SN74F1056

8-BIT SCHOTTKY BARRIER DIODE

BUS-TERMINATION ARRAY

SDFS085A – AUGUST 1992 – REVISED JULY 1997

3

POST OFFICE BOX 655303

APPLICATION INFORMATION

Large negative transients occurring at the inputs of memory devices (DRAMs, SRAMs, EPROMs, etc.) or on the

CLOCK lines of many clocked devices can result in improper operation of the devices. The SN74F1056 diode

termination array helps suppress negative transients caused by transmission-line reflections, crosstalk, and

switching noise.

Diode terminations have several advantages when compared to resistor termination schemes. Split resistor or

Thevenin equivalent termination can cause a substantial increase in power consumption. The use of a single resistor

to ground to terminate a line usually results in degradation of the output high level, resulting in reduced noise immunity.

Series damping resistors placed on the outputs of the driver reduce negative transients, but they also can increase

propagation delays down the line, as a series resistor reduces the output drive capability of the driving device. Diode

terminations have none of these drawbacks.

The operation of the diode arrays in reducing negative transients is explained in the following figures. The diode

conducts current when the voltage reaches a negative value large enough for the diode to turn on. Suppression of

negative transients is tracked by the current-voltage characteristic curve for that diode. A typical current versus

voltage plot for the SN74F1056 is shown in Figure 1.

To illustrate how the diode arrays act to reduce negative transients at the end of a transmission line, the test setup

in Figure 2(a) was evaluated. The resulting waveforms with and without the diode are shown in Figure 2(b).

The maximum effectiveness of the diode arrays in suppressing negative transients occurs when the diode arrays are

placed at the end of a line and/or the end of a long stub branching off a main transmission line. The diodes also can

be used to reduce the negative transients that occur due to discontinuities in the middle of a line. An example of this

is a slot in a backplane that is provided for an add-on card.

VF – Forward Voltage – V

Variable 1:

VIN

–Ch 1

Linear Sweep:

Start

0.000 V

Stop

–2.000 V

Step

–0.010 V

Constants:

VHI

–Vs1

3.5000 V

VLO –Vs2

0.0000 V

DIODE FORWARD CURRENT

vs

DIODE FORWARD VOLTAGE

–50

–40

–20

–10

0

–90

–30

0

0.2

0.4

0.6

0.8

1

1.2

–70

–60

–80

–100

1.4

1.6

1.8

2

TA = 25°C

Figure 1. Current Versus Voltage for the SN74F1056