Rainbow Electronics MAX9552 - Manual (Page 6)

MAX9550/MAX9551/MAX9552
High-Current VCOM Drive Buffers
for TFT LCDs
6 _______________________________________________________________________________________
MAX9550
16V
SUPPLY
16V
SUPPLY
*C1 = 4.7µF
C L = 1µF
*C2 = 0.1µF
*PLACE 4.7µF AND 0.1µF CAPACITORS AS CLOSE TO PIN AS POSSIBLE.
**R
S INCLUDES THE GENERATOR RESISTANCE.
NOTE: PLACE C L AS CLOSE TO OUT PIN AS POSSIBLE AND KELVIN
SENSE THE FEEDBACK LOOP CONNECTION AS SHOWN.
0V TO 1.5V AT 50kHz
C S = 0.1µF **R S = 2.2Ω
IN+
OUT
V OUT
TEST LOAD
V REF
V DD
IN-
GND
Figure 1. Settling Time Test Circuit
Detailed Description
The MAX9550/MAX9551/MAX9552 operational transcon-
ductance amplifiers (OTA) hold the VCOM voltage sta-
ble while providing the ability to source and sink a high
current quickly (800mA typ) into a capacitive load such
as the backplane of a TFT-LCD panel. The output settles
to within 0.1% in less than 2µs. The fast settling time is
achieved by increasing the transconductance of the
buffer as the output current increases (see the Typical
Operating Characteristics).
In addition, the MAX9550/MAX9551/MAX9552 directly
drive the capacitive load in the VCOM layer of the
TFT LCD without the need for a series resistor.
The MAX9550/MAX9551/MAX9552 unity-gain band-
width is:
GBW = g M / 2πC OUT
where C OUT is the capacitive load at the output and g M
is the transconductance.
To insure buffer stability, place a 1µF low-ESR capaci-
tor as close to the OUT pin as possible. However, this
value may be reduced if the TFT-LCD panel load pro-
vides some of the capacitance and the resistance in
series when this capacitance is low. Connect the feed-
back at OUT using a Kelvin connection at the low-ESR
capacitor.
Thermal Shutdown with Temperature
Hysteresis
The MAX9550/MAX9551/MAX9552 are capable of high
output currents and therefore, feature thermal-shutdown
protection with temperature hysteresis. When the die tem-
perature reaches +160°C, the devices shut down. When
the die cools down by 15°C, the devices turn on again.