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DS276S
Low Power Transceiver Chip
FEATURESLow-power serial transmitter/receiver for
battery-backed systemsTransmitter steals power from receive signalline to save powerSingle 3V or 5V operationFull duplex operation up to 20k bpsUltra-low static currentCompatible with RS-232-E signals
PIN ASSIGNMENT
PIN DESCRIPTION
RXOUT RS-232 Receiver Output
VDRV+ Transmit Driver Positive Supply
TXIN RS-232 Driver Input
GND System Ground (0V)TXOUT RS-232 Driver Output
VDRV- Transmit Driver Negative Supply
RXIN RS-232 Receiver Input
VCC System Logic Supply (3-5V)
ORDERING INFORMATION
DS276 8-Pin DIPDS276S 8-Pin SOIC
DESCRIPTION
The DS276 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 can be used with independently supplied positive and negativesupplies, but in most cases will be used with the positive supply, sharing the logic supply and the negative
supply stolen from the receive RS-232 signal when that signal is in a negative state (marking). By using
an external reservoir capacitor and Schottky diode (see Figure 4) this negative supply can be maintained
even during full-duplex operation. Since most serial communication ports remain in a negative state
statically, using the receive signal for negative power greatly reduces the DS276’s static powerconsumption. 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 DS276’s
transmitter will use system power (3-12 volts) for positive transitions while still employing the receive
signal for negative transitions.
DS276
Low Power Transceiver Chip
DS276
OPERATION
Designed for the unique requirements of battery-backed systems, the DS276 provides a low-power full-or
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 DS276eliminates 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 DS276 will not consume
system power in this condition. Sys-tem 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 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 areinverted and then level-shifted to normal +3 or +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 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 improvenoise 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 data from the user system. A logic 1 at TXIN produces amark (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 can use the RXIN line for
swinging to negative levels. RXIN can be connected via external circuitry to VDRV- to allow stored charge
to supply this voltage during marking (or idle) states. 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 3 to 12 volts, and inmany situations it can be tied directly to the 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 9V battery in order to provide a higher voltage level
when TXOUT is in a space state. When VCC is shut off to the DS276 and VDRV+ is still powered (as mighthappen 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 = 3 or 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, dependingupon 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 prevents damage, even if continuously shorted.
RS-232 COMPATIBILITY
DS276
feet. As a prime example, the DS276 will not meet the RS-232 requirement that the signal levels be at
least ±5 volts minimum when terminated by a 3 kΩ=load and VDRV+ = +3-5 volts. Typically 2.5 to 4 volts
will be present at TXOUT when spacing under this condition, depending on the supply voltage. However,
since most RS-232 receivers will correctly interpret any voltage over 2 volts as a space, there will be noproblem transmitting data.
DS276 BLOCK DIAGRAM Figure 1
APPLICATIONS INFORMATION
The DS276 is designed as a low-cost, RS-232-E interface expressly tailored for the unique requirementsof battery-operated handheld products. As shown in the electrical specifications, the DS276 draws
exceptionally low operating and static current. During normal operation when data from the handheld
system is sent from the TXOUT output, the DS276 only draws significant VDRV+ current when TXOUT
transitions positively (spacing). This current flows primarily into the RS-232 receiver’s 3-7 kΩ=load at
the other end of the attaching cable. When TXOUT is marking (a negative data signal), the VDRV+ currentfalls 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 DS276, observe the following guidelines: First, to
minimize VDRV+ current when connected to an RS-232 port, always maintain TXIN at a logic 1 when datais not being transmitted (idle state). This will force TXOUT into the marking state, minimizing VDRV+
current. Second, VDRV+ current will drop significantly when VCC is grounded. Therefore, if VDRV+ is
derived independently from VCC (for example connected to a 9V battery), the logic supply voltage can be
turned off to achieve the lowest possible power state.
FULL-DUPLEX OPERATION
The DS276 is intended for full-duplex operation using the full-duplex circuit shown in Figure 4 to
generate a negative rail from RXIN. The 22 μF capacitor forms a negative-charge reservoir; consequently,
when the TXD line RXIN 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).
SUPPLY VOLTAGE OPTIONS
The DS276 is intended primarily for use in single supply 3- or 5- volts systems. However, several supply
configurations are possible.
3V OPERATION
DS276
two similar devices over short distances, and into larger loads than the 3 kΩ=RS-232 standard (Figure 2).
If Vdrv+ is increased to 5V, and Vdrv- decreased (to less than -2V) communication with standard RS-232
devices is possible, although of course the output voltage swing of the DS276 remains below the RS-232
specification. The Vdrv- supply can be derived using the “stealing” technique shown in Figure 4.
5V OPERATION
The use of a single 5V supply for VCC and Vdrv+, and Vdrv- derived using the circuit in Figure 4 can
produce reliable communication with standard RS-232 devices, although the DS276 output voltage
swings are below the RS-232 minimum (Figure 3).
Increasing the magnitude of the voltage to Vdrv+ to 10 volts or more will result in “true” RS-232 output
voltage levels.
SINGLE 3V OPERATION Figure 2
(See Note 3)
SINGLE 5V OPERATION Figure 3
(not true RS-232)
(See Note 1 and 3)
"STEALING" NEGATIVE SUPPLY Figure 4
(See Note 2)
DS276
NOTES:
1. This circuit as shown does not meet the RS-232 requirement for signal levels (high-level output
voltage). However, as most RS-232 receivers will interpret any voltage over 2V as a space this will
normally be of no consequence. Alternatively, VDRV+ can be supplied independently from a higher
voltage supply.
2. The capacitor is charged negatively whenever RXIN is in a marking (or idle) state. When the DS276 is
transmitting marking data and RXIN is spacing the capacitor will discharge towards ground with a
time constant determined by the capacitor value and the value of the load resistance. The value shown
should store sufficient charge for reliable operation up to 20 kbps.
3. RXIN must never be allowed to reach a negative voltage with respect to VDRV- or excessive currents
will be drawn. Therefore, if negative voltage swings are present on RXIN, VDRV- should not be
connected to ground and the circuit shown in Figure 4 should be used.
DS276
ABSOLUTE MAXIMUM RATINGS*
VCC -0.3V to +7.0V
VDR+ -0.3V to +13V
VDR- -13V to +0.3V
RXIN -15V to +15V
TXIN -0.3V to VCC+0.3VTXOUT -15V to +15V
RXOUT -0.3V to VCC+0.3V
Operating Temperature 0°C to 70°C
Storage Temperature -55°C to +125°C
Soldering Temperature 260°C for 10 secondsThis 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 CONDITIONS (tA = 0°C to 70°C)
ELECTRICAL CHARACTERISTICS-3V OPERATION (tA = 0°C to 70°C)

Partno	Mfg	Dc	Qty	Available	Descript
DS276S	DALLAS	N/a	4	avai	Low Power Transceiver Chip
DS276S	MAXIM	N/a	2500	avai	Low Power Transceiver Chip