Operation
Why the output is 5.1V
Section titled “Why the output is 5.1V”The 5.1V output is intentional. USB cables introduce resistance, commonly 0.1–0.5 Ω per meter. Under load, this resistance causes a voltage drop:
- Voltage drop = Current × Resistance
- At 3A through 0.1 Ω of total cable resistance, the drop is 0.3V.
Starting at 5.1V helps the connected device remain above the 4.75V USB minimum under load. In the example above, the device receives approximately 4.8V. A cable with 0.3 Ω of resistance would drop 0.9V at 3A and is not suitable for that load.
STATUS LED
Section titled “STATUS LED”The STATUS LED on top of the enclosure is driven by the TPS56A37 power-good output.
| LED state | Meaning | Action |
|---|---|---|
| 🟠 Solid orange | Normal operation; output is stable | None |
| ⚫ Off | Input or output fault | Follow the troubleshooting guide |
The LED turns off when:
- Input power is disconnected.
- Input polarity is reversed.
- Output voltage is outside its acceptable range, commonly because of an overload or short circuit.
- The regulator enters thermal shutdown when its junction temperature exceeds 165°C.
- Input voltage falls below the regulator’s undervoltage-lockout threshold.
The Power Good circuit includes deglitch timing to avoid false fault indications during short transients.
FRC brownouts
Section titled “FRC brownouts”During high-current maneuvers, a robot battery can sag from 12V to 6–7V for several seconds. Many inexpensive automotive and general-purpose 12V-to-5V converters specify minimum input voltages around 8–9V and are not designed to remain regulated through a deep FRC battery sag. A loss of regulated output can reboot a vision co-processor and disrupt NetworkTables communication.
The TPS56A37 buck converter operates with input voltages as low as 6V. Marigold-5 can therefore maintain its regulated output during the battery-voltage sags that commonly occur in an FRC match.