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1 of 11 112299 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 pin description rx out rs-232 receiver output v drv+ transmit driver positive supply tx in rs-232 driver input gnd system ground (0v) tx out rs-232 driver output v drv- transmit driver negative supply rx in rs-232 receiver input v cc 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 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 www.dalsemi.com ds276 8-pin dip (300-mil) ds 276s 8-pin soic ( 150-mil ) rx out v drv+ tx in gnd v cc rx in v drv- 1 2 3 4 8 7 6 5 tx out
ds276 2 of 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. 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 ds276 eliminates this static current drain by stealing current from the receive line (rx in ) 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 (tx out ) 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 rx in 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 rx in is rx out which swings from v cc to ground. therefore, a mark on rx in produces a logic 1 at rx out ; a space produces a logic 0. the input threshold of rx in 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 rx out to switch states. a negative-going signal must now be lower than 1.3 volts (typically) to cause rx out to switch again. an open on rx in is interpreted as a mark, producing a logic 1 at rx out . transmitter section tx in is the cmos/ttl-compatible input for data from the user system. a logic 1 at tx in produces a mark (negative data signal) at tx out 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 rx in line for swinging to negative levels. rx in can be connected via external circuitry to v drv- to allow stored charge to supply this voltage during marking (or idle) states. when tx out needs to transition to a positive level, it uses the v drv+ power pin for this level. v drv+ can be a voltage supply between 3 to 12 volts, and in many situations it can be tied directly to the v cc supply. it is important to note that v drv+ must be greater than or equal to v cc at all times. the voltage range on v drv+ permits the use of a 9v battery in order to provide a higher voltage level when tx out is in a space state. when v cc is shut off to the ds276 and v drv+ is still powered (as might happen in a battery-backed condition), only a small leakage current (about 50-100 na) will be drawn. if tx out is loaded during such a condition, v drv+ will draw current only if rx in is not in a negative state. during normal operation (v cc = 3 or 5 volts), v drv+ will draw less than 2 ua when tx out is marking. of course, when tx out is spacing, v drv+ will draw substantially more current ? about 3 ma, depending upon its voltage and the impedance that tx out sees. the tx out output is slew rate-limited to less than 30 volts/us in accordance with rs-232 specifications. in the event tx out should be inadvertently 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 3 of 11 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 k ? = load and v drv+ = +3-5 volts. typically 2.5 to 4 volts will be present at tx out 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 diagram figure 1 applications information 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 tx out output, the ds276 only draws significant v drv+ current when tx out transitions positively (spacing). this current flows primarily into the rs-232 receiver?s 3-7 k ? = load at the other end of the attaching cable. when tx out is marking (a negative data signal), the v drv+ 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 v drv+ current when connected to an rs-232 port, always maintain tx in at a logic 1 when data is not being transmitted (idle state). this will force tx out into the marking state, minimizing v drv+ current. second, v drv+ current will drop significantly when v cc is grounded. therefore, if v drv+ is derived independently from v cc (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 rx in . the 22 f capacitor forms a negative-charge reservoir; consequently, when the txd line rx in is spacing (positive), tx out still has a negative source available for a time period determined by the capacitor and the load resistance at the other end (3-7 k ? ). 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 the simplest configuration is to use a single 3v supply for v cc and v drv+ , and connect v drv- to ground. this will result in the lowest power consumption and will give adequate serial communication between
ds276 4 of 11 two similar devices over short distances, and into larger loads than the 3 k ? = rs-232 standard (figure 2). if v drv+ is increased to 5v, and v drv- 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 v drv- supply can be derived using the ?stealing? technique shown in figure 4. 5v operation the use of a single 5v supply for v cc and v drv+ , and v drv- 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 v drv+ 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 5 of 11 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, v drv+ can be supplied independently from a higher voltage supply. 2. the capacitor is charged negatively whenever rx in is in a marking (or idle) state. when the ds276 is transmitting marking data and rx in 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. rx in must never be allowed to reach a negative voltage with respect to v drv- or excessive currents will be drawn. therefore, if negative voltage swings are present on rx in , v drv- should not be connected to ground and the circuit shown in figure 4 should be used.
ds276 6 of 11 absolute maximum ratings* v cc -0.3v to +7.0v v dr+ -0.3v to +13v v dr- -13v to +0.3v rx in -15v to +15v tx in -0.3v to vcc+0.3v tx out -15v to +15v rx out -0.3v to vcc+0.3v operating temperature 0c to 70c storage temperature -55c to +125c soldering temperature 260c 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 (t a = 0 c to 70 c) parameter symbol condition min typ max units notes logic supply voltage v cc 2.7 3-5 5.5 v 1 transmit driver supply v dr+ v dr+ v cc =5v 10% v cc 2.7-3.6v v cc v cc 5-12 3-5 13 5.5 v v 1 1 transmit driver supply v dr- -15 0 v 1 high-level input voltage v ih 2v cc +0.3 v low-level input voltage v il -0.3 0.8 v rx in input voltage v rs -15 +15 v 1 electrical characteristics-3v operation (t a = 0 c to 70 c) parameter symbol min typ max units notes logic supply voltage v cc 2.7 3.6 v dynamic supply current i drv1 i cc1 i drv1 i cc1 tx in = v cc tx in = v cc tx in = gnd tx in = gnd 400 40 3.8 40 800 100 5 100 ua ua ma ua 2 2 2 2 static supply current i drv2 i cc2 i drv2 i cc2 tx in = v cc tx in = v cc tx in = gnd tx in = gnd 1.5 10 3.8 10 10 15 5 20 ua ua ma ua 3 3 3 3 driver leakage current i drv3 v cc = 0 0.05 1 ua 4 v drv+ =v cc = 2.7v v drv- =0 2 2.4 v 5 tx out level high v otxh v drv+ =4.5v, v drv- =-12v 3.8 4 v 6
ds276 7 of 11 electrical characteristics-3v operation cont?d (t a = 0 c to 70 c) parameter symbol min typ max units notes v drv += v cc =2.7v v drv- =0 0.2 0.3 v 5 tx out level low v otxl -11 -10 v 6 tx out short circuit current i sc v drv+ = 5.5v, v drv- = -12v 85 ma 7 tx out output slew rate t sr 30 v/us propagation delay t pd 5us8 rx in input threshold low v tl 0.8 1.0 1.6 v rx in input threshold high v th 1.2 2.0 2.4 v rx in threshold hysteresis v hys 0.4 1.0 v 9 rx out output current high i oh v cc = 2.7v v oh = 2v -0.5 ma rx out output current low i ol v cc = 2.7v v ol = 0.4v 0.5 ma notes: 1. v drv+ must be greater than or equal to v cc , rx in must be greater than v drv- . 2. see test circuit in figure 5. 3. see test circuit in figure 6. 4. see test circuit in figure 7. 5. r l = 3k ? = to ground. max data rate = 20k bps. 6. r l = 3k ? = to ground. max data rate = 50k bps. 7. tx in = v il . 8. see test circuit in figure 8. 9. v hys = v th - v tl .
ds276 8 of 11 + electrical characteristics-5v operation (t a = 0 c to 70 c) parameter symbol min typ max units notes logic supply voltage v cc 4.5 5 5.5 v dynamic supply current i drv1 i cc1 i drv1 i cc1 tx in = v cc tx in = v cc tx in = gnd tx in = gnd 400 40 3.8 40 800 100 5 100 ua ua ma ua 1 1 1 1 static supply current i drv2 i cc2 i drv2 i cc2 tx in = v cc tx in = v cc tx in = gnd tx in = gnd 1.5 10 3.8 10 10 15 5 20 ua ua ma ua 2 2 2 2 driver leakage current i drv3 v cc = 0 0.05 1 ua 3 v drv+ =v cc = 4.5v v drv- =0 3.3 3.8 v 4 tx out level high v otxh v drv+ =12v, v drv- =-12v 10 11 v 5 v drv += v cc =12v v drv- =12v -11 -10 v 4 tx out level low v otxl v drv+ =v cc v drv- =0 0.2 0.3 v 5 tx out short circuit current i sc v drv+ = 12v, v drv- = -12v 85 ma 6 tx out output slew rate t sr 30 v/us propagation delay t pd 5us7 rx in input threshold low v tl 0.8 1.2 6 v rx in input threshold high v th 1.6 2 2 v rx in threshold hysteresis v hys 0.5 0.8 v 8 rx out output current high i oh v cc = 4.5v v oh = 2.4v -1 ma rx out output current low i ol v cc = 4.5v v ol = 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. r l = 3 k ? to ground. max data rate = 20 kbps. 5. r l = 3 k ? to ground. max data rate = 100 kbps.
ds276 9 of 11 6. tx in = v il . 7. see test circuit in figure 12. 8. v hys = v th - v tl . dynamic operating current test circuit figure 5 static operating current test circuit figure 6
ds276 10 of 11 driver leakage test circuit figure 7 propagation delay test circuit figure 8 dynamic operating current test circuit figure 9
ds276 11 of 11 static operating current test circuit figure 10 driver leakage test circuit figure 11 propagation delay test circuit figure 12

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