FS401LF datasheet(100/104 Pages) ETC2

100 / 104 page

FS401, FS403

PRODUCT SPECIFICATION

REV. NO. 1.7

100

JANUARY 24, 2007

For applications with more than one supply, combinations of the above two circuits can be used.

However, the simplest approach to this problem is to limit the loading on the SIO bus when possible.

When this is not possible, some of the SIO passive loads can be replaced with active ones. This will

increase the SIO access speed without increasing the SIO output low drive current.

In lightly loaded SIO busses (where VOL is easily reached), a single MOSFET can be used for translation

on the SIO SDA line (Since the FS400’s input for SCL is 5 Volt tolerant, no additional parts are required

on that input). Pull-ups on both supply sides assure that the output on each device can reach its

corresponding upper limit. The pass gate (Q1) provides a low impedance connection during the low

switching voltage.

5 Volt Device

OC Output

+5V

FS400

OC Output

+3.3V

10K

Figure 27. SIO (Open Collector) Translation using a MOSFET

Q1 = BSS138

Q1 also provides isolation when the 3.3 Volt power to the FS400 is removed. The pass gate (Q1) gate to

source voltage is negative and the bulk diode is off. This effectively removes the FS400’s Serial Data line

from the SIO bus system and allows the higher voltage SIO section to operate normally.

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