Checking a Power-Good Pull-Up Across Power Domains

A high level on a power-good wire is useful only when the circuit driving that wire can establish what the level means. Pulling an open-drain output to an always-on supply can reveal an unpowered interval in which the signal looks valid even though the monitored rail is absent.

Read the unpowered behavior

TI’s power-good discussion describes this issue for TPS53315 and TPS53319 circuits with an external pull-up supply present before regulator bias. Its examples are device-specific; other regulators can provide different guarantees for an unpowered power-good output.

Find the selected part’s power-good pin voltage limits, leakage behavior and conditions for a guaranteed low state. Record the minimum supply conditions under which its internal status logic is valid. Do not infer these conditions from the normal-operation timing diagram alone.

Map three domains

Identify the monitored rail, the regulator’s bias rail and the receiving logic’s supply. Add the pull-up source as a separate entry when necessary. For each startup and shutdown order, mark which domains are energized and what voltage the receiver can observe.

A useful state table includes external supply insertion, regulator disabled, rail rising, valid regulation, input removal and residual output charge. Include a programming cable or backup supply that can keep the receiving controller alive after the monitored regulator loses power.

Check electrical and logical compatibility

Calculate pull-up current in the asserted-low state and confirm the receiver’s input thresholds. Check whether the receiving pin tolerates voltage while its own supply is off. Account for any level translator, filter or wired combination with other status outputs.

Then define the logic rule that authorizes downstream operation. A status wire that is high before its source is valid must not accidentally enable a load or release reset. The appropriate gating or pull-up arrangement depends on the actual documented behavior of the selected parts.

Capture the ambiguous interval

Measure the pull-up supply, regulator bias, monitored output and status signal together. Trigger on supply insertion and removal, using enough record length to reveal slow discharge. Confirm the receiver’s interpretation at the moment it begins executing or accepting an enable.

Save the power-state table and annotated traces with the schematic. Identify which high states represent verified regulation and which merely reflect an undriven wire. This makes a later pull-up change reviewable and prevents a seemingly harmless resistor move from changing the product’s sequencing behavior. Include a slow supply ramp when that sequence can occur in the intended installation.