Datasheet

SCBA015A
7–35
Fast GTLP Backplanes With the GTLPH1655
V
TH
Max Peak With Precharge
Possible Peak Without Precharge
Point of Live Insertion
During High on Bus
Point of Live Insertion
During Low on Bus
Max Peak With Precharge
Possible Peak
Without Precharge
Time
Precharge Level = V
TH
Low Data on Bus
Volts
High Data on Bus
Figure 27. Influence of Precharge Function on Bus Signal
The SN74GTLPH1655 has the characteristics discussed above, which are necessary for the
successful development of a live-insertion application.
Using the OE control input, it is possible to set the outputs of the SN74GTLPH1655 on both
sides simultaneously to a high-resistance state. As a result of the integrated power-up 3-state
circuit, the device is definitely inactive at a V
CC
of less than 1.5 V.
To ensure that there is also a definite high-resistance state at a supply voltage between 1.5 V
and the operating voltage, it is recommended that OE be connected to V
CC
via a pullup resistor.
High-impedance outputs can be precharged to a definite voltage level by means of the
precharge circuitry (BIAS V
CC
). Disturbances to the active bus arising from insertion
(charging/discharging of the I/O capacitance) will thus be kept to a minimum.
In a similar fashion, in modern applications, particular parts of a system are switched off from the
source of power without having first removed them from the complete system. This is a partial
switching off of the system, or a partial power down.
If a device is used in a partial power-down application, the inputs and outputs for V
CC
= 0 V must
be at high impedance, and thus be able to tolerate active bus signals.
The property of being partial-power-down compatible is reflected in the parameter I
OFF
, which
specifies the maximum leakage current in an input or output.
I
OFF
is defined as:
• The device is disconnected from the operating voltage (V
CC
= 0 V), and
• A logic level is applied to the input or output.
With the SN74GTLPH1655, the maximum value of I
OFF
is 100 µA.
Refer to the TI application report
Live Insertion, literature number SDYA012, which discusses
this subject in detail.