Datasheet

MAX17094
Internal-Switch Boost Regulator with Integrated
7-Channel Driver, VCOM Calibrator, Op Amp, and LDO
______________________________________________________________________________________ 19
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 the
POS terminal of the op amp and sinks a programmable
current (I
OUT
), which sets the VCOM levels (Figure 5).
The DAC setting that controls the wiper setting of the
potentiometer is directly determined by the value stored
in the wiper register (WR). Changing the value in WR
allows the user to change the wiper position to increase
or decrease the VCOM levels. The user can store a WR
setting into the nonvolatile initial value register (IVR)
such that on power-up, WR is preset to the last stored
value in IVR. The 2-wire I
2
C interface between the sys-
tem controller and the programming circuit adjusts WR
and programs IVR. The resistive voltage-divider and
AVDD 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 (R
SET
) sets the full-scale sink current
and the minimum value of VCOM.
The GON2 input provides the high voltage required to
program IVR. GON2 is connected to the TFT LCD V
GON2
supply. V
GON2
should be between 12V and 30V. IVR pro-
gramming is guaranteed only when GON2 is greater than
7V. Bypass GON2 to GND (which is bypassed to GND)
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 T
J
= +160°C, a ther-
mal sensor immediately activates the fault protection,
which shuts down the step-up regulator, LDO, and the
operational amplifiers, allowing the device to cool down.
Once the device cools down by approximately 15°C,
cycle the input voltage (below the UVLO falling thresh-
old) to clear the fault latch and reactivate the device.
The thermal-overload protection protects the IC in the
event of fault conditions. For continuous operation, do
not exceed the absolute maximum junction temperature
rating of T
J
= +150°C.
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak current rating,
and series resistance are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output-load capability,
transient-response time, and output-voltage ripple.
Physical size and cost are also important factors to be
considered.
SDA
SCL
ADDR0
ADDR1
AVDD
R
SET
SET
OUT
VGON
GON2
R4
R3
VCOM
V
MAIN
DAC
00
7F
19R
R
7
RSB
1
7
V
IN
POS
NEG
VCOM
I
2
C BUS
I
2
C
CONTROL
INTERFACE
DATA
REGISTER
00H
WR
CONTROL
REGISTER
02H
MAX17094
IVR
Figure 5. VCOM Calibrator Functional Diagram