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LT1331

3V RS562 or 5V/3V RS232 Transceiver with One Receiver Active in Shutdown

Linear Technology 

LT1331

PI FU CTIO S

V+ (Pin 1): Positive Supply Output. V+ ≅ 2VCC – 1.5V. This

pin requires an external capacitor for charge storage,

chosen to minimize ripple to acceptable levels. A mini-

mum size of 0.1µF is recommended.

VCC (Pin 2): Power Supply for Charge Pump and Drivers.

Proper circuit operation is insured for VCC = 3V to 6V.

VCC = 5V operation gives full RS232 compliant perfor-

mance, 3V operation results in lower driver output ampli-

tude. The VCC pin should be decoupled with a 0.1µF

ceramic capacitor.

C1+, C1–, C2+, C2– (Pins 3, 4, 26, 27): These pins require

two external capacitors C ≥ 0.2µF. One from C1+ to C1–,

and another from C2+ to C2–. To maintain charge pump

efficiency, the capacitor’s effective series resistance should

be less than 2Ω. Low ESR tantalum capacitors work well

in this application, small value ceramic capacitors may

also be used with minimal reduction in charge pump

compliance.

DRIVER OUT (Pins 5, 7, 11): RS232 Driver Outputs.

Outputs are in a high impedance state when in shutdown,

driver disable, or VCC = 0V. Outputs are fully short-circuit

protected from V – + 30V to V + – 30V. Although the

outputs are protected, short circuits on one output can

load the power supply generator and may disrupt the

signal levels of the other outputs. The driver outputs are

protected against ESD to ±10kV for human body model

discharges. Output levels of – 3.3V to 4V are achieved

when the circuit is operated with VCC = 3.3V.

RX IN (Pins 6, 8, 9, 10): Receiver Inputs with 0.4V of

Hysteresis for Noise Immunity. These pins accept RS232

level signals (±30V) into a protected 5k terminating

resistor. The receiver inputs are protected against ESD to

±10kV for human body model discharges.

LOW-Q RX IN (Pin 12): Low Power Receiver Input. This

receiver remains active in shutdown mode, consuming

only 60µA from supply VL. This receiver has the same

input and protection characteristics as receivers RX1

through RX4.

ON/OFF (Pin 13): A TTL/CMOS Compatible Operating

Mode Control. A logic low puts the device in the shutdown

mode. All drivers and four of the receivers go to a high

impedance state, and the VCC supply may be turned off. A

logic high fully enables the transceiver.

VL (Pin 14): Power Supply for Receivers. This pin should

be powered to the same voltage as the logic circuits

connected to the receiver outputs, either 5V or 3V. The VL

pin should be decoupled with a 0.1µF ceramic capacitor.

DRIVER DISABLE (Pin 16): A logic high shuts down the

charge pump, placing all drivers in a high impedance state.

All receivers remain active. Floating the pin, or driving it

to a logic low, fully enables the transceiver. A low voltage

on the ON/OFF pin supersedes the state of the driver

disable control.

GND (Pin 17): Ground.

LOW-Q RX OUT (Pin 18): Low Power Receiver Output.

This pin produces the same output levels as standard

receivers, with slightly decreased speed and short-circuit

current.

DRIVER IN (Pins 19, 23, 25): RS232 Driver Inputs.

Inputs are TTL/CMOS compatible. Tie unused inputs

to VCC.

RX OUT (Pins 20, 21, 22, 24): Receiver Outputs. RX1

through RX4 outputs are in a high impedance state when

in shutdown mode to allow data line sharing. Outputs,

including LOW-Q RX OUT, are fully short-circuit pro-

tected to ground, VCC, or VL. Output voltage levels are

determined by the choice of power supply VL.

V – (Pin 28): Negative Supply Output. V – ≅ – (2VCC –

2.5V). This pin requires an external charge storage

capacitor, chosen to minimize ripple on V –. A minimum

value of 0.1µF is recommended.

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