3 Description

The ISO721-Q1 and ISO722-Q1 devices are digital isolators with a logic input and output buffer separated by a silicon dioxide (SiO₂) insulation barrier. This barrier provides galvanic isolation of up to 4000 V_PK per VDE 0884-17. Used in conjunction with isolated power supplies, these devices prevent noise currents on a data bus or other circuits from entering the local ground, and interfering with or damaging sensitive circuitry.

A binary input signal is conditioned, translated to a balanced signal, then differentiated by the capacitive isolation barrier. Across the isolation barrier, a differential comparator receives the logic transition information, then sets or resets a flip-flop and the output circuit accordingly. A periodic update pulse is sent across the barrier to provide the proper dc level of the output.

If this dc-refresh pulse is not received for more than 4μs, the input is assumed to be unpowered or not being actively driven, and the failsafe circuit drives the output to a logic-high state.

These devices require two supply voltages of 3.3V, 5V, or any combination. All inputs are 5V tolerant when supplied from a 3.3V supply and all outputs are 4mA CMOS.

The ISO722-Q1 devices include an active-low output enable that when driven to a high logic level, places the output in a high-impedance state and turns off internal bias circuitry to conserve power.

The ISO721-Q1 and ISO722-Q1 devices have TTL input thresholds and a noise filter at the input that prevent transient pulses of up to 2 ns in duration from being passed to the output of the device.

The ISO721-Q1 and ISO722-Q1 devices are characterized for operation over the ambient temperature range of –40°C to +125°C.

Simplified Schematic