ST2149 datasheet(9/23 Pages) STMICROELECTRONICS

9 / 23 page

ST2149

Supplementary notes

Doc ID 15949 Rev 2

9/23

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

maximum current of (VCC/4) mA to/from the weak 4 kΩ output buffer.

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

the level translator. The device is automatically disabled when VCC supply is turned OFF,

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

I/OVCC

I/OVL

H(1)

High-level VL power supply referred.

H(2)

High-level VCC power supply referred.

H(1)

LZ(3)

Z = high impedance.

Z(3)

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