FS401LF datasheet(99/104 Pages) ETC2

99 / 104 page

FS401, FS403

PRODUCT SPECIFICATION

REV. NO. 1.7

JANUARY 24, 2007

For most applications this voltage difference is not an issue as the output drive low specification (VOL) of

the SIO bus and the FS400 are both 0.4 Volts. However, in heavily loaded SIO busses the output VOL is

not always preserved.

An easy way to regain the 0.7 Volt difference in the VIL specification of the FS400 and the SIO bus is to

bias the FS400’s input negative by a diode drop (D1). The diode can be biased by a long-tail resistor pair

or a current source pair. Shown below is the long-tail pair:

+12V

-12V

22K

27K

I2C

FS400

Figure 25. SIO Translation Using Long-tail Resistors

D1 = 1N4148

The long-tail pair is a simple circuit but has the disadvantage of requiring higher voltage power supplies.

Also, these supplies may have to power up in a specific sequence so the surround circuits are not over-

voltage.

The translation circuit below requires only one 5 Volt power supply and has no special sequence

requirements. In addition, the circuit offers a high impedance load (Q1 become reverse biased) to the

SIO bus when its power supply is removed. Unfortunately, it requires more parts. In applications where

transistors are more readily available, R2 and R3 can be replaced with diode connected transistors.

+5V

2.9K

1.3K

1.2K

I2C

FS400

Figure 26. SIO Translation Using Current Mirrors

D1 = 1N4148, Q1 = 2N3906, Q2 = 2N3904

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