Datasheet

LTC2872
17
2872f
Overview
The LTC2872 is a flexible multiprotocol transceiver sup-
porting RS485/RS422 and RS232 protocols. It can be
powered from a single 3.0V to 5.5V supply with optional
logic interface supply as low as 1.7V. An integrated DC/
DC converter provides the positive and negative supply
rails needed for RS232 operation. Automatically selected
integrated termination resistors for both RS232 and
RS485 protocols are included, eliminating the need for
external components and switching relays. Both parts
include loopback control for self-test and debug as well
as logically-switchable half- and full-duplex control of the
RS485 bus interface.
The LTC2872 offers two ports that can be independently
configured as either two RS232 receivers and drivers or
one RS485/RS422 receiver and driver depending on the
state of its 485/232 pins. Control inputs DXEN and RXEN
provide independent control of driver and receiver opera-
tion for either RS232 or RS485 transceivers, depending
on the selected operating protocol.
The LTC2872 features rugged operation with an ESD rating
of ±15kV HBM on the receiver inputs and driver outputs,
both powered and unpowered. All other pins offer protec-
tion exceeding ±4kV.
DC/DC Converter
The on-chip DC/DC converter operates from the V
CC
input,
generating a 7V V
DD
supply and a charge pumped –6.3V
V
EE
supply, as shown in Figure 13. V
DD
and V
EE
power
the output stage of the RS232 drivers and are regulated
to levels that guarantee greater than ±5V output swing.
The DC/DC converter requires a 22µH inductor (L1) and a
bypass capacitor (C4) of 2.2µF or larger. The charge pump
capacitor (C1) is 470nF and the storage capacitors (C2 and
C3) are 2.2µF. Larger storage capacitors up to 4.7µF may
be used if C1 and C4 are scaled proportionately. Locate
C1-C4 close to their associated pins.
Bypass capacitor C5 on the logic supply pin can be omitted
if V
L
is connected to V
CC
. See the V
L
Logic Supply section
for more details about the V
L
logic supply.
Inductor Selection
An inductor with a value of 22µH ±20% is required. It
must have a saturation current (I
SAT
) rating of at least
200mA and a DCR (copper wire resistance) of less than
1.3Ω. Some small inductors meeting these requirements
are listed in Table 10.
Table 10. Recommended Inductors
PART NUMBER
L
(µH)
I
SAT
(mA)
MAX
DCR
(Ω) SIZE (mm) MANUFACTURER
BRC2016T220M
CBC2518T220M
22
22
310
320
1.3
1.0
2 × 1.6 × 1.6
2.5 × 1.8 × 1.8
Taiyo Yuden
t-yuden.com
LQH32CN220K53 22 250 0.92 3.2 × 2.5 × 1.6 Murata
murata.com
Capacitor Selection
The small size of ceramic capacitors makes them ideal for
the LTC2872. Use X5R or X7R dielectric types; their ESR is
low and they retain their capacitance over relatively wide
voltage and temperature ranges. Use a voltage rating of
at least 10V.
applicaTions inForMaTion
Figure 13. DC/DC Converter with Required External Components
2872 F13
BOOST
REGULATOR
V
CC
3V TO 5.5V
V
L
1.7V TO V
CC
C1
470nF
L1
22µH
21
18
C4
2.2µF
V
CC
V
DD
V
EE
SW
GND
GND
CAP
C5
0.1µF
C2
2.2µF
19
C3
2.2µF
17
20
39
34
V
L
35