4

408-432-9100/www.impweb.com

© 2000 IMP, Inc.

Application Information

IMP5218 Maximizes Line Current

Cable transmission theory suggests to optimize signal speed and

quality, the termination should act both as an ideal voltage refer-

ence when the line is released (deasserted) and as an ideal

current source when the line is active (asserted). Common active

terminators which consist of linear regulators in series with resis-

tors (typically 110

Ω) are a compromise. With coventional linear

terminators as the line voltage increases the amount of current

decreases linearly by the equation;

The IMP5218, with its unique new architecture, applies the maxi-

mum amount of current regardless of line voltage until the

termination high threshold (2.85V) is reached.

Acting as a near ideal line terminator, the IMP5218 closely repro-

duces the optimum case when the device is enabled. To enable the

device the Disconnect 1 and Disconnect 2 pins must be driven per

Table 1. When enabled, quiescent current is 6mA and the device

will respond to line demands by delivering 24mA on assertion

and by imposing 2.85V on de-assertion.

Disable/Sleep Mode

Disable/Sleep mode can be used for power conservation or to

remove the terminator from the SCSI chain.

The IMP5218 is disabled when both disconnect pins are driven

logic LOW.

Disable mode places the device in a sleep state, where quiescent

current is typically 375

µA. When disabled, all outputs are in a

high impedance state and each output has only 3pF of capaci-

tance.

An additional feature of the IMP5218 is its compatibility with

active negation drivers.

VV

R

I

REF

LINE

−

()

= .

Figure 3.

5218_03.eps

1 Meter, AWG 28

IMP5218

Receiver

Driver

IMP5218

Figure 1. Receiving Waveform – 20MHz

Figure 2. Driving Waveform – 20MHz

IMP5

IMP52