Datasheet

Output Enable Control www.ti.com
With regard to capacitive loads, TXB translators are designed to drive up to 70 pF without issue. If
capacitive loading is larger than 70 pF, the O.S. will time-out after 10 ns and subsequently turn off before
the output voltage is driven fully high or low. From there, the output will continue to rise or fall based on
the RC time constant determined by the 4-kΩ buffer, load resistance and capacitive loading. To ensure
reliable operation, system designers should keep capacitive loading for the TXB-type devices to 70 pF or
less. With no external loading, the one-shot circuits will remain on for approximately 4.5 nsec. The
duration of the one-shots is related to the output voltage level. The output high voltage at which O.S.
switches off is approximately 95% of full scale, while the output low voltage at which O.S. switches off is
approximately 5%.
4 Output Enable Control
The TXB devices offer low power consumption of 5-mA maximum I
CC
when the output enable is high.
When the output enable is low, the TXB translator buffer will be disabled and the outputs are put into a
high impedance state for increased power savings. The OE input circuit is referenced to the V
CCA
power-supply and when the device is disabled, the 4-kΩ buffer and the O.S. for both the A-port and B-Port
are also disabled. In this state, output leakage (I
OZ
) will be less than ±2 mA. If the application does not
require output enable control, The OE pin should be tied to the V
CCA
supply. One should never leave the
OE pin floating in an indeterminate state as this can cause undesirable quiescent current to flow in the
device, which subsequently increases its overall power dissipation.
Under partial power down conditions, the outputs are also disabled and put into a high-Impedance state.
This feature is referred to as V
CC
isolation. If V
CCB
= 0 V, the A-port is disabled. Likewise, if V
CCA
= 0V, the
B-port will be disabled. In addition, the TXB type translators are fully specified for partial-power-down
applications using the I
OFF
feature. This I
OFF
circuitry disables the outputs, preventing damaging current
backflow through the device when these devices are powered-down.
5 Conclusion
The TXB translators offer system designers a versatile solution for remedying mixed-voltage system
incompatibilities. These translators eliminate the need for provisioning a processor GPIO, since they
change the direction of the data flow automatically without the use of a direction control pin. This simplifies
system level design and allows for solutions in smaller packages.
Please visit www.ti.com for data sheets and additional information on all bit-width TXB translators along
with the full line of Texas Instruments voltage-level translators.
6
A Guide to Voltage Translation With TXB-Type Translators SCEA043–March 2010
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