
output is controlled by the variable N-channel on-resis-
tance.

Low-Dropout Linear Regulator (MAX1778/

MAX1881/MAX1883/MAX1884 Only)

The MAX1778/MAX1881/MAX1883/MAX1884 contain a
low-dropout linear regulator (Figure 5) that uses an
internal PNP pass transistor (Q

) to supply loads up to

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

, which reduces the amount of current to the

output. If the feedback voltage is too low, the device

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.

VCOM Buffer

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:

GBW = gm/C

OUT

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

SUPL

4.5V TO 15V

LDOOUT

1.25V TO (V

SUPL

- 0.3V)

FBL

REF

1.25V

ERROR
AMPLIFIER

GND

CURRENT

LIMIT

THERMAL

SENSOR

LDOOUT

(

1 +

)

REF

= 1.25V

R7
R8

MAX1778
MAX1881
MAX1883
MAX1884

Figure 5. Low-Dropout Linear Regulator Block Diagram