2.5V, ±15kV ESD-Protected RS-232

Transceivers for PDAs and Cell Phones

High Data Rates

The MAX3388E/MAX3389E maintain RS-232-compatible

transmitter output voltages even at high data rates. Figure

5 shows a transmitter loopback test circuit. Figure 6 shows

a loopback test result at 250kbps, and Figure 7 shows the

same test at 460kbps. For Figure 6, all transmitters were

driven simultaneously at 250kbps into RS-232 loads in

parallel with 1000pF. For Figure 7, a single transmitter was

driven at 460kbps, and all transmitters were loaded with

an RS-232 receiver in parallel with 150pF.

Power Switch

The MAX3388E/MAX3389E contain an internal switch

for powering external circuitry. This can be used to

power hot-sync circuitry or other low-power circuitry.

The switch on- resistance is typically 62

Ω. The SWIN

side of the switch is ESD protected to ±15kV.

Logic-Level I/O

In addition to the traditional RS-232 I/O, the

MAX3388E/MAX3389E have a logic-level transceiver

from the RS-232 connector side to the CMOS-logic

side. The input impedance is typically 30k

Ω, and the

output is open drain. The logic level I/O is active during

shutdown for the MAX3389E.

This I/O transceiver is useful for hot syncing or other

dedicated communication capability. The input is ESD

protected to ±15kV.

Typical PDA/Cell-Phone Application

The MAX3388E/MAX3389E designed with PDA applica-

tions in mind. Two transmitters and two receivers handle

standard full-duplex communication protocol, while an

extra transmitter allows a ring indicator (RI) signal to alert

the UART on the PC. Without the ring indicator transmit-

ter, solutions for these applications would require soft-

ware-intensive polling of the cradle inputs.

The RI signal is generated when a PDA, cellular phone, or

other “cradled” device is plugged into its cradle. This

generates a logic-low signal to RI transmitter input, creat-

ing +3.7V at the ring indicate pin. The PC’s UART RI input

is the only pin that can generate an interrupt from signals

arriving through the RS-232 port. The interrupt routine for

this UART will then service the RS-232 full-duplex com-

munication between the PDA and the PC.

As cell phone design becomes more like that of PDAs,

cell phones will require similar docking ability and com-

munication protocol. Cell phones operate on a single

lithium-ion (Li+) battery and generate a regulated out-

put voltage of +2.35V to +3V from the phone connector.

The baseband logic coming from the phone connector

can be as low as 1.8V at the transceivers. To prevent

forward biasing of a device internal to the cell phone,

the MAX3388E/MAX3389E come with a logic power-

MAX3388E

MAX3389E

5k

Ω

R_ IN

R_ OUT

C2-

C2+

C1-

C1+

V-

V+

C4

C3

C1

C2

0.1

µF

SHDN

T_ OUT

T_ IN

GND

Figure 5. Loopback Test Circuit

1

µs/div

T1IN

T1OUT

2V/div

5V/div

R1OUT

5V/div

Figure 6. Loopback Test Results at 250kbps

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