to maintain the nominal contrast is -16mV/°C. In this case,
data for a spread of nominal bias voltages is not avail-
able, so a range of ±1V is chosen by experimentation.
Feedback Resistor (R
FB
)
The first step in designing the MAX3325 LCD bias is to
select a feedback resistor. This can be arbitrary, but
values between 220kΩ to 1MΩ are a good starting
point. We will choose 330kΩ. If the design can’t reach
its target range in later calculations, the feedback resis-
tor can be adjusted accordingly.
DAC Output Resistor (R
OUT
)
Given the above criterion of a ±1V output range, the
DAC’s output should be multiplied by the ratio of the
desired output swing (±1V) divided by the available
output from the DAC (0 to 1.2V). Assuming that we’ve
used a 330kΩ feedback resistor, this corresponds to a
total DAC resistance of 200kΩ. Because the DAC has
an intrinsic output impedance of 50kΩ, set R
OUT
to
200kΩ - 50kΩ = 150kΩ.
Temperature Compensation Resistor (R
TEMP
)
Next, the temperature compensation resistor is select-
ed. Because the MAX3325 regulates FB to virtual
ground, adding or removing the remaining resistors in
this design does not affect the transfer function set in
the previous section. The TEMP output has a tempera-
ture coefficient of -17.5mV per °C, and the LCD’s is
-16mV/°C. To scale these two values, multiply the feed-
back resistor (330kΩ) by the ratio of the TEMP coeffi-
cient divided by the display’s coefficient. For this
example, the result is 360kΩ.
Reference Resistance (R
REF_
)
To complete the design, the DC output is biased to the
final desired value at DAC midscale. Because the
previous steps concentrated on the transfer function
only, we now have a large offset of +1.94V. This is cal-
culated from the entire equation, where the reference
resistors are assumed to be infinite, the DAC voltage is
+0.6V, and V
TEMP
is -3.2V. Connecting a 130kΩ resis-
tor from REF+ to FB forces V
LCD
to -1.1V, resulting in a
nominal contrast voltage (V
REG
- V
LCD
) of +6.1V. This
is close to the target value of +6V.
Actual Performance
The graph in Figure 2 shows the actual LCD display’s
data curve, along with the MAX3325’s performance
with various DAC codes. Note that changing the DAC
code does not affect the slope of the temperature com-
pensation. If a wider scale of contrast adjustments is
desired, change the DAC output resistor, and readjust
the offset voltage.
Interfacing to the UP and DOWN Inputs
The UP and DOWN inputs to the MAX3325 are edge-
triggered digital inputs. For proper operation, the sig-
nals must be standard logic signals. Mechanical switch
outputs, (toggle or membrane types) are unsuitable
and require proper debouncing before connecting to
the MAX3325. The best solution is to use the MAX6817
dual switch debouncer. This sends the correct signal
levels to the UP and DOWN inputs, and provides a
robust interface to the switch inputs. The UP and
DOWN inputs can be driven directly from a micro-
processor.
System Considerations
Because the MAX3325 is the temperature transducer
for the LCD bias compensation, optimal performance is
obtained by placing the IC as close as possible to the
LCD.
MAX3325
3V Dual RS-232 Transceiver with
LCD Supply and Contrast Controller
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