ISL41334IRZ datasheet(19/28 Pages) INTERSIL

19 / 28 page

FN6202.3

November 21, 2007

RS-485 Slew Rate Limited Data Rates

The SOIC and SSOP versions of this IC operate with Tx

output transitions optimized for a 20Mbps data rate. These

fast edges may increase EMI and reflection issues, even

though fast transitions aren’t required at the lower data rates

used by many applications. The ISL41334 (QFN version)

solves this problem by offering two additional, slew rate

limited, data rates that are optimized for speeds of 115kbps,

and 460kbps.The slew limited edges permit longer

unterminated networks, or longer stubs off terminated

busses, and help minimize EMI and reflections.

Nevertheless, for the best jitter performance when driving

long cables, the faster speed options may be preferable,

even at lower data rates. The faster output transitions deliver

less variability (jitter) when loaded with the large capacitance

associated with long cables. Figures 43, 44, and 45 detail

the jitter performance of the three speed options while

driving three different cable lengths. The figures show that

under all conditions the faster the edge rate, the better the

jitter performance. Of course, faster transitions require more

attention to ensuring short stub lengths, and quality

terminations, so there are trade-offs to be made. Assuming a

jitter budget of 10%, it is likely better to go with the slow

speed option for data rates of 115kbps or less, to minimize

fast edge effects. Likewise, the medium speed option is a

good choice for data rates between 115kbps and 460kbps.

For higher data rates, or when the absolute best jitter is

required, use the high speed option.

Speed selection is via the SPA and SPB pins (see Table 3),

and the selection pertains to each port programmed for

RS-485 mode.

Evaluation Board

An evaluation board, part number ISL41334EVAL1, is

available to assist in assessing the dual protocol IC’s

performance. The evaluation board contains a QFN

packaged device, but because the same die is used in all

packages, the board is also useful for evaluating the

functionality of the other versions. The board’s design allows

for evaluation of all standard features, plus the QFN specific

features. Refer to the eval board application note for details,

and contact your sales rep for ordering information.

Typical Performance Curves VCC = VL = 5V, TA = +25°C; Unless Otherwise Specified.

FIGURE 14. RECEIVER OUTPUT CURRENT vs RECEIVER

OUTPUT VOLTAGE

FIGURE 15. RECEIVER HIGH OUTPUT VOLTAGE vs LOGIC

SUPPLY VOLTAGE (VL)

FIGURE 16. RS-485, DRIVER OUTPUT CURRENT vs

DIFFERENTIAL OUTPUT VOLTAGE

FIGURE 17. RS-485, DRIVER DIFFERENTIAL OUTPUT

VOLTAGE vs TEMPERATURE

0123

RECEIVER OUTPUT VOLTAGE (V)

VOH, +25°C

VOH, +85°C

VOL, +25°C

VOL, +85°C

VL (V)

0123

IOH = -1mA

IOH = -4mA

IOH = -8mA

DIFFERENTIAL OUTPUT VOLTAGE (V)

100

-40

TEMPERATURE (°C)

-25

RDIFF = 54Ω

RDIFF = 100Ω

3.0

3.1

3.2

3.3

3.4

3.5

3.6

ISL81334, ISL41334

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