TSC2200_16 Datasheet, PDF (33/45 Pages) Texas Instruments – PDA ANALOG INTERFACE CIRCUIT

OPERATION—D/A CONVERTER
The TSC2200 has an onboard 8-bit D/A converter, config-
ured as shown in Figure 27. This configuration yields a
current sink (A_OUT) controlled by the value of a resistor
connected between the ARNG pin and ground. The D/A
converter has a control register that controls whether or not
the converter is powered up. The 8-bit data is written to the
D/A converter through the D/A converter data register.
8 Bits
D/A Converter
V+
R₁
R₂
A_OUT
V_BIAS
ARNG
RRNG
FIGURE 27. D/A Converter Configuration.
This circuit is designed for flexibility in the output voltage at the
V_BIAS point shown in Figure 27 to accommodate the widely
varying requirements for LCD contrast control bias. V+ can be
a higher voltage than the supply voltage for the TSC2200. The
only restriction is that the voltage on the A_OUT pin can never go
above the absolute maximum ratings for the device, and
should stay above 1.5V for linear operation.
The D/A converter has an output sink range that is limited to
1mA. This range can be adjusted by changing the value of
RRNG shown in Figure 27. As this D/A converter is not
designed to be a precision device, the actual output current
range can vary as much as ±20%. Furthermore, the current
output will change due to variations in temperature; the D/A
converter has a temperature coefficient of approximately
–2µA/°C. To set the full-scale current, RRNG can be deter-
mined from the graph shown in Figure 28.
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
10k
100k
1M
10M
ARNG Resistor (Ω)
100M
FIGURE 28. D/A Converter Output Current Range versus
RRNG Resistor Value.
For example, consider an LCD that has a contrast control
voltage V_BIAS that can range from 2V to 4V, which draws
400µA when used, and an available +5V supply. Note that
this is higher than the TSC2200 supply voltage, but it is within
the absolute maximum ratings.
The maximum V_BIAS voltage is 4V, and this occurs when the
D/A converter current is 0, so only the 400µA load current
I_LOAD will be flowing from 5V to V_BIAS. This means 1V will be
dropped across R₁, so R₁ = 1V/400µA = 2.5kΩ.
The minimum V_BIAS is 2V, which occurs when the D/A
converter current is at its full scale value, I_MAX. In this case,
5V – 2V = 3V will be dropped across R₁, so the current
through R₁ will be 3V/2.5K = 1.2mA. This current is
I_MAX + I_LOAD = I_MAX + 400uA, so I_MAX must be set to 800µA.
Looking at Figure 28, this means that RRNG should be
around 1MΩ.
Since the voltage at the A_OUT pin should not go below 1.5V,
this limits the voltage at the bottom of R₂ to be 1.5V minimum;
this occurs when the D/A converter is providing its maximum
current, I_MAX. In this case, I_MAX + I_LOAD flows through R₁, and
I_MAX flows through R₂. Thus,
R₂I_MAX + R₁(I_MAX + I_LOAD) = 5V – 1.5V = 3.5V (8)
We already have found R₁ = 2.5kΩ, I_MAX = 800µA,
I_LOAD = 400µA, so we can solve this for R₂ and find that it
should be 625Ω.
TSC2200
33
SBAS191F
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