ADD8754-EVAL

Rev. A | Page 5 of 12

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 data sheet. See Table 1 for a list of factory voltage

settings.

When reviewing this section, keep the following points in mind:

•

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).

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

(

)

21

.

1

21

.

1

_

−

×

=

OUT

BOOST

R2

R1

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.

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

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 AD5259 256-position digital

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.