DS275EN+ - Maxim Integrated Products, Inc.

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FEATURES

 Low-power serial transmitter/receiver for

battery-backed systems

 Transmitter steals power from receive signal

line to save power

 Ultra-low static current, even when connected

to RS-232-E port

 Variable transmitter level from +5 to +12

volts

 Compatible with RS-232-E signals

 Available in 8-pin, 150 mil wide SOIC

package (DS275S)

 Low-power CMOS

ORDERING INFORMATION

DS275 8-pin DIP

DS275S 8-pin SOIC

DS275E 14-pin TSSOP

PIN ASSIGNME NT

PIN DESCRIPTION

RXOUT - RS-232 Receiver Output

VDRV - Transmit driver +V

TXIN - RS-232 Driver Input

GND - System Ground (0V)

TXOUT - RS-232 Driver Output

NC - No Connection

RXIN - RS-232 Receive Input

VCC -System Logic Supply (+5V)

DESCRIPTION

The DS275 Line-Powered RS-232 Transceiver Chip is a CMOS device that provides a low-cost, very

low-power interface to RS-232 serial ports. The receiver input translates RS-232 signal levels to common

CMOS/TTL levels. The transmitter employs a unique circuit which steals current from the receive RS-

232 signal when that signal is in a negative state (marking). Since most serial communication ports

remain in a negative state statically, using the receive signal for negative power greatly reduces the

DS275’s static power consumption. This feature is especially important for battery-powered s ystems such

as laptop computers, remote sensors, and portable medical instruments. During an actual communication

session, the DS275’s transmitter will use system power (5-12 volts) for positive transitions while still

employing the receive signal for negative transitions.

DS275

Line-Powered RS-232 Transceiver Chip

www.dalsemi.com

DS275 8-Pin DIP (300-mil)

DS275 8-Pin SOIC (150-mil)

TXIN

VCC

VDRV

GND

RXOUT

RXIN

TXOUT

DS275E 14-Pin TSSOP

VDRV

TXIN

GND

VCC

RXIN

TXOUT

RXOUT NC

DS275

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DS275 BLOCK DIAGR AM Figure 1

OPERATION

Designed for the unique requirements of battery-backed systems, the DS275 provides a low-power half-

duplex interface to an RS-232 serial port. Typically, a designer must use an RS-232 device which uses

system power during both negative and positive transitions of the transmit signal to the RS-232 port. If

the connector to the RS-232 port is left connected for an appreciable time after the communication

session has ended, power will statically flow into that port, draining the battery capacity. The DS275

eliminates this static current drain by stealing current from the receive line (RXIN) of the RS-232 port

when that line is at a negative level (markin g). Since most asynchronous communication over an RS-232

connection t ypicall y remains in a marking state when dat a is not being sent, the DS275 will not consume

system power in this condition. System power would only be used when positive-going transitions are

needed on the transmit RS-232 output (TXOUT) when data is sent. However, since synchronous

communication sessions typically exhibit a very low duty-cycle, overall system power consumption

remains low.

RECEIVER SECTION

The RXIN pin is the receive input for an RS-232 signal whose levels can range from ±

±±

±3 to ±

±±

±15 volts. A

negative data signal is called a mark while a positive data signal is called a space. These signals are

inverted and then level-shifted to normal +5-volt CMOS/TTL logic levels. The logic output associated

with RXIN is RXOUT which swings from +VCC to ground. Therefore, a mark on RXIN pro du ces a logic 1 at

RXOUT; a space produces a logic 0.

The input threshold of RXIN is typically around 1.8 volts with 500 millivolts of hysteresis to improve

noise rejection. Therefore, an input positive-going signal must exceed 1.8 volts to cause RXOUT to switch

states. A negative-going signal must now be lower than 1.3 volts (typically) to cause RXOUT to switch

again. An open on RXIN is interpreted as a mark, producing a logic 1 at RXOUT.

TRANSMITTER SECTION

TXIN is the CMOS/TTL-compatible input for digital data from the user system. A logic 1 at TXIN

produces a mark (negative data signal) at TXOUT while a logic 0 produces a space (positive data signal).

As mentioned earlier, the transmitter section employs a unique driver design that uses the RXIN line for

swinging to negative levels. The RXIN line must be in a marking or idle state to take advantage of this

design; if RXIN is in a spaci ng stat e, TXOUT will onl y swing to ground. When TXOUT needs to transition to

a positive level, it uses the VDRV power pin for this level. VDRV can be a voltage supply between 5 to 12

DS275

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volts, and in many situations it can be tied directly to the +5 volt VCC supply. It is important to note that

VDRV must be greater than or equal to VCC at all times.

The voltage range on VDRV permits the use of a 9-volt battery in order to provide a higher voltage level

when TXOUT is in a space state. When VCC is shut off to the DS275 and VDRV is still powered (as might

happen in a battery-backed condition), only a small leakage current (about 50-100 nA) will be drawn. If

TXOUT is loaded during such a condition, VDRV will draw current only if RXIN is not in a negative state.

During normal operation (VCC=5 volts), VDRV will draw less than 2 uA when TXOUT is marking. Of

course, when TXOUT is spacing, VDRV will draw substantially more current



=

=about 3 mA, depending

upon its voltage and the impedance that TXOUT sees.

The TXOUT output is slew rate-limited to less than 30 volts/us in accordance with RS-232 specifications.

In the event TX OUT should be inadvertentl y shorted to ground, internal cu rrent-limiting circuitry prevents

damage, even if continuously shorted.

RS-232 COMPATIBILITY

The intent of the DS275 is not so much to meet all the requirements of the RS-232 specification as to

offer a low-power solution that will work with most RS-232 ports with a connector length of less than 10

feet. As a prime example, the DS275 will not meet the RS-232 requirement that the signal levels be at

least ±

±±

±5 volts minimum when terminated by a 3 kΩ

ΩΩ

Ω=

=load and VDRV = +5 volts. Typically a voltage of 4

volts will be present at TXOUT when spacing. However, since most RS-232 receivers will correctly

interpret any voltage over 2 volts as a space, there will be no problem transmitting data.

APPLICATIONS INFORM ATION

The DS275 is designed as a low-cost, RS-232-E interface expressly tailored for the unique requirements

of battery-operated handheld products. As shown in the electrical specifications, the DS275 draws

exceptionally low operating and static current. During normal operation when data from the handheld

system is sent from the TXOUT output, the DS275 only draws significant VDRV current when TXOUT

transitions positively (spacing). This current flows primarily into the RS-232 receiver’s 3-7 kΩ

ΩΩ

Ω=

=load at the

other end of the attaching cable. When TXOUT is marking (a negative data signal), the VDRV current falls

dramaticall y since the negative voltage is provided by the transmit signal from the other end of the cable.

This represents a large reduction in overall operating current, since typical RS-232 interface chips use

charge-pump circuits to establish both positive and negative levels at the transmit driver output.

To obtain the lowest power consumption from the DS275, observe the following guidelines. First, to

minimize VDRV current when connected to an RS-232 port, always maintain TXIN at a logic 1 when data is

not being transmitted (idle state). This will force TXOUT into the marking state, minimizing VDRV current.

Second, VDRV current will drop to less than 100 nA when VCC is grounded. Therefore, if VDRV is tied

directly to the system battery, the logic +5 volts can be turned off to achieve the lowest possible power

state.

FULL-DUPLEX OPERATION

The DS275 is intended primarily for half-duplex operation; that is, RXIN should remain idle in the

marking state when transmitting data out TXOUT and visa versa. However, the part can be operated full-

duplex with most RS-232-E serial ports since signals swinging between 0 and +5V will usually be

correctly interpreted by an RS-232-E receiver device. The 5-volt swing occurs when TXOUT attempts to

swing negative while RXIN is at a positive voltage, which turns on an internal weak pulldown to ground

for the TXOUT driver’s negative reference. So, transmit mark signals at TXOUT may have voltage jumps

from some negative value (corresponding to RXIN marking) to approximately ground. One possible

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problem that may occur in this case is if the receiver at the other end requires a negative voltage for

recognizing a mark. In this situation, the full-duplex circuit shown in Figure 3 can be used as

analternative. The 22 µ

µµ

µF capacitor forms a negative-charge reservoir; consequently, when the TXD line is

spacing (positive), TXOUT still has a negative source available for a time period determined by the

capacitor and the load resistance at the other end (3-7 kΩ

ΩΩ

Ω). This circuit was tested from 150-19,200 bps

with error-free operation using a SN75154 Quad Line R eceiver as the receiver for the TXOUT signal. Not e

that the SN75154 can have a marking input threshold below ground; hen ce the re is the need for T XOUT to

swing both positive and negative in full-duplex operation with this device.

HANDHELD RS-232-C APPLICATION USING A STEREO MINI-JACK Figure 2

FULL-DUPLEX CIRCUIT USING NEGATIVE-CHARGE STORAGE Figure 3

NOTE:

The capacitor stores negative charge whenever the TXD signal from the PC serial port is in a marking

data state (a negative voltage that is typically -10 volts). The top DS275’s TXOUT uses this negative

charge reservoir when it is in a marking state. The capacitor will discharge to 0 volts when the TXD line

is spacing (and TXOUT is still marking) at a time constant determined by its value and the value of the load

resistance reflected back to TXOUT. However, when TXD is marking the capacitor will quickly charge

back to -10 volts. Note that TXD remains in a marking state when idle, which improves the performance

of this circuit.

A BSOLUTE MAXIMUM RATINGS*

VCC -0.3 to +7.0 volts

VDRV -0.3 to +13.0 volts

DS275

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RXIN ±15 volts

TXIN -0.3 to VCC + 0.3 volts

TXOUT ±15 volts

RXOUT -0.3 to VCC + 0.3 volts

Storage Temperature -55°C to +125°C

Operating Temperature 0°C to 70°C

* This is a stress rating only and functional operation of the device at these or any other conditions

above those indicated in the operation sections of this specification is not implied. Exposure to

absolute maximum rating conditions for extended periods of time may affect reliability.

RECOMMENDED DC OPERATING CONDITI ONS

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Logic Supply VCC 4.5 5.0 5.5 V 1

Transmit Driver Supply VDRV 4.5 5-12 13.0 V 1

Logic 1 Input VIH 2.0 VCC+0.3 V 2

Logic 0 Input VIL -0.3 +0.8 V

RS-232 Input Range (RXIN)V

RS -15 +15 V

Dynamic Supply Current

TXIN = VCC

TXIN = GND

IDRV1

ICC1

IDRV1

ICC1

400

3.8

800

100

5.0

100

µA

Static Supply Current

TXIN = VCC

TXIN = GND

IDRV2

ICC2

IDRV2

ICC2

1.5

10.0

3.8

10.0

15.0

5.0

20.0

µA

Driver Leakage

Current (VCC=0V) IDRV3 0.05 1.0 µA5

DS275

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DC ELECTRICAL CHARACTERISTICS (0°

°°

°C to 70°

°°

°C; VCC = VDRV = 5V ±

±±

±=

=10%)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

TXOUT Level High VOTXH 3.5 4.0 5.0 V 6

TXOUT Lev el Low VOTXL -8.5 -9.0 V 7

TXOUT Short Circuit Current ISC +60 +85 mA

TXOUT Output Slew Rate tSR 30 V/µs

Propagation Delay tPD 5µs8

RXIN Input Threshold Low VTL 0.8 1.2 1.6 V

RXIN Input Threshold High VTH 1.6 2.0 2.4 V

RXIN Threshold Hysteresis VHYS 0.5 0.8 V 9

RXOUT Output Current @ 2.4V IOH -1.0 mA

RXOUT Output Current @ 0.4V IOL 3.2 mA

NOTES:

1. VDRV must be greater than or equal to VCC.

2. VCC = VDRV = 5V ±

±±

±=

=10%.

3. See test circuit in Figure 4.

4. See test circuit in Figure 5.

5. See test circuit in Figure 6.

6. TXIN = VIL and TX OUT loaded by 3 kΩ

ΩΩ

Ω=

=to ground.

7. TXIN = VIH, RXIN = -10 volts and TXOUT loaded by 3 kΩ

ΩΩ

Ω=

=to ground.

8. TXIN to TXOUT - see Figure 7.

9. VHYS = VTH - VTL.

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DYNAMIC OPER ATING CURRENT

TEST CIRCUIT Figure 4

STATIC OPERATING CURRENT

TEST CIRCUIT Figure 5

DRIVER LE AKAGE TEST CIRCUIT

Figure 6

DS275

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PROPAGATION DELAY TEST CIRCUIT Figure 7

DS275E 14-PIN TSSOP 14-PIN

DIM MIN MAX

A MM - 1.10

A1 MM 0.05 -

A2 MM 0.75 1.05

B MM 0.18 0.30

C MM 0.09 0.18

D MM 4.90 5.10

E MM 4.40 NOM

e1 MM 0.65 BSC

G MM 0.25 REF

H MM 6.25 6.55

L MM 0.50 0.70

phi 0°8°