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DS275E-DS275S
Line-Powered RS-232 Transceiver Chip
FEATURESLow-power serial transmitter/receiver for
battery-backed systemsTransmitter steals power from receive signalline to save powerUltra-low static current, even when connected
to RS-232-E portVariable transmitter level from +5 to +12
voltsCompatible with RS-232-E signalsAvailable in 8-pin, 150 mil wide SOIC
package (DS275S)Low-power CMOS
ORDERING INFORMATIONDS275 8-pin DIP
DS275S 8-pin SOICDS275E 14-pin TSSOP
PIN ASSIGNMENT
PIN DESCRIPTIONRXOUT- RS-232 Receiver OutputVDRV - Transmit driver +V
TXIN - RS-232 Driver Input
GND- System Ground (0V)
TXOUT- RS-232 Driver Output - No ConnectionRXIN - RS-232 Receive Input
VCC-System Logic Supply (+5V)
DESCRIPTIONThe 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 systems 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 stillemploying the receive signal for negative transitions.
DS275
Line-Powered RS-232 Transceiver ChipDS275 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
DS275
DS275 BLOCK DIAGRAM Figure 1
OPERATIONDesigned 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 DS275eliminates 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 (marking). Since most asynchronous communication over an RS-232
connection typically remains in a marking state when data 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 synchronouscommunication sessions typically exhibit a very low duty-cycle, overall system power consumption
remains low.
RECEIVER SECTIONThe 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 associatedwith RXIN is RXOUT which swings from +VCC to ground. Therefore, a mark on RXIN produces 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 switchstates. 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 SECTIONTXIN 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
DS275
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 levelwhen 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 TXOUT should be inadvertently shorted to ground, internal current-limiting circuitry preventsdamage, even if continuously shorted.
RS-232 COMPATIBILITYThe 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 kΩΩΩΩ=
=load and VDRV = +5 volts. Typically a voltage of 4volts 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 INFORMATIONThe 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 handheldsystem 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 kΩΩΩΩ=
=load at the
other end of the attaching cable. When TXOUT is marking (a negative data signal), the VDRV current falls
dramatically 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, tominimize 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 OPERATIONThe DS275 is intended primarily for half-duplex operation; that is, RXIN should remain idle in themarking 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
DS275
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 μμμμF 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 kΩΩΩΩ). This circuit was tested from 150-19,200 bps
with error-free operation using a SN75154 Quad Line Receiver as the receiver for the TXOUT signal. Note
that the SN75154 can have a marking input threshold below ground; hence there is the need for TXOUT 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 loadresistance 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.
DS275
RXIN ±15 volts
TXIN -0.3 to VCC + 0.3 volts
TXOUT ±15 volts
RXOUT -0.3 to VCC + 0.3 voltsStorage Temperature -55°C to +125°C
Operating Temperature 0°C to 70°CThis 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 toabsolute maximum rating conditions for extended periods of time may affect reliability.
RECOMMENDED DC OPERATING CONDITIONS
DS275
DC ELECTRICAL CHARACTERISTICS (0°°°°C to 70°°°°C; VCC = VDRV = 5V ±±±±=
=10%)
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 kΩΩΩΩ=
=to ground.
7. TXIN = VIH, RXIN = -10 volts and TXOUT loaded by 3 kΩΩΩΩ=
=to ground.
8. TXIN to TXOUT - see Figure 7.9. VHYS = VTH - VTL.
