ISL76534 Datasheet, PDF (19/24 Pages) Intersil Corporation – 14-channel gamma references

ISL76534
Data Word (WRITE/READ)
Data Words contain the data written or read from the
10-bit DACs. Each 10-bit DAC data is transmitted in one word
(e.g. two bytes, 16-bits), as shown in Table 5. Bits [b₉:b₀]
represent the 10-bit data. Bits [b₁₅:b₁₀] are “do not cares” and
will default to zero when reading data, however, when writing
data bit b₁₄ indicates the type of write. If b₁₄ = ‘0’: WRITE data
to the DAC register; if b₁₄ = ‘1’: WRITE data to the DAC and
EEPROM (program).
For any DAC, the first data byte of the word is called the Upper
Byte (or MSB), and the two LSBs of this byte represent the MSBs
of the 10-bit data, [b₉:b₈]. The second data byte of the word, or
Lower Byte (LSB), contains the remaining 8-bits (LSBs) of the
10-bit DAC data, [b₇:b₀]. These data words provide the DAC
values that ultimately determine the output voltages of the
ISL76534.
Refer to Table 2 for more information about the byte structure,
and refer to the following sections for information about the
expected DAC output voltage.
DAC Transfer Function of OUT1-OUT14
Equation 1 shows the transfer function for each 10-bit DAC
channel (expected output voltage). The transfer function relates
the REF_IN voltage and a DAC_CODE to a DAC output voltage,
“V_OUT.” The DAC_CODE is the decimal value of the 10-bit data
written to a given DAC channel.
V_OUT = REF_IN  ^D-----^A-----^C-₁----_{0_}---_2C---₄-^O-----^D----^E---
(EQ. 1)
DAC_CODE: 0 ~ 1023
Example calculation to find the expected V_OUT:
A_VDD = 15V
REF_IN = 14.75V
1LSB = ¹----₁₄---_0.--⁷-₂--⁵--₄--^V-- = 14.40mV
V_OUT = 14.40mV  DAC_CODE
V_OUTDAC_CODE = 512 = 14.40mV  512 = 7.375V
DAC Output Accuracy
The relationship between the actual/measured DAC output
voltage and the expected voltage is the “Output Accuracy”
(OUT_AC). Equation 2 shows how to determine output accuracy:
OUT_AC = V_{OUT expected} – V_{OUTmeasured}
(EQ. 2)
The ISL76534 features are very good and have consistent output
accuracy across all DAC codes and REF_IN voltages, ±15mV
(typical). Some competitor devices have diverging accuracy
performance near the high rail and have significantly larger
output accuracy variances compared to ISL76534.
The output accuracy performance of the ISL76534 provides
highly accurate reference voltages ideal for TFT-LCD applications.
This is important in TFT-LCD applications that require the
DACs/EEPROM to be programmed with the same digital codes
(e.g. gamma references determined from gamma calibration) on
a production line, and especially when using reflected code
gamma calibration methods. Ensuring accurate gamma
references is also important for optimizing pixel/panel reliability.
DAC Reference Voltage
The REF_IN pin is the reference voltage input for the 10-bit DACs. It
is a high impedance input. The voltage can be set using an
external resistor divider, regulated voltage, or tied directly to the
A_VDD power rail. The REF_IN pin should always be well bypassed to
minimize noise, and provide the best DAC performance. Use a
0.1µF ceramic capacitor (to GND), placed as close to the pin as
possible.
See example of the typical application circuits in Figures 30 and
31.
DAC Channel Outputs
The DAC output buffers are optimized to drive rail-to-rail for
optimal flexibility. Generally, in a TFT-LCD half of the required
gamma reference voltages will lie between V_COM and A_VDD (or
REF_IN), and the other half will lie between V_COM and GND. For
maximum flexibility, all ISL76534 outputs (OUT1-OUT14) can
drive to within 100mV of REF_IN (A_VDD = REF_IN) and to within
85mV of GND (with ±5mA load). See “Electrical Specifications”
table on page 6 for more details about output channel
capabilities.
Each DAC is updated as soon as the entire 10-bit value for that
DAC is received via I²C. DAC data is latched, and the respective
DAC responds, on the falling edge of the 8th SCL clock (which
corresponds to the DAC LSB data bit, bit b₀).
V_COM Amplifier
The ISL76534 V_COM amplifier is capable of rail-to-rail output
swings and the ability to drive highly capacitive loads. It can
source/sink up to 100mA of continuous current and over 500mA
of peak current. The output capability of the V_COM amplifier is
described in detail in “Electrical Specifications” table on page 6.
The V_COM amplifier is powered from a separate power supply
than the rest of the IC, called A_VDD_AMP. This allows A_VDD_AMP
to be set at a voltage lower than AVDD, to save system power.
Though it is acceptable to set A_VDD_AMP = A_VDD.
A_VDD_AMP is not required for the rest of the IC to operate, so if
an application is not utilizing the V_COM Amplifier in the ISL76534
(OUTCOM), then A_VDD_AMP can be tied to GND to disable the
function and save IC and system power. If A_VDD_AMP is powered
but not connected in the application, then the V_COM amplifier
should be set in a buffer (A_V = +1) configuration (INN_COM tied
to OUTCOM). See example typical application circuits in
Figures 30 and 31.
The device offers access to the inverting pin of the amplifier,
INN_COM, which enables various types of circuit configurations
depending on the requirements of the TFT-LCD panel
architecture. Most common applications either buffer the
amplifier for direct driving and response (INN_COM tied to
OUTCOM), or they may utilize feedback from the TFT-LCD panel
as an input to the INN_COM pin.
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July 27, 2016