LTC2802 datasheet(17/32 Pages) AD

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LTC2873

REV B

For more information www.analog.com

Figure 16. Simplified DC/DC Converter with Required External Components

The LTC2873 is a flexible multiprotocol transceiver sup-

porting RS485/RS422 and RS232 protocols.

Thisdevicecanbepoweredfromasingle3Vto5.5Vsupply

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

andRS485protocolsareincluded,eliminatingtheneedfor

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

andreceivercanbeenabledindependentlywiththeDE485/

F232 and RE485 pins, or by tying these signals together, a

single control selects transmit or receive modes. A 120Ω

terminationresistorisautomaticallyengagedbetweenpins

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.

APPLICATIONS INFORMATION

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.

2873 F16

BOOST

REGULATOR

VCC

3V TO 5.5V

1.7V TO VCC

220nF

10µH

2.2µF

VCC

VDD

VEE

GND

CAP

0.1µF

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.

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