Debug an Imported Altium Board From the First-Spin Symptom

Flux can diagnose a first-spin fault from the symptom on an imported Altium design, correct the circuit, and make the BOM match. On a LimeSDR-USB 1v4 test import, it traced a nominal 3.3 V rail reading about 6 V to a feedback resistor and corrected the divider to predict 3.31 V.

Import the design. Give Flux the symptom.

We used the public LimeSDR-USB 1v4 Altium project: a 12-layer SDR board with 858 parts. Before import, we deliberately changed one regulator feedback resistor, R180, from 10 kΩ to 20 kΩ. This fault was planted for the test. It was not a fault in the shipped LimeSDR design.

After import, we gave Flux the bring-up symptom without identifying the component or the cause. Use a prompt like this:

With a current-limited 12 V supply, the nominal 3.3 V regulator output measures about 6.0 V with the downstream load disconnected. Diagnose the cause from this design, check the vendor datasheet, and fix it. Explain the predicted voltage before and after the change. Verify the changed component and connectivity.

The symptom describes the test scenario. Flux had to find the relevant regulator and its feedback network in the imported design.

1. Diagnose. Flux identified IC27, an ADP2386 regulator, and traced its feedback divider through R180 and R184. It obtained and read the ADP2386 datasheet, then calculated the output from the saved resistor values.
2. Fix the value. The 20 kΩ top resistor predicted 6.03 V. Flux changed R180 to 10 kΩ, which predicted 3.31 V with the existing 2.21 kΩ bottom resistor.
3. Make the BOM match. The first change saved the correct resistance, but the imported part name still said 20 kΩ. We sent one short follow-up to correct the purchasing identity.

The follow-up was direct:

The BOM still says 20k. Please fix it so the build is right.

Flux replaced the incorrectly named instance with the 10 kΩ part and verified the saved BOM. The sequence was diagnose, fix the value, then make the BOM match. It took the symptom prompt and one follow-up.

Why one resistor explains the voltage

The ADP2386 datasheet, Rev. C, page 15, gives the output-voltage equation and the recommended divider values. The regulator holds its feedback input at a nominal 0.6 V:

VOUT = 0.6 × (1 + RTOP / RBOTTOM)

• Before: 0.6 × (1 + 20 / 2.21) = 6.03 V.
• After: 0.6 × (1 + 10 / 2.21) = 3.31 V.

Table 6 specifies 10 kΩ and 2.21 kΩ for a 3.3 V output. Flux used that datasheet evidence to explain both the symptom and the correction, rather than selecting a resistor from the rail name alone.

Check the saved circuit and the BOM

A corrected value and a purchasing record must agree. We checked both after the follow-up:

• Resistance: R180 is saved as 10,000 Ω, ±1%.
• BOM: the line names a 10 kΩ 0402 resistor, Yageo RC0402FR-0710KL.
• Connectivity: R180 still connects VCC3P3 to the IC27 feedback net. The feedback net still connects IC27:FB, R180, and R184; R184 still connects to ground.

Start with the evidence you have

Bring the original design and the symptom into the same project. State the supply conditions, measured voltage, and load state. Ask Flux to explain the cause, make the targeted change, and verify the saved value, purchasing identity, and connections.

For a circuit-response question, see SPICE simulation on imported Altium and Cadence boards. That workflow starts with an existing circuit and produces DC, frequency-response, and transient results.

Source design: LimeSDR-USB by MyriadRF, licensed under CC BY 3.0. The test modification was the R180 value change described above.

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