Specifying a Digital Isolator's Default Output State

1. Define the Input-Loss Scenario

A digital isolator's normal signal transfer and its response to an input-loss condition are different requirements. Begin by identifying the condition the application needs to address. The product-description example concerns loss of input power or signal. Preserve that language rather than treating an ordinary logic state, a disconnected connection and an unspecified supply event as though they necessarily describe the same documented operating mode.

Identify which side loses the relevant input condition and what power context remains on the receiving side. A statement about the receiver's default output must stay connected to that situation. It should not be extended to a circuit with both sides unpowered. Where the application requires a different power-state combination, its behavior needs the applicable exact-device evidence instead of an assumption based on the word default.

Describe why the receiving circuit needs a particular disposition under that condition. The relevant question may be whether the receiver expects a high or low logical default, rather than the channel direction used during normal communication. Keep that requirement distinct from the rail-monitoring function of a voltage supervisor. An isolator's response to lost input is not a description of threshold detection and RESET release timing.

2. Retain the Suffix-Dependent Default

TI's ISO7721 product description states that, in the described input-power or signal-loss event, devices without suffix F default high and devices with suffix F default low. This is a functional distinction within the named family. Keep the suffix in the requirement and in any candidate comparison. Removing it from a shortened ordering description can remove the very choice the application is trying to preserve.

Compare the documented default with the receiver's required logical disposition under the same condition. High and low are not interchangeable labels, but neither is automatically the correct application choice. The engineer must relate the signal meaning to the receiving circuit. A product-summary default should not be promoted into a universally safe output merely because the variant offers the level requested in an incomplete sourcing description.

Keep the exact candidate's default evidence separate from its channel-direction allocation. Direction concerns which way a channel carries information; default concerns the documented output under the specified loss condition. A suitable direction arrangement does not decide the required default, and matching a default does not establish the required routes. Both distinctions need to survive when the proposed part is compared with the released device.

3. Keep Enable and Unread Modes Pending

The TI product description directs the reader to Device Functional Modes for further detail. Treat that reference as a boundary of this limited discussion. The summary supports the stated suffix-dependent input-loss distinction; it does not supply a complete enable truth table. Do not assign a default to every control combination, or invent an enable connection, from the short description alone.

Distinguish a default logic value from an output-driver specification. A high or low label does not, by itself, establish the complete output-interface requirement of another component. The supervisor guide separately considers output polarity and driver type for a RESET connection. Here, retain whatever exact-device output-interface evidence the receiver assessment needs without importing a supervisor's open-drain behavior into the isolator discussion.

Keep transitions into and out of the described input-loss situation separate from the steady condition being compared. If the application depends on those transitions, identify its relevant requirement for engineering review. This article supplies no delay, recovery sequence or waveform interpretation. A default-state selection should remain a precise functional decision, rather than an implied assessment of every event that might occur around a supply change.

4. Record Receiver Disposition

Bring the exact variant, the documented loss condition and the receiver's required logical disposition into one comparison. The useful engineering question is whether those descriptions refer to the same situation. If the application requirement concerns another power combination or an enable state not established by the available evidence, retain that difference explicitly rather than assigning the family-summary default to it.

Preserve the distinction between recognising the manufacturer's default option and accepting the receiver's resulting behavior. A logical high might be interpreted differently by different receiving functions, just as a low need not establish the application's desired response. The design owner must assess the actual receiving requirement. This discussion gives no functional-safety conclusion, verified application state or measured output for the customer circuit.

Keep default selection separate from isolation qualification as the requirement moves into sourcing. A suffix-dependent high or low description does not establish barrier certification, lifetime or common-mode performance. Identify any such requirements under their own evidence. The final comparison should make the input-loss output choice visible while leaving broader qualification and application approval with the responsible reviewer, without implying present stock or an approved substitute.

Sources and further reading

• Texas Instruments — ISO7721 Product Description

TI's product-description core supports the suffix-dependent default under its described input-loss situation. Device Functional Modes and the complete datasheet are outside this review; no physical connection, actual output result, certification or current supply is established.