FS401LF datasheet(98/104 Pages) ETC2

98 / 104 page

FS401, FS403

PRODUCT SPECIFICATION

REV. NO. 1.7

JANUARY 24, 2007

7.3 Interfacing to the FS400 in a Mixed Voltage Environment

As many devices designed today, the FS400 is powered by +3.3 Volts. However, 5 Volt devices are still

very common today and will continue to be used for some time in the future. To meet this interface

requirement the FS400 has 5 Volt tolerant inputs. Unfortunately, the FS400 does not have 5 Volt tolerant

outputs. When a bi-directional I/O pin is used on the FS400, care must be taken when driving the device.

7.3.1 5 to 3.3 Volt Translation

A series resistor is the simplest way to safely drive an I/O pin of the FS400 . This input resistor limits the

input current to under the 10mA maximum limit when the input is raised towards the 5 Volt supply.

270

FS400

+3.3V

5 Volt

Device

+5V

Figure 23. 5 to 3.3 Volt Translation using a Resistor

In applications where a series resistor limits the speed or the drive capability of the I/O, a pass gate can

be used instead of a resistor. The pass gate (Q1) provides a low impedance path between the devices

when the switching voltage is low and a high impedance path as the switching voltage approaches the

supply rail.

In the circuit below, D1 biases the gate of the MOSFET (Q1) down to 4.3 Volts so Q1 will turn off when its

input exceeds 3.3 Volts. This limits the high state input current, into the FS400, to under the 10mA

maximum value.

Note that the MOSFET source is connected to the lower supply device and the MOSFET drain is

connected to the higher supply device. This configuration reverse biases the MOSFET drain to bulk

diode in the high switching state.

FS400

+3.3V

5 Volt

Device

+5V

3.9K

Figure 24. 5 to 3.3 Volt Translation using a MOSFET

Q1 = BSS138, D1 = 1N4148

7.3.2 SIO Bus Interfacing

The SIO bus was developed previous to 3.3V logic processes. The SIO bus input voltage specification is

1.5 Volts for VIL and 3.0 Volts for VIH. The FS400 is built on a 3.3 Volt process and has 5 Volt tolerant

inputs with a VIL of 0.8 Volts and a VIH of 2.0 Volts.

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