1) Choose the maximum VCOM level (VMAX), the mini-
mum VCOM level (VMIN
voltage (VAVDD).
2) Select R1 between 10k
and 500k based on the
acceptable power loss from the AVDD supply rail.
3) Calculate R2:
4) Calculate RSET:
5) Verify that ISET(MAX) does not exceed 120A:
6) If ISET exceeds 120A, return to step 2 and choose
a larger value for R1.
7) The resulting resolution is:
A complete design example is given below:
1) VMAX = 5V, VMIN = 3V, VAVDD = 10V.
2) If R1 = 200k
, then R2 = 200k and
RSET = 24.9k
.
3) ISET(MAX) = 20A.
4) Resolution = 15.75mV.
Translating Existing Potentiometer
Circuits
Existing VCOM adjustment circuits using conventional
mechanical potentiometers can be translated into
MAX8738 circuits. Figures 4 and 5 show two common
adjustment circuits and their equivalent MAX8738 circuits.
(
)
VV
MAX
MIN
127
I
V
R
A
SET MAX
AVDD
SET
() =
×
≤
20
120
R
V
VV
R
SET
MAX
MIN
(
)
=
×
20
1
R
V
VV
R
MAX
AVDD
MAX
21
(
)
×
MAX8738
EEPROM-Programmable TFT VCOM
Calibrator with I2C Interface
_______________________________________________________________________________________
9
MAX8738
AVDD
RSET
R2
R1
VCOM
SET
GND
AVDD
OUT
MAX8738
EQUIVALENT CIRCUIT
MECHANICAL
POTENTIOMETER
RA
RB
RC
R1 = RA
R2 = RB + RC
RSET =
RA x (RB + RC)
20 x RB
Figure 4. Replacement of Mechanical Potentiometer Circuit