MAX218EAP Maxim Integrated Products, MAX218EAP Datasheet - Page 5

• Manufacturer Part Number: MAX218EAP
• Description: IC TXRX DUAL RS232 TRUE 20-SSOP
• Manufacturer: Maxim Integrated Products
• Type: Transceiverr

Specifications of MAX218EAP

• Number Of Drivers/receivers: 2/2
• Protocol: RS232
• Voltage - Supply: 1.8 V ~ 4.25 V
• Mounting Type: Surface Mount
• Package / Case: 20-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

The MAX218 line driver/receiver is intended for battery-
powered EIA/TIA-232 and V.28/V.24 communications
interfaces that require two drivers and two receivers.
The operating voltage extends from 1.8V to 4.25V, yet
the device maintains true RS-232 and EIA/TIA-562
transmitter output voltage levels. This wide supply volt-
age range permits direct operation from a variety of
batteries without the need for a voltage regulator. For
example, the MAX218 can be run directly from a single
lithium cell or a pair of alkaline cells. It can also be run
directly from two NiCd or NiMH cells from full-charge
voltage down to the normal 0.9V/cell end-of-life point.
The 4.25V maximum supply voltage allows the two
rechargeable cells to be trickle- or fast-charged while
driving the MAX218.
The circuit comprises three sections: power supply,
transmitters, and receivers. The power-supply section
converts the supplied input voltage to 6.5V, providing the
voltages necessary for the drivers to meet true RS-232
levels. External components are small and inexpensive.
Figure 1. Single-Supply Operation
_______________Detailed Description
4.25V
ON/OFF
ENABLE
1.8V
TO
1 F
C4
10
3
7
8
9
6
15 H
V
SHDN
T1IN
T2IN
R1OUT
R2OUT
_______________________________________________________________________________________
CC
EN
1
LX
4
MAX218
1N6050
T1
T2
D1
R1
R2
19
GND
V+
5, 17, 20
T1OUT
T2OUT
R1IN
R2IN
C1+
C1-
V-
15
18
16
13
12
11
14
True RS-232 Dual Transceiver
1 F
1 F
C2
C3
0.47 F
C1
1.8V to 4.25V-Powered,
The transmitters and receivers are guaranteed to oper-
ate at 120kbps data rates, providing compatibility with
LapLink™ and other high-speed communications soft-
ware. A shutdown mode extends battery life by reduc-
ing supply current to 0.04µA. While shut down, all
receivers can either remain active or be disabled under
logic control. With this feature, the MAX218 can be in
low-power shutdown mode and still monitor activity on
external devices. Three-state drivers are provided on
both receiver outputs.
The switch-mode power supply uses a single inductor
with one diode and three small capacitors to generate
±6.5V from an input voltage in the 1.8V to 4.25V
range.
Use a 15µH inductor with a saturation current rating of at
least 350mA and less than 1Ω resistance. Table 1 lists
suppliers of inductors that meet the 15µH/350mA/1Ω
specifications.
Key diode specifications are fast recovery time (<10ns),
average current rating (>100mA), and peak current rat-
ing (>350mA). Inexpensive fast silicon diodes, such as
the 1N6050, are generally recommended. More expen-
sive Schottky diodes improve efficiency and give slightly
better performance at very low V
lists suppliers of both surface-mount and through-hole
diodes. 1N914s are usually satisfactory, but specifica-
tions and performance vary widely with different manu-
facturers.
Use capacitors with values at least as indicated in
Figure 1. Capacitor C2 determines the ripple on V+,
but not the absolute voltage. Capacitors C1 and C3
determine both the ripple and the absolute voltage of
V-. Bypass V
close to pins 5 and 6. If the V
elsewhere (e.g., at the power supply), increase C4 to
4.7µF.
You may use ceramic or polarized capacitors in all
locations. If you use polarized capacitors, tantalum
types are preferred because of the high operating fre-
quency of the power supplies (about 250kHz). If alu-
minum electrolytics are used, higher capacitance val-
ues may be required.
™ LapLink is a trademark of Traveling Software, Inc.
CC
to GND with at least 1µF (C4) placed
Switch-Mode Power Supply
CC
CC
Capacitor Selection
line is not bypassed
Inductor Selection
voltages. Table 1
Diode Selection
5