MAX8798 datasheet(21/31 Pages) MAXIM

21 / 31 page

Internal-Switch Boost Regulator with

Integrated 3-Channel Scan Driver for TFT LCDs

______________________________________________________________________________________

SDA

SCL

VDD

BOOST

RSET

SET

VDD

OUT

VGON

GON

VCOM

VMAIN

EEPROM

BLOCK

DAC

19R

VDD

LINEAR

REGULATOR

POS

NEG

WPN

WPP

SCLS

VCOM

I2C BUS

I2C

CONTROL

INTERFACE

Figure 6. VCOM Calibrator Functional Diagram

GOFF Rapid Discharge Function

(DISH Input)

The DISH input controls a switch between GOFF and

GND. When DISH is pulled below ground by at least

1V, GOFF is rapidly discharged to GND. Typically,

DISH is capacitively coupled to IN so that if IN falls

suddenly, GOFF is discharged to blank the display

(Figure 3).

VCOM Calibrator

The VCOM calibrator is a solid-state alternative to

mechanical potentiometers used for adjusting the LCD

backplane voltage (VCOM) in TFT LCD displays. OUT

attaches to the external resistive voltage-divider at POS

and sinks a programmable current (IOUT), which sets the

VCOM level (Figure 6). An internal 7-bit DAC controls the

sink current and allows the user to increase or decrease

the VCOM level. The DAC is ratiometrically relative to

VBOOST and is monotonic over all operating conditions.

The user can store the DAC setting in an internal

EEPROM. On power-up, the EEPROM presets the DAC

to the last stored setting. The 2-wire I2C interface

between the system controller and the programming cir-

cuit adjusts the DAC and programs the EEPROM when

WPN is high.

The resistive voltage-divider and the BOOST supply set

the maximum value of VCOM. OUT sinks current from the

voltage-divider to reduce the POS voltage level and

VCOM output. The external resistor at SET (RSET) sets the

full-scale sink current and the minimum value of VCOM.

The GON input provides the high voltage required to

program the EEPROM. To allow programming, VGON is

connected to the TFT LCD VGON supply. VGON should

be between 16.1V and 35V. EEPROM programming is

disabled when VGON is below 15.5V (typ). Bypass

VGON to PGND or BOOST (that is bypassed to PGND)

with a 0.1µF or greater capacitor.

Thermal-Overload Protection

The thermal-overload protection prevents excessive

power dissipation from overheating the device. When

the junction temperature exceeds TJ = +160°C, a ther-

mal sensor immediately activates the fault protection

that shuts down the step-up regulator, switch control

block, operational amplifier, and the internal linear reg-

ulator, allowing the device to cool down. Once the

device cools down by approximately 15°C, cycle the

input voltage (below the UVLO falling threshold) to

clear the fault latch and reactivate the device.

The thermal-overload protection protects the controller

in the event of fault conditions. For continuous opera-

tion, do not exceed the absolute maximum junction

temperature rating of TJ = +150°C.

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