BUF08800 Datasheet, PDF (12/23 Pages) Burr-Brown (TI) – Programmable Reference Generator and 400mA VCOM Driver

BUF08800
SBOS380A − FEBRUARY 2007 − REVISED MAY 2007
OUTPUT LATCH
Because the BUF08800 features a double-buffered
register structure, updating a DAC register is not the same
as updating the DAC output voltage. There are three
methods for latching transferred data from the storage
registers into the DACs to update the DAC output voltage.
Method 1 requires externally setting the latch pin (LD) =
LOW, which updates each DAC output voltage whenever
its corresponding register is updated.
Method 2 externally sets LD = HIGH to allow all DAC
output voltages to retain their values during data transfer
and until LD = LOW, which simultaneously updates the
output voltages of all DACs to the new register values.
Method 3 uses software control. LD is maintained HIGH,
and all DACs are updated when the master writes a ‘1’ in
bit 15 of any DAC register. The update occurs after
receiving the 16-bit data for the currently-written register.
Use methods 2 and 3 to transfer a future data set into the
first bank of registers in advance to prepare for a very fast
update of DAC output voltages.
The General Call Reset and the power-up reset update the
DACs regardless of the state of the latch pin (LD).
REPLACEMENT OF TRADITIONAL GAMMA
BUFFER
Traditional gamma buffers rely on a resistor string (often
using expensive 0.1% resistors) to set the gamma
voltages. During development, the optimization of these
gamma voltages can be time-consuming. Programming
these gamma voltages with the BUF08800 can
significantly reduce the time required for gamma voltage
optimization. The final gamma values can be written into
the internal OTP memory to replace a traditional gamma
buffer solution. Figure 16a shows the traditional resistor
string; Figure 16b shows the more efficient alternative
method using the BUF08800.
The BUF08800 uses the most advanced high-voltage
CMOS process available today, allowing it to be
competitive with traditional gamma buffers.
Programmability offers the following advantages:
D Shortens development time significantly.
D Increases reliability by eliminating more than 18
external components.
D Eliminates manufacturing variance between panels.
D Allows a single panel to be built for multiple
customers, with loading of customer-dependent
gamma curves during final production. This method
significantly lowers inventory cost and risk, and
simplifies inventory management.
D Allows demonstration of various gamma curves to
LCD monitor makers by simply uploading a different
set of gamma values.
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D Allows simple adjustment of gamma curves during
production to accommodate changes in the panel
manufacturing process or end-customer require-
ments.
D Decreases cost and space.
V_COM ADJUSTMENT
The output of the V_COM digital-to-analog converter (DAC)
is internally connected to the input of the V_COM buffer. As
a result of the high 10-bit resolution, the V_COM voltage can
directly be adjusted without the need for external circuitry.
The integrated V_COM driver can deliver up to 400mA of
peak current. In addition, the negative input is brought out
as a separate pin on the package to facilitate V_COM
compensation or equalization of the V_COM voltage across
the panel (see Figure 17).
Traditional V_COM adjustment uses a mechanical
potentiometer and a voltage divider for adjustment, as
shown in Figure 15. The programmable V_COM channel
integrated in the BUF08800 is also able to use an external
voltage divider connected to +IN. It can be used to set the
initial V_COM voltage as well as the adjustment range (see
Figure 17). Using this method, even at power-on the initial
V_COM setting is close to the optimized V_COM value, without
any programming. The external voltage divider also limits
the adjustment range that typically leads to a smaller
number of adjustment steps. In addition, the V_COM output
voltage is limited only to the adjustment range, thereby
protecting the panel from undesirable V_COM voltages.
AVDD
R_A
R_B
R_C
V_COM
Figure 15. Traditional V_COM Adjustment. External
voltage divider sets initial V_COM voltage as well as
adjustment range.
The 10-bit DAC acts as a voltage source with a nominal
250kΩ output impedance; see Figure 17. For example, at
code 000h, the lowest V_COM voltage is achieved because
the 250kΩ impedance is now in parallel with R₂, which
lowers the impedance of the lower side of the voltage
divider. Consequently, code 3FFh results in the highest
adjustable V_COM voltage.
However, an external voltage divider is not required for
correct function of the V_COM channel integrated in the
BUF08800. Once the desired output level (that is,
minimum flicker) is obtained, the corresponding code can
be stored in the external EEPROM memory.