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FEATURES
Low-power serial transmitter/receiver for
battery-backed systems
Transmitter steals power from receive signal
line to save power
Single 3V or 5V operation
Full duplex operation up to 20k bps
Ultra-low static current
Compatible with RS-232-E signals
PIN ASSIGNME NT
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 DIP
DS276S 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 negative
supplies, 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 power
consumption. This feature is especiall y important for battery-powered s ystems such as laptop computers,
remote sensors, and portable medi cal 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
www.dalsemi.com
DS276 8-Pin DIP (300-mil)
DS 276S 8-Pin SOIC
(
150-mil
)
RXOUT
VDRV+
TXIN
GND
VCC
RXIN
VDRV-
1
2
3
4
8
7
6
5TXOUT
DS276
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OPERATION
Designed for the unique requirements of batte r y-backed s ystems, the DS276 provides a low-pow er full-or
half-duplex interface to an RS-232 serial port. Typically, a designer must use an RS-232 device which
uses system pow er 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 DS276
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 (marking). Since most as ynchronous communication over an RS-232
connection t ypicall y remains in a m arking 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 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 +3 or +5 volt CMOS/TTL logic levels. The logic output
associated with R XIN 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 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 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 can use the RXIN line for
swinging to negative levels. RXIN can be connect ed via ex ternal ci rcuitr y 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 in
many 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 spa ce state. When VCC is shut off to the DS276 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 = 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 currentabout 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 in advertentl y shorted to
ground, internal current-limiting circuitry prevents damage, even if continuously shorted.
RS-232 COMPATIBILITY
The intent of the DS276 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
DS276
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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. T ypically 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 no
problem transmitting data.
DS276 BLOCK DIAGR AM Figure 1
APPLICATIONS INFORM ATION
The DS276 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 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+ 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 DS276, observe the following guidelines: First, to
minimize VDRV+ current when connected to an RS-232 port, alwa ys maintain TXIN at a lo gic 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 significantly when VCC is grounded. Therefore, if VDRV+ is
derived independentl y from VCC (for example connected to a 9V batter y), the logic supply voltage can b e
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 ne gative-charge reservoir; consequentl y,
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 primaril y for use in single supply 3- or 5- volts systems. However, several suppl y
configurations are possible.
3V OPERATION
The simplest configuration is to use a single 3V supply for VCC and Vdrv+, and connect Vdrv- to ground.
This will result in the lowest power consumption and will give adequate serial communication between
DS276
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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
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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 capacit or is cha r ged ne gatively whenever RXIN i s in a markin g (or idle) state. W hen 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
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ABSOLUTE MAXIMUM RA TINGS*
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+0.3V
TXOUT -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 seconds
* 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 CONDITIONS (tA = 0°C to 70°C)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES
Logic Supply Voltage VCC 2.7 3-5 5.5 V 1
Transmit Driver Supply VDR+
VDR+
VCC=5V±10%
VCC 2.7-3.6V VCC
VCC
5-12
3-5 13
5.5 V
V1
1
Transmit Driver Supply VDR- -15 0 V 1
High-level Input Voltage VIH 2V
CC
+0.3 V
Low-level Input Volta ge VIL -0.3 0.8 V
RXIN Input Volta ge VRS -15 +15 V 1
ELECTRICAL CHARACTERISTICS-3V OPERATION (tA = 0°C to 70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Logic Supply Voltage VCC 2.7 3.6 V
Dynamic Supply Current
IDRV1
ICC1
IDRV1
ICC1
TXIN = VCC
TXIN = VCC
TXIN = GND
TXIN = GND
400
40
3.8
40
800
100
5
100
uA
uA
mA
uA
2
2
2
2
Static Supply Current
IDRV2
ICC2
IDRV2
ICC2
TXIN = VCC
TXIN = VCC
TXIN = GND
TXIN = GND
1.5
10
3.8
10
10
15
5
20
uA
uA
mA
uA
3
3
3
3
Driver Leakage Current IDRV3 VCC = 0 0.05 1 uA 4
VDRV+=VCC=
2.7V
VDRV-=0 2 2.4 V 5
TXOUT Level High VOTXH VDRV+=4.5V,
VDRV-=-12V 3.8 4 V 6
DS276
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ELECTRICAL CHAR ACTERISTICS-3V OPERATION cont’d (TA = 0°C to 70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
VDRV +=
VCC=2.7V
VDRV-=0 0.2 0.3 V 5
TXOUT Lev el Low V OTXL
-11 -10 V 6
TXOUT Short Circuit
Current ISC VDRV+ = 5.5V,
VDRV- = -12V 85 mA 7
TXOUT Output Slew Rate tSR 30 V/us
Propagation Delay tPD 5us8
RXIN Input Threshold Low VTL 0.8 1.0 1.6 V
RXIN Input Threshold
High VTH 1.2 2.0 2.4 V
RXIN Threshold Hysteresis VHYS 0.4 1.0 V 9
RXOUT Output Current
High IOH VCC = 2.7V
VOH = 2V -0.5 mA
RXOUT Output Current
Low IOL VCC = 2.7V
VOL = 0.4V 0.5 mA
NOTES:
1. VDRV+ must be greater than or equal to VCC, RXIN must be greater than VDRV-.
2. See test circuit in Figure 5.
3. See test circuit in Figure 6.
4. See test circuit in Figure 7.
5. RL = 3k Ω=to ground. Max data rate = 20k bps.
6. RL = 3k Ω=to ground. Max data rate = 50k bps.
7. TXIN = VIL.
8. See test circuit in Figure 8.
9. VHYS = VTH - VTL.
DS276
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+ELECTRICA L CHA RA CTERISTIC S-5V OPERATION (tA = 0°C to 70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Logic Supply Voltage VCC 4.5 5 5.5 V
Dynamic Supply Current
IDRV1
ICC1
IDRV1
ICC1
TXIN = VCC
TXIN = VCC
TXIN = GND
TXIN = GND
400
40
3.8
40
800
100
5
100
uA
uA
mA
uA
1
1
1
1
Static Supply Current
IDRV2
ICC2
IDRV2
ICC2
TXIN = VCC
TXIN = VCC
TXIN = GND
TXIN = GND
1.5
10
3.8
10
10
15
5
20
uA
uA
mA
uA
2
2
2
2
Driver Leakage Current IDRV3 VCC = 0 0.05 1 uA 3
VDRV+=VCC=
4.5V
VDRV-=0 3.3 3.8 V 4
TXOUT Level High VOTXH VDRV+=12V,
VDRV-=-12V 10 11 V 5
VDRV +=
VCC=12V
VDRV-=12V -11 -10 V 4
TXOUT Lev el Low VOTXL VDRV+=VCC
VDRV-=0 0.2 0.3 V 5
TXOUT Short Circuit Current ISC VDRV+ = 12V,
VDRV- = -12V 85 mA 6
TXOUT Output Slew Rate tSR 30 V/us
Propagation Delay tPD 5us7
RXIN Input Threshold Low VTL 0.8 1.2 6 V
RXIN Input Threshold High VTH 1.6 2 2 V
RXIN Threshold Hysteresis VHYS 0.5 0.8 V 8
RXOUT Output Current High IOH VCC = 4.5V
VOH = 2.4V -1 mA
RXOUT Output Current Low IOL VCC = 4.5V
VOL = 0.4V 3 mA
NOTES:
1. See test circuit in Figure 9.
2. See test circuit in Figure 10.
3. See test circuit in Figure 11.
4. RL = 3 kΩ to ground. Max data rate = 20 kbps.
5. RL = 3 kΩ to ground. Max data rate = 100 kbps.
DS276
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6. TXIN = VIL.
7. See test circuit in Figure 12.
8. VHYS = VTH - VTL.
DYNAMIC OPER ATING CURRENT TEST CIRCUIT Figure 5
STATIC OPERATING CURRENT TEST CIRCUIT Figure 6
DS276
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DRIVER LE AKAGE TEST CIRCUIT Figure 7
PROPAGATION DELAY TEST CIRCUIT Figure 8
DYNAMIC OPER ATING CURRENT TEST CIRCUIT Figure 9
DS276
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STATIC OPERATING CURRENT TEST CIRCUIT Figure 10
DRIVER LE AKAGE TEST CIRCUIT Figure 11
PROPAGATION DELAY TEST CIRCUIT Figure 12
