USB-C Temperature and Humidity Sensor Node

E

enjoychul

"Design a robust, industrial-grade power supply circuit. Input is DC 24V with a PTC resettable fuse and a reverse polarity protection diode. Step down to 5V (3A max) using the LM2596S-5.0 with high-quality input/output capacitors and a shielded inductor. Then, add a high-reliability secondary LDO regulator to convert 5V into a clean, ripple-free 3.3V for an MCU."

flux

I’ll harden the existing 24V→5V→3.3V power section: inspect the current LM2596/LDO wiring, verify datasheet-critical passives, update sizing/properties, and run power/ERC checks.

flux

I reviewed and annotated the existing industrial power section against the LM2596S-5.0 and TLV1117LV33 datasheets.
Completed:
  • Confirmed existing parts: 24V screw terminal, SMBJ33A TVS, LM2596S-5.0, 33µH shielded inductor, SS34 Schottky, and TLV1117LV33 LDO.
  • Updated power-part properties and net metadata for current, temperature, thermal, and layout constraints.
  • Updated the Design Notes with the full power-section review and required fixes.
Important findings:
  • The existing 33µH inductor and SS34 diode are marginal for a robust 24V/3A industrial LM2596 design.
  • The schematic still needs explicit PTC fuse, reverse-polarity protection diode or ideal-diode MOSFET stage, and real LM2596 VIN/5V bulk capacitors.
  • The 3.3V LDO is electrically valid, but thermally risky above a few hundred mA continuous because it dissipates (5V − 3.3V) × IOUT. For a heavy RP2350B/W5500/USB-hub system, a 3.3V buck or buck-plus-LDO architecture is safer.
Fix power stage
Use buck for 3.3V
Run power review
Choose LCSC parts
Create power schematic
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Schematic

Layout

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Design Notes — RP2350B Industrial Ethernet Controller
Architecture status
This project is a wired-only controller/data-acquisition board based on RP2350B and W5500. No Wi-Fi/Bluetooth circuitry is included or planned.
Added in prior pass
  • U16 24AA025E48T-I/SN I2C identity EEPROM for pre-programmed EUI-48 MAC identity.
  • U17 DS3231 RTC on I2C for timestamping after power recovery.
  • J15 microSD socket on SPI for local fail-safe buffering.
  • J16 USB-C receptacle for native USB debug/flashing.
  • 14 visual status LEDs were added as LED1–LED15 range with one existing skipped designator reassignment.
  • Four M3 mounting holes were added as H1, U18, U19, U20 due automatic designator assignment.
Power supply review — 2026-06-06
Requested architecture: 24V DC input with PTC resettable fuse and reverse-polarity protection diode, LM2596S-5.0 buck to 5V/3A, then clean 3.3V LDO rail for MCU/W5500/logic.
What exists now
  • J2: 5.08mm 24V screw terminal, -40°C to +105°C, 24A rating.
  • D1: SMBJ33A TVS on 24V input for surge/ESD clamping.
  • U3: LM2596S-5.0/NOPB primary buck regulator.
  • L1: 33µH shielded inductor, 3A rating.
  • D6: SS34 Schottky catch diode, 3A/40V.
  • U4: TLV1117LV33DCYR 3.3V LDO, -40°C to +125°C, SOT-223.
  • C21/C22: 1µF ceramic output capacitors on the 3.3V rail.
Datasheet-grounded findings
  • LM2596 supports 4.5–40V operation and 3A load, with 5V output specified 4.75–5.25V across -40°C to +125°C when designed correctly.
  • TI LM2596 guidance for 5V/3A with 40V-class input margin points to a 47µH inductor and ~270–330µF low-ESR output capacitor; for 5V/2A/40V it points to 68µH. The present 33µH inductor is not the best 24V/industrial-surge-margin choice.
  • LM2596 catch diode must be at least 1.3× load current and reverse voltage at least 1.25× VIN_MAX. For 3A output, use a 5A, >=40V Schottky for robust short-circuit/thermal margin. Present SS34 is marginal.
  • LM2596 input capacitor must be a low-ESR aluminum/tantalum capacitor close to VIN/GND with RMS ripple current about 0.5× load current. For 3A, design for at least 1.5A RMS ripple; 50V rating is appropriate for 24V industrial input.
  • TLV1117LV33 requires >=1µF effective ceramic output capacitance and recommends 0.1–1µF input capacitance close to VIN/GND. It is stable with ceramic caps.
  • TLV1117LV33 thermal dissipation is Pd=(5V-3.3V)*Iout=1.7W/A. At 500mA it dissipates 0.85W; at 1A it dissipates 1.7W. This requires large copper/heatsinking and may be too hot in an 85°C ambient enclosure. If the 3.3V rail exceeds roughly 200–300mA continuous, consider replacing the LDO with a low-noise buck or buck-plus-LDO architecture.
Required fixes before layout/manufacture
  1. Add explicit PTC resettable fuse in series with J2 24V input. Size hold current from load budget with hot-temperature derating.
  2. Add explicit reverse-polarity protection diode or, preferably, a P-channel/eFuse ideal-diode front end to avoid high dissipation at 24V/3A input power.
  3. Add a real LM2596 input bulk capacitor on U3 VIN to GND: 470–680µF, >=50V, 105°C low-ESR aluminum electrolytic, with ripple-current rating >=1.5A.
  4. Add a real LM2596 output bulk capacitor on 5V to GND close to L1/U3/D6: 270–330µF, >=10–35V, 105°C low-ESR aluminum electrolytic depending on selected part and ripple spec.
  5. Replace L1 with a 47µH shielded inductor for 5V/3A/40V-class margin, saturation current >= LM2596 peak current limit margin.
  6. Replace D6 SS34 with a 5A, >=40V Schottky catch diode for full 3A/short-circuit robustness.
  7. Add/verify U4 input capacitor: 0.1–1µF ceramic directly from 5V to GND at U4 VIN.
  8. Decide whether U4 LDO thermal budget is acceptable. If the RP2350B/W5500/USB hub/logic rail can approach >300mA continuous, use a 5V-to-3.3V buck regulator and optionally post-filter/LDO only sensitive analog domains.
Important open issues before production
  1. USB architecture conflict: RP2350B has one native USB interface; the existing design already routes it to U6 USB2512B hub upstream. J16 USB-C has been tied to the same USB_UP_DP/USB_UP_DM nets, which is not a production-safe simultaneous direct-debug plus hub-upstream architecture. Use a USB 2.0 mux, choose either USB-C direct device mode or hub upstream, or redesign around a supported host/device strategy.
  2. USB-C CC1/CC2 require independent 5.1k pull-downs to GND for sink/device attach. Generic resistor insertion failed during this pass, so this remains open.
  3. The visual LED bank lacks individual current-limit resistors because generic resistor insertion failed. Do not power LEDs directly from GPIO/rails.
  4. DI3 optocoupler insertion failed due library duplicate lookup failure. Add a third EL817-compatible industrial optocoupler and wire it with FIELD_GND isolation.
  5. Strict isolation is not yet fully enforced. Existing isolated I/O must be refactored so field-side optocoupler inputs/relay contacts use FIELD_GND/24V_FIELD, while MCU logic stays on GND. Do not tie FIELD_GND and GND unless through the intended isolation/safety network.
  6. The selected microSD connector is the closest LCSC socket found, but the search result indicated -25°C to +85°C rather than full -40°C compliance. Substitute an industrial -40°C microSD connector before military/industrial release.
  7. Blue LED candidate requires temperature verification/substitution for full -40°C lower limit.
  8. ERC currently reports many floating-pin warnings; review and remediate before layout.
Power budget estimate
  • 24V input feeds LM2596S-5.0 5V/3A primary rail.
  • 5V rail loads: USB hub downstream ports dominate. If two USB-A ports can source 500mA each, reserve at least 1.0A plus hub losses.
  • 3.3V rail loads: RP2350B, W5500, QSPI flash, USB2512B 3.3V domains, RS485 transceiver, EEPROM, RTC, microSD, LEDs. Budget at least 700mA peak before final datasheet-current summing.
  • LDO thermal check is mandatory: Pd=(5V-3.3V)*Iout. At 700mA this is 1.19W, high for SOT-223 unless copper area and airflow are adequate; use a buck for high continuous load.
Net additions
  • I2C_SDA: U1 GPIO0 to U16 SDA and U17 SDA.
  • I2C_SCL: U1 GPIO1 to U16 SCL and U17 SCL.
  • SD_CS_N: U1 GPIO20 to J15 CD/DAT3.
  • SPI1_SCK/SPI1_MOSI reused for microSD CLK/CMD.
  • SPI1_MISO_SD created for J15 DAT0; connect it to a selected RP2350B GPIO before firmware completion.

  • Architecture status

  • Added in prior pass

  • Power supply review — 2026-06-06

  • What exists now

  • Datasheet-grounded findings

  • Required fixes before layout/manufacture

  • Important open issues before production

  • Power budget estimate

  • Net additions