DS276
Low Power Transceiver Chip
DS276
021998 1/11
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 ASSIGNMENT
8
7
6
5
1
2
3
4
RXOUT
VDRV+
TXIN
GND
VCC
RXIN
VDRV–
TXOUT
DS276 8–PIN DIP (300 MIL)
DS276S 8–PIN SOIC (150 MIL)
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 nega-
tive 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 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
021998 2/11
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. T ypi-
cally, 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 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 asynchronous communica-
tion 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 synchro-
nous 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 lev-
el–shifted to normal +3 or +5 volt CMOS/TTL logic lev-
els. The logic output associated with RXIN is RXOUT
which swings from VCC to ground. Therefore, a mark on
RXIN produces a logic 1 at RX OUT; 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 rejec-
tion. 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 connected via external circuitry to VDRV– to
allow stored charge to supply this voltage during mark-
ing (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 bat-
tery 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 might happen in a
battery–backed condition) , only a small leakage current
(about 50–100 nA) will be drawn. If TXOUT is loaded dur-
ing 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
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 speci-
fications. In the event TXOUT should be inadvertently
shorted to ground, internal current–limiting circuitry pre-
vents 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 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 3KΩ 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 no problem transmitting data.
DS276
021998 3/11
DS276 BLOCK DIAGRAM Figure 1
VDRV+
VCC
TXIN
VDRV–
RXOUT
GND
TXOUT
RXIN
APPLICATIONS INFORMATION
The DS276 is designed as a low–cost, RS–232–E inter-
face 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 opera-
tion when data from the handheld system is sent from
the TXOUT output, the DS276 only draws significant
VDRV+ current when TXOUT transitions positively (spac-
ing). This current flows primarily into the RS–232
receiver’s 3–7KΩ 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 inter-
face chips use charge–pump circuits to establish both
positive and negative levels at the transmit driver out-
put. To obtain the lowest power consumption from the
DS276, observe the following guidelines: First, to mini-
mize 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 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 low-
est 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
(3–7KΩ).
SUPPLY VOLTAGE OPTIONS
The DS276 is intended primarily for use in single supply
3– or 5– volts systems. However several supply config-
urations 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 two similar
devices over short distances, and into larger loads than
the 3KΩ 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 volt-
age 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.
DS276
021998 4/11
SINGLE 3V OPERATION Figure 2
RXOUT
VDRV+
TXIN
GND
VCC
RXIN
VDRV–
TXOUT
3V
TTL/
CMOS RS–232
CONNECTOR
–2 to –13V
(See Note 3)
SINGLE 5V OPERATION Figure 3
(not true RS–232)
RXOUT
VDRV+
TXIN
GND
VCC
RXIN
VDRV–
TXOUT
5V
TTL/
CMOS RS–232
CONNECTOR
or 0 to –13V
(See Note 1 and 3)
“STEALING” NEGATIVE SUPPLY Figure 4
RXOUT
VDRV+
TXIN
GND
VCC
RXIN
VDRV–
TXOUT
VCC (3 – 5V)
TTL/
CMOS RS–232
CONNECTOR
22 µF
(See Note 2)
NOTES:
1. This circuit as shown does not meet the RS–232 requirement for signal levels (High–level output voltage). How-
ever, as most RS–232 receivers will interpret any voltage over 2V as a space this will normally be of no conse-
quence. 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 20K bps.
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
021998 5/11
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+0.3V
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
T ransmit Driver Supply VDR+
VDR+ VCC=5V±10%
VCC=2.7–3.6V VCC
VCC 5–12
3–5 13
5.5 V
V1
1
T ransmit Driver Supply VDR– –15 0 V 1
High–level Input Voltage VIH 2Vcc +
0.3 V
Low–level Input Voltage VIL –0.3 0.8 V
RXIN Input Voltage 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
TXOUT Level High VOTXH
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
021998 6/11
ELECTRICAL CHARACTERISTICS–3V OPERATION cont’d (tA = 0°C to 70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
TXOUT Level Low
VOTXL
VDRV +=
VCC=2.7V
VDRV–=0 0.2 0.3 V 5
TX
OUT
L
eve
l
L
ow
V
OTXL VDRV+=VCC to
5.5V
VDRV–=–12V –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 5us 8
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 = 3KΩ to ground. Max data rate = 20K bps.
6. RL = 3KΩ to ground. Max data rate = 50K bps.
7. TXIN = VIL.
8. See test circuit in Figure 8.
9. VHYS = VTH – VTL.
DS276
021998 7/11
+ELECTRICAL CHARACTERISTICS–5V OPERATION (tA = 0°C to 70°C)
PARAMETER SYMBOL CONDITION 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
TXOUT Level High
VOTXH
VDRV+=VCC=
4.5V
VDRV–=0 3.3 3.8 V 4
TX
OUT
L
eve
l
Hi
g
h
V
OTXH VDRV+=12V,
VDRV–=0 to
–12V 10 11 V 5
TXOUT Level Low VOTXL
VDRV+=VCC to
12V,
VDRV–=–12V –11 –10 V 4
TXOUT Level 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 5us 7
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 = 3KΩ to ground. Max data rate = 20K bps.
5. RL = 3KΩ to ground. Max data rate = 100K bps.
6. TXIN = VIL.
7. See test circuit in Figure 12.
8. VHYS = VTH – VTL.
DS276
021998 8/11
DYNAMIC OPERATING CURRENT TEST CIRCUIT Figure 5
ICC1 IDRV1
+3.6V +5.5V
VDRV+
VCC
+5V
TXIN TXOUT
RXOUT RXIN
VDRV– GND VPULSE
C=2500 pFR = 3KΩ
+15V
–15V
tt = 100 µ sec
slew rate < 30V/µ sec
VPULSE
–15V
STATIC OPERATING CURRENT TEST CIRCUIT Figure 6
ICC2 IDRV2
+3.6V +5.5V
VDRV+
VCC
+5V
TXIN TXOUT
RXOUT RXIN
VDRV– GND
R = 3KΩ
–15V
–15V
DS276
021998 9/11
DRIVER LEAKAGE TEST CIRCUIT Figure 7
IDRV3
+5.5V
VDRV+
VCC
TXIN TXOUT
RXOUT RXIN
VDRV– GND
R = 3KΩ
–3 to –15V
–15V
PROPAGATION DELAY TEST CIRCUIT Figure 8
ICC1 IDRV1
+2.7V +2.7V
VDRV+
VCC
TXIN TXOUT
RXOUT RXIN
VDRV– GND
C=2500 pFR = 3KΩ
2.0V
0.8V
VOTXH
VOTXL
TXIN
TXOUT
50% 50%
tPD tPD
90% 90%
10% 10%
tFtR
0.8(VOTXH – VOTXL)
tF or tR
tSR =
DS276
021998 10/11
DYNAMIC OPERATING CURRENT TEST CIRCUIT Figure 9
ICC1 IDRV1
+5.5V +13V
VDRV+
VCC
+5V
TXIN TXOUT
RXOUT RXIN
VDRV– GND VPULSE
C=2500 pFR = 3KΩ
+15V
–15V
tt = 100 µ sec
slew rate < 30V/µ sec
VPULSE
–15V
STATIC OPERATING CURRENT TEST CIRCUIT Figure 10
ICC2 IDRV2
+5.5V +13V
VDRV+
VCC
+5V
TXIN TXOUT
RXOUT RXIN
VDRV– GND
R = 3KΩ
–15V
–15V
DS276
021998 11/11
DRIVER LEAKAGE TEST CIRCUIT Figure 11
IDRV3
+13V
VDRV+
VCC
TXIN TXOUT
RXOUT RXIN
VDRV– GND
R = 3KΩ
–3 to –15V
–15V
PROPAGATION DELAY TEST CIRCUIT Figure 12
ICC1 IDRV1
+4.5V +4.5V
VDRV+
VCC
TXIN TXOUT
RXOUT RXIN
VDRV– GND
C=2500 pFR = 3KΩ
2.0V
0.8V
VOTXH
VOTXL
TXIN
TXOUT
50% 50%
tPD tPD
90% 90%
10% 10%
tFtR
0.8(VOTXH – VOTXL)
tF or tR
tSR =
