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 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 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, 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 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
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
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DALLAS [ DALLAS SEMICONDUCTOR ]
