Detailed Description

Dual Charge-Pump Voltage Converter

The MAX3325’s internal power supply consists of a reg-

ulated dual charge pump that provides output voltages

of +5.5V (doubling charge pump) and -5.5V (inverting

charge pump operates in discontinuous mode; if the

output voltages are less than 5.5V, the charge pump is

enabled; if the output voltages exceed 5.5V, the charge

pump is disabled. Each charge pump requires a flying

capacitor (C1, C2) and a reservoir capacitor (C3, C4)

to generate the V+ and V- supplies (Figure 1).

RS-232 Transmitters

The transmitters are inverting level translators that con-

vert logic levels to ±5.0V EIA/TIA-232 levels. The

MAX3325 transmitters guarantee a 250kbps data rate

with worst-case loads of 3k

Ω in parallel with 1000pF,

providing compatibility with PC-to-PC communication

software (such as LapLink™).

The MAX3325’s transmitters are disabled and the out-

puts are forced into a high-impedance state when the

RS-232 circuitry is in shutdown (

SD232 = low). The

MAX3325 permits the outputs to be driven up to ±13V

in shutdown.

The transmitter inputs do not have pull-up resistors.

RS-232 Receivers

The receivers convert RS-232 signals to logic output

The receiver outputs are always active, regardless of

the shutdown state.

Positive Voltage Regulator

The MAX3325 has a regulated +5V output suitable for

powering +5V LCD modules or other circuits. The out-

put of the boost charge pump is regulated with an LDO

linear regulator. The REG output sources up to 11mA of

current to external circuitry.

Adjustable LCD Supply

The LCD output provides a flexible output voltage to

adjust the contrast of LCD modules. The output voltage

range is determined by the external circuitry connected

to LCD, FB, DAC, REF+ (or REF-, depending on con-

trast polarity). Additionally, the TEMP output can be

used to automatically compensate the contrast adjust-

ment for temperature variance.

The LCD output is a linear regulator powered by the neg-

ative charge pump. It is capable of sinking up to 3mA of

current. Although the LCD regulator can be adjusted to

positive voltages, it is not capable of sourcing current. A

minimum output current of 100µA is required.

6-Bit DAC

The MAX3325’s DAC output is an unbuffered inverted

R2R structure with an output voltage range of 0 to

+1.2V. The DAC output impedance is typically 50k

Ω,

and can be connected through a series resistor to the

FB input of the LCD regulator. An internal power-on

reset circuit sets the DAC to midscale on power-up.

DAC Control Inputs

The DAC code is controlled by

UP and DOWN to adjust

the contrast of the LCD module. These inputs are

intended to interface to digital signals, but do not

include debounce circuitry. See the

Applications sec-

tion. See Table 1 for the truth table.

Temperature Compensation

The MAX3325’s TEMP output is used to minimize devia-

tion in LCD contrast level due to temperature changes.

The TEMP output is capable of sinking or sourcing up

to 22µA to the external resistor network.

Shutdown Mode

Supply current falls below 1µA in shutdown mode

(

SDLCD = SD232 = low). When shut down, the device’s

V- is pulled to ground, and the transmitter outputs are

disabled (high impedance). The LCD section is also

powered down. The REG, LCD, and both reference out-

puts become high impedance. The time required to exit

shutdown is typically 100µs, as shown in the

Typical

Operating Characteristics. However, the TEMP output

requires 50ms to fully stabilize. Connect

SDLCD and

Table 2.

3V Dual RS-232 Transceiver with

LCD Supply and Contrast Controller

8

_______________________________________________________________________________________

LapLink is a trademark of Traveling Software.

X = Don’t care

Table 1. DAC Truth Table

Table 2. Shutdown Truth Table

UP

0

1

↓

DOWN

0

↓

1

FUNCTION

DAC set to midscale

DAC register decrements 1 count

DAC register increments 1 count

SDLCD

0

1

X

SD232

0

X

1

FUNCTION

Low-power shutdown mode

LCD bias and REG outputs enabled

RS-232 transmitters enabled