ADD8754-EVAL AD [Analog Devices], ADD8754-EVAL Datasheet - Page 5

• Manufacturer Part Number: ADD8754-EVAL
• Description: Evaluation Kit for LCD Power, VCOM, and Gate Modulation
• Manufacturer: AD [Analog Devices]

FUNCTIONAL BLOCK OVERVIEW
This section provides an overview of the important features
of each functional block to assist in evaluating the ADD8754
device. For detailed information about the ADD8754, see the
ADD8754
settings.
When reviewing this section, keep the following points in mind:
•
•
•
•
STEP-UP SWITCHING REGULATOR
The step-up switching regulator, or boost converter, converts
a dc voltage input of 5 V to an output of 10 V (adjustable up to
20 V). With its factory setting, the boost converter can output
up to 750 mA of load current. Typically, a TFT-LCD monitor
requires about 500 mA of current at 10 V.
For an accurate evaluation of the step-up regulator, it is recom-
mended that an equivalent panel load (20 Ω to 40 Ω) be applied
from BOOST_OUT to GND.
Resetting the Boost Output Voltage
BOOST_OUT is determined by the divider resistors R1 and R2.
To obtain a given output, set R2 = 10 kΩ and calculate
Loop Compensation
Resistor RC and Capacitor CC set the loop compensation for
stability. If the input voltage, output voltage, inductor, or output
capacitor values need to be changed, see the ADD8754 data
sheet for a method tPo recalculate the compensation values for
stable operation of the step-up switching regulator.
Inductor Current Rating
The factory-installed inductor is rated for 1.5 A. If the output
current and/or output voltage make the inductor current higher
than 1.5 A, then the inductor should be replaced with an appro-
priate inductor to prevent saturation.
Soft Start
The soft start time is adjusted by Capacitor CSS. See the
ADD8754 data sheet for instructions on recalculating a
new soft start time.
The ADD8754 has an undervoltage lockout circuit that
prevents operation below a VIN of 2.8 V.
The high-side charge pump has a Zener diode regulated
output (VCPH) of 28 V.
The low-side charge pump has a Zener diode regulated
output (VCPL) of −5 V.
The evaluation board shuts down when the SHDN pin is
connected to ground (JP1 is shorted).
R1
=
R2
data sheet. See Table 1 for a list of factory voltage
×
(
BOOST
_
. 1
OUT
21
−
. 1
21
)
Rev. A | Page 5 of 12
LDO
The LDO generates a selectable logic voltage output for the
digital logic needs of a TFT-LCD display. Typically, this voltage
is applied to the timing controller (TCON) and gate drivers. For
instructions on selecting the 3.3 V, 2.85 V, or 2.5 V voltage
output, see Table 2.
VCOM AMPLIFIER
Buffer and Feedback Configurations
The VCOM amplifier of the ADD8754 can be used as a unity-
gain buffer or with panel feedback. To set the ADD8754 in a
unity-gain buffer configuration, short Jumper JP4
that if using model ADD8754ACPZ, buffer mode is not recom-
mended.
To use panel feedback, open Jumper JP4 and use the VCOM_S
pin, Resistor R16, Resistor R17, Capacitor C16, and Capacitor C17
to configure the feedback network.
To add VCPL feedback, R14, R15, C14, and C15 can be changed.
R13, R21, C13, and C22 are used to set the VCOM amplifier
gain. R10 and C9 are used if a filtered output is desired from the
VCOM amplifier.
Adjusting VCOM
The reference voltage for VCOM is adjustable by turning
the on-board potentiometer (Potentiometer R12) using a
small screwdriver. The VCOM voltage range depends on the
boost output voltage and how it is divided by Resistor R24,
Resistor R4, and Resistor R12.
If your board includes an
potentiometer, this device is configured for use (the analog
potentiometer is disabled). See the AD5259 Digital Potentiometer
(Optional) section for details about setting and disabling this
device. For more information about the AD5259, see the
AD5259 data sheet.
GATE PULSE MODULATOR
The gate pulse modulator takes a high voltage (VGH) and ramps
this voltage down to a lower voltage (VDD_1). The VFLK signal
determines when the voltage begins ramping. The CE capacitor
and RE resistor determine the delay and the ramp rate.
VDPM must be pulled high to enable GPM (Jumper JP5 should be
shorted). A startup delay for VGH_M can be set with C8.
AD5259
256-position digital
ADD8754-EVAL
.
Please note