To calculate the maximum practical frequency component, or the knee frequency (fknee), use the following

equations:

fknee = 0.5/RT (10–80%)

(2)

fknee = 0.4/RT (20–80%)

(3)

For signals with rise time characteristics based on 10- to 90-percent thresholds, fknee is equal to 0.5 divided by

the rise time of the signal. For signals with rise time characteristics based on 20% to 80% thresholds, which is

very common in many of today's device specifications, fknee is equal to 0.4 divided by the rise time of the signal.

Some guidelines to follow that will help maximize the performance of the device:

• Keep trace length to a minimum by placing the LSF family close to the I2C output of the processor.

• The trace length should be less than half the time of flight to reduce ringing and line reflections or non-

monotonic behavior in the switching region.

• To reduce overshoots, a pullup resistor can be added on the 1.8 V side; be aware that a slower fall time is to

be expected.

10.2.1.3 Application Curve

Input

Output

Time (ns)

±1

1

2

4

0

0

50

100

150

200

250

350

400

500

450

3

300

Figure 10-3. Captured Waveform From Above I2C Set-Up (1.8 V to 3.3 V at 2.5 MHz)

LSF0204, LSF0204D

SLVSCP5H – JULY 2014 – REVISED APRIL 2021

www.ti.com

16

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