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LT1537

Advanced Low Power 5V RS232 Transceiver with Small Capacitors

Linear Technology 

LT1537

TYPICAL PERFOR A CE CHARACTERISTICS

Shutdown to Driver Output

Driver Output Waveforms

DRIVER 10

OUTPUT HIGH

RL = 3k 5

0

DRIVER – 5

OUTPUT LOW

RL = 3k – 10

ON/OFF PIN

DRIVER OUTPUT

RL = 3k

DRIVER OUTPUT

RL = 3k

CL = 2500pF

INPUT

LT1537 • TPC12

LT1537 • TPC13

PI FU CTIO S

VCC: 5V Input Supply Pin. Supply current drops to zero in

the shutdown mode. This pin should be decoupled with a

0.1µF ceramic capacitor close to the package pin. Insuffi-

cient supply bypassing can result in low output drive levels

and erratic charge pump operation.

GND: Ground Pin.

ON/OFF: TTL/CMOS Compatible Operating Mode Control.

A logic LOW puts the device in the shutdown mode which

reduces input supply current to zero and places all of the

drivers and receivers in high impedance state. A logic

HIGH fully enables the transceiver.

DRIVER DISABLE: This pin provides an alternate control

for the charge pump and RS232 drivers. A logic HIGH on

this pin shuts down the charge pump and places all drivers

in a high impedance state. Receivers remain active under

these conditions. Floating the driver disable pin or driving

it to a logic LOW level fully enables the transceiver. A logic

LOW on the On/Off pin supersedes the state of the Driver

Disable pin. Supply current drops to 1.5mA when in

DRIVER DISABLE mode.

V+: Positive Supply Output (RS232 Drivers). V+ ≈ 2VCC –

1.5V. This pin requires an external charge storage capaci-

tor C ≥ 0.1µF, tied to ground or VCC. Larger value capaci-

tors may be used to reduce supply ripple. With multiple

transceivers, the V + and V – pins may be paralleled into

common capacitors. For large numbers of transceivers,

increasing the size of the shared common storage capaci-

tors is recommended to reduce ripple.

V–: Negative Supply Output (RS232 Drivers). V – ≈

–(2VCC – 2.5V). This pin requires an external charge

storage capacitor C ≥ 0.1µF. V – is short-circuit proof for

30 seconds.

C1+, C1–, C2+, C2–: Commutating Capacitor Inputs. 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 ceramic

capacitors work well in this application.

DRIVER IN: RS232 Driver Input Pins. These inputs are

TTL/CMOS compatible. Inputs should not be allowed to

float. Tie unused inputs to VCC.

DRIVER OUT: Driver Outputs at RS232 Voltage Levels.

Driver output swing meets RS232 levels for loads up to 3k.

Slew rates are controlled for lightly loaded lines. Output

current capability is sufficient for load conditions up to

2500pF. Outputs are in a high impedance state when in

shutdown mode, VCC = 0V or when the driver disable pin

is active. Outputs are fully short-circuit protected from V –

+ 25V to V+ – 25V. Applying higher voltages will not

damage the device if the overdrive is moderately current

limited. 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 ±5kV for human body model discharges.

5

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