LTC2873H Просмотр технического описания (PDF) - Analog Devices
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LTC2873H Datasheet PDF : 32 Pages
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LTC2873
APPLICATIONS INFORMATION
The LTC2873 is a flexible multiprotocol transceiver sup-
porting RS485/RS422 and RS232 protocols.
This device can be powered from a single 3V to 5.5V supply
with optional logic interface supply as low as 1.7V. An inte-
grated 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 termination components and switching relays. A
logic loopback control is included for self-test and debug.
The LTC2873 bus interface is a single two-pin port that can
be configured as either an RS232 driver/receiver pair or a
differential RS485 (and RS422) transceiver depending on
the state of the 485/232 pin. In RS485 mode, the driver
and receiver can be enabled independently with the DE485/
F232 and RE485 pins, or by tying these signals together, a
single control selects transmit or receive modes. A 120Ω
termination resistor is automatically engaged between pins
A/DO and B/RI in RS485 mode if TE485 is low.
When the LTC2873 is in RS232 mode, the RS232 driver
and receivers are both active and a 5k resistor is engaged
at the receiver input to ground. The slew rate in RS232
mode can be set to support 1Mbps or 250kbps operation
using the DE485/F232 pin.
The LTC2873 features rugged operation with ESD ratings
of ±26kV HBM on the RS232 and RS485 receiver inputs
and driver outputs, both unpowered and powered. All
other pins offer protection exceeding ±4kV.
DC/DC Converter
The on-chip DC/DC converter operates from the VCC
input, generating a 7.0V VDD supply and a charge
pumped –6.3V VEE supply, as shown in Figure 16. VDD
and VEE 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 10µH inductor (L1)
and a bypass capacitor (C4) of 2.2µF. The charge pump
capacitor (C1) is 220nF and the storage capacitors (C2
and C3) are 1µF. Locate C1 – C4 close to their associated
pins shown in Figure 16. Refer to Layout Considerations
section for guidance on circuit board layout.
Bypass capacitor C5 on the logic supply pin can be omitted
if VL is connected to VCC. See the VL Logic Supply section
for more details about the VL logic supply.
VCC
3V TO 5.5V
L1
10µH
C1
220nF
C4
VL
1.7V TO VCC
2.2µF
14
VCC
22 VL
12
SW
9
CAP
VDD
13
C2
C5
0.1µF
BOOST
REGULATOR
20 GND
1µF
VEE
10
GND
11
C3
2873 F16
1µF
NOTE: NOT ALL PINS SHOWN. IN THE CASE OF DUPLICATE PINS FOR VCC,
GND, AND VEE, EXTERNAL COMPONENTS SHOULD BE POSITIONED
CLOSEST TO THE NUMBERED PIN SHOWN ABOVE.
Figure 16. Simplified DC/DC Converter with Required External Components
For more information www.analog.com
REV B
17
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