ST2149

Supplementary notes

Doc ID 15949 Rev 2

9/23

3

Supplementary notes

3.1

Driver requirements

For proper operation, the driver from each side of the device must be able to source and

sink a minimum of 1 mA current. The device architecture requires the driver to source/sink

3.2

Load driving capability

To support the architecture that allows level translation without the direction pin, the one-

shot transistor is turned ON only during state transition at the output side. After the one-shot

transistor is turned OFF, only the 4 k

Ω resistor maintains the state. So, a resistive load or

pull-up resistor less than 50 k

Ω is not recommended for proper operation.

3.3

Power-off feature

In some applications it may be required to turn off one of the power supplies powering up

even if the OE pin is set to HIGH (enabled). In this mode, all I/Os are in high impedance

state.

3.4

Truth table

Table 3.

Truth table

Enable

Bi-directional Input/output

OE

1.

2.

H(1)

H(1)

LL

3.

Z = high impedance.

Z(3)