2.5V, ±15kV ESD-Protected RS-232

Transceivers for PDAs and Cell Phones

6

_______________________________________________________________________________________

Detailed Description

Dual Charge-Pump

Voltage Converter

The MAX3388E/MAX3389E’s internal power supply

consists of a regulated dual charge pump that provides

output voltages of +4.2V (doubling charge pump) and -

4.2V (inverting charge pump), regardless of the input

pumps operate in a discontinuous mode: if the output

voltages are less than 4.2V, the charge pumps are

enabled; if the output voltages exceed 4.2V, the charge

pumps are disabled. Each charge pump requires flying

capacitors (C1, C2) and reservoir capacitors (C3, C4)

to generate the V+ and V- supplies.

RS-232 Transmitters

The transmitters are inverting level translators that convert

CMOS-logic levels to ±3.7V EIA/TIA-232-compatible

levels.

The MAX3388E/MAX3389E’s transmitters guarantee a

250kbps data rate with loads of 3k

Ω in parallel with

1000pF and 460kbps data rate with loads of 3k

Ω in paral-

lel with 150pF. Figure 1 shows a complete system con-

nection.

These RS-232 output stages are turned off (high

impedance) when the devices are in shutdown mode.

When the power is off, the MAX3388E/MAX3389E permit

the outputs to be driven up to ±12V.

The transmitter inputs do not have pullup resistors.

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic

output levels. The MAX3388E/MAX3389E’s receivers

have inverting outputs. The outputs are high imped-

ance in shutdown.

Shutdown Mode

Supply current falls to less than 1µA when the

MAX3388E/MAX3389E are placed in shutdown mode

(SHDN logic low). When shut down, the device’s charge

ground, the switch is opened, and the transmitter outputs

are disabled (high impedance). The time required to exit

MAX3388E

MAX3389E

I/O

CHIP

WITH

UART

CPU

RS-232

POWER-

MANAGEMENT

UNIT OR

KEYBOARD

CONTROLLER

SHDN

I/O CHIP

POWER SUPPLY

Figure 1. Interface Under Control of PMU

10

µs/div

5V/div

2V/div

T1

T2

C1–C4 = 0.1

µF

Ω

Figure 2. Transmitter Outputs when Exiting Shutdown

Table 1. Shutdown Logic Truth Table

LOUT

SHDN

TRANSMITTER

OUTPUTS

RECEIVER

OUTPUTS

CHARGE

PUMP

SWITCH

MAX3388E

MAX3389E

L

High-Z

High-Z

Inactive

Open

High-Z

LIN

H

Active

Active

Active

Closed

LIN

LIN