3.0V, ±15kV ESD-Protected RS-232
Transceiver for PDAs and Cell Phones
High Data Rates
The MAX3386E maintains the RS-232 ±5.0V minimum
transmitter output voltage even at high data rates.
Figure 5 shows a transmitter loopback test circuit. Figure 6
shows a loopback test result at 120kbps, and Figure 7
shows the same test at 250kbps. For Figure 6, all transmit-
ters were driven simultaneously at 120kbps into RS-232
loads in parallel with 1000pF. For Figure 7, a single trans-
mitter was driven at 250kbps, and all transmitters were
loaded with an RS-232 receiver in parallel with 1000pF.
V
CC
0.1µF
MAX3386E
V
CC
C1+
C1
C1-
C2+
C2
C2-
V+
MAX3386E
V-
C3
Interconnection with
3V and 5V Logic
The MAX3386E can directly interface with various 5V
logic families, including ACT and HCT CMOS. The logic
voltage power-supply pin V
L
sets the output voltage
level of the receivers and the input thresholds of the
transmitters.
C4
T_ IN
T_ OUT
Typical PDA/Cell-Phone Application
The MAX3386E is designed with PDA applications in
mind. Two transmitters and two receivers handle stan-
dard 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 transmitter,
solutions for these applications would require software-
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 +6V 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 work with a power-supply
voltage of +2.7V to +4V. 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 MAX3386E comes with a
logic power-supply pin (V
L
) that limits the logic levels
presented to the phone. The receiver outputs will sink
to zero for low outputs, but will not exceed V
L
for logic
highs. The input logic levels for the transmitters are also
altered, scaled by the magnitude of the V
L
input. The
device will work with V
L
as low as 1.8V before the
charge-pump noise will begin to cause the transmitter
outputs to oscillate. This is useful with cell phones and
other power-efficient devices with core logic voltage
levels that go as low as 1.8V.
R_ OUT
R_ IN
5k
1000pF
V
CC
SHDN
GND
Figure 5. Loopback Test Circuit
T1IN
5V/div
5V/div
T1OUT
R1OUT
V
CC
= 3.3V
2µs/div
5V/div
Figure 6. Loopback Test Results at 120kbps
_______________________________________________________________________________________
9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
