DS275
3 of 8
volts, and in many situations it can be tied directly to the +5 volt 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 9-volt battery in order to provide a higher voltage level
when TXOUT is in a space state. When VCC is shut off to the DS275 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
TX
OUT is loaded during such a condition, VDRV will draw current only if RXIN is not in a negative state.
During normal operation (V
CC=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 specifications.
In the event TXOUT should be inadvertently shorted to ground, internal current-limiting circuitry prevents
damage, even if continuously shorted.
RS-232 COMPATIBILITY
The intent of the DS275 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 DS275 will not meet the RS-232 requirement that the signal levels be at
least ±
±±
±5 volts minimum when terminated by a 3 kΩ
ΩΩ
Ω=
==
=load and VDRV = +5 volts. Typically a voltage of 4
volts will be present at TXOUT when spacing. However, since most RS-232 receivers will correctly
interpret any voltage over 2 volts as a space, there will be no problem transmitting data.
APPLICATIONS INFORMATION
The DS275 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 DS275 draws
exceptionally low operating and static current. During normal operation when data from the handheld
system is sent from the TXOUT output, the DS275 only draws significant VDRV current when TXOUT
transitions positively (spacing). This current flows primarily into the RS-232 receiver’s 3-7 kΩ
ΩΩ
Ω=
==
=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 DS275, 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 TX
OUT into the marking state, minimizing VDRV current.
Second, VDRV current will drop to less than 100 nA when VCC is grounded. Therefore, if VDRV is tied
directly to the system battery, the logic +5 volts can be turned off to achieve the lowest possible power
state.
FULL-DUPLEX OPERATION
The DS275 is intended primarily for half-duplex operation; that is, RXIN should remain idle in the
marking state when transmitting data out TX
OUT and visa versa. However, the part can be operated full-
duplex with most RS-232-E serial ports since signals swinging between 0 and +5V will usually be
correctly interpreted by an RS-232-E receiver device. The 5-volt swing occurs when TXOUT attempts to
swing negative while RXIN is at a positive voltage, which turns on an internal weak pulldown to ground
for the TX
OUT driver’s negative reference. So, transmit mark signals at TXOUT may have voltage jumps
from some negative value (corresponding to RX
IN marking) to approximately ground. One possible
