Applications Information
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; polarized or nonpolarized capacitors
can be used. Ceramic chip capacitors with an X7R
dielectric provide the best combination of performance,
cost, and size. The charge pump requires 0.22µF
capacitors for 3.3V operation. Do not use values small-
er than those listed in Figure 1. Increasing the capacitor
values (e.g., by a factor of 2) reduces ripple on the
transmitter outputs, slightly reduces power consump-
tion, and increases the available output current from
V
REG
and V
LCD
. C2, C3, and C4 can be increased
without changing C1’s value. However, do not
increase C1 without also increasing the values of
C2, C3, C4, and C5 to maintain the proper ratios.
When using the minimum required capacitor values,
make sure the capacitor value does not degrade exces-
sively with temperature or voltage. This is typical of Y5V
and Z5U dielectric ceramic capacitors. If in doubt, use
capacitors with a larger nominal value, or specify X7R
dielectric. The capacitor’s equivalent series resistance
(ESR), which usually rises at low temperatures, influences
the amount of ripple on V+ and V-.
Power-Supply Decoupling
In most circumstances, a 0.22µF V
DD
bypass capacitor
(C5) is adequate. Choosing larger values for C5
increases performance and decreases the induced rip-
ple on the V
DD
supply line. Note that capacitor C2, con-
nected to V+, is returned to C5. This connection also
improves the performance of the MAX3325. Locate all
bypass capacitors as close as possible to the IC. Keep
metal traces as wide as possible. Return all capacitor
ground connections directly to a solid-copper ground
plane.
Transmitter Outputs
when Exiting Shutdown
The
Typical Operating Characteristics
show the
MAX3325 transmitter outputs when exiting shutdown
mode. As they become active, the two transmitter out-
puts are shown going to opposite RS-232 levels (one
transmitter input is high, the other is low). Each trans-
mitter is loaded with 3kΩ in parallel with 2500pF. The
transmitter outputs display no ringing or undesirable
transients as they come out of shutdown. Note that the
transmitters are enabled only when the magnitude of V-
exceeds approximately -3V.
High Data Rates
The MAX3325 maintains the RS-232 ±5.0V minimum
transmitter output voltage even at high data rates.
Figure 1 shows a transmitter loopback test circuit. The
Typical Operating Characteristics
show loopback test
results at 120kbps and 250kbps. For 120kbps, all trans-
mitters were driven simultaneously at 120kbps into RS-
232 loads in parallel with 1000pF. For 250kbps, a single
transmitter was driven at 250kbps, and all transmitters
were loaded with an RS-232 receiver in parallel with
1000pF.
Interconnection with
Lower Logic Voltages
The MAX3325 provides a separate supply for the logic
interface to optimize input and output levels. Connect
V
L
to the system’s logic supply voltage, and bypass it
with a 0.1µF capacitor to GND. If the logic supply is the
same as V
DD
, connect V
L
to V
DD
. The V
L
pin can be
operated from +1.8V to +5.0V to accommodate various
logic levels.
Setting V
LCD
Output Voltage
The LCD output can be configured in a variety of ways
to suit the requirements of the LCD display. First, deter-
mine the nominal voltage range that the LCD will
require for adequate contrast adjustment. If the display
requires temperature compensation for contrast,
include the TEMP output in all calculations. The output
voltage is defined by:
where code is the current digital code in the DAC, and
R
O
is the nominal DAC output impedance (50kΩ). The
other terms in the equation are due to external resis-
tances connected to the indicated pins. A spreadsheet
program is an excellent tool for helping to select compo-
nents and evaluate their effect on the output voltage
range.
Although the above equation has terms for both REF+
and REF- offset resistors, only one or the other is used.
Design Example
The first step in designing for a particular display is to
obtain the manufacturer’s device specifications for the
nominal values as well as the temperature characteristics.
For example, consider the Optrex DMC series of dot
matrix LCD modules. The manufacturer specifies a nomi-
nal contrast bias voltage of 6V at +25°C, where bias volt-
age is V
REG
- V
LCD
. The temperature coefficient needed
