Robot Arm PCB

USB-C PD Powered ESP32 Servo Controller

This document consolidates the power architecture, wiring, BOM additions, and system diagram for an ESP32-based robot arm controller using:

• USB-C Power Delivery (PD) via STUSB4500QTR
• ESP32 MCU
• PCA9685 PWM servo controller
• CH340X USB-to-UART programmer
• Cheap / low-load servos (budget-friendly design)

1. System Overview

The board negotiates a higher-voltage USB-C PD contract, then generates two isolated rails:

• SERVO_6V – high-current rail for servos
• +3V3 – logic rail for ESP32, PCA9685, and STUSB4500

Key design goal: prevent ESP32 brownout caused by servo current spikes.

2. Power Domains

Rails

• VBUS_IN: USB-C VBUS (5–20V after PD negotiation)
• VSYS: Post power-path rail
• SERVO_6V: Buck-regulated servo rail
• +3V3: Logic rail

Critical Rules

• ESP32 must only see 3.3V
• Servo power must not be drawn from 3.3V
• Servo current must not share thin traces or return paths with ESP32 logic ground

3. Wiring Map (Net-Level)

USB-C → STUSB4500QTR (PD)

• USBC_CC1 (A5) -> STUSB_CC1
• USBC_CC2 (B5) -> STUSB_CC2
• USBC_VBUS -> VBUS_IN
• USBC_GND -> GND

USB-C → CH340X (USB2)

• USBC_D+ (A6/B6 tied) -> CH340_UD+
• USBC_D− (A7/B7 tied) -> CH340_UD−

CH340X → ESP32 (UART)

• CH340_TXD -> ESP32_U0RXD (GPIO3)
• CH340_RXD -> ESP32_U0TXD (GPIO1)
• CH340_DTR/RTS -> ESP32_EN / ESP32_IO0 (auto-program circuit)

ESP32 ↔ STUSB4500QTR (I2C + Status)

• ESP32_GPIO21 (SDA) <-> STUSB_SDA (pull-up to +3V3)
• ESP32_GPIO22 (SCL) <-> STUSB_SCL (pull-up to +3V3)
• STUSB_ALERT -> ESP32_GPIO (input, optional)
• ESP32_GPIO -> STUSB_RESET (optional; otherwise pull-up)

ESP32 ↔ PCA9685 (I2C)

• ESP32_GPIO21 (SDA) <-> PCA_SDA
• ESP32_GPIO22 (SCL) <-> PCA_SCL
• PCA_VDD -> +3V3
• PCA_V+ -> SERVO_6V
• PCA_GND -> GND

Power Chain

• VBUS_IN -> Power Path -> VSYS
• VSYS -> Buck (6V) -> SERVO_6V
• VSYS -> Buck/LDO -> +3V3

4. Layout Notes (Brownout Prevention)

• Route SERVO_6V with wide copper pours
• Return servo current directly to power entry ground (star-like topology)
• Place bulk capacitors close to servo headers
• Isolate logic rail with ferrite bead if needed

5. BOM Additions (Budget / Low-Load Servo Design)

5.1 Servo Rail Buck (VSYS → 6.0V)

Recommended IC

• MP1584EN-LF-Z – 3A buck regulator (cheap, common)

Typical Supporting Parts

• Inductor: 4.7µH, ≥3A shielded
• Input caps: 22–47µF MLCC
• Output caps: 2× 47µF MLCC + bulk electrolytic
• Feedback resistors: set to 6.0V

5.2 Logic Rail (VSYS → 3.3V)

Option A (quieter, recommended)

• Buck to 5V: MP1584EN
• LDO to 3.3V: AP2112K-3.3TRG1 (600mA)

Option B (fewer parts)

• Single MP1584EN configured for 3.3V

5.3 USB Protection

• USB D+/D− ESD: USBLC6-2SC6
• CC line ESD (optional): PESD5V0S1UL (one per CC)

5.4 Ferrite Bead (Logic Isolation)

• BLM21PG600SN1D (0603)

5.5 Bulk Capacitors

Servo Rail

• 470µF – 2200µF electrolytic or polymer (≥10V)

Logic Rail

• 22µF bulk
• 0.1µF decoupling near ESP32 VDD

5.6 I2C Pull-ups

• SDA: 4.7kΩ to +3V3
• SCL: 4.7kΩ to +3V3

5.7 Optional Servo Signal Conditioning

• 33–100Ω series resistor on each PCA9685 PWM output

6. System Wiring Diagram (Mermaid)

flowchart LR
  USBC["USB-C Receptacle P1"]
  PD["STUSB4500QTR U2"]
  PPATH["Power-Path / Protection<br/>(eFuse or Load Switch)"]
  CH340["CH340X U5"]
  ESP["ESP32 U1"]
  PCA["PCA9685 U3"]
  LDO33["3.3V Regulator<br/>(LDO or Buck)"]
  SERVOS["Servo Headers / Servos"]
  ESDUSB["USB ESD<br/>(D+/D−)"]
  ESDCC["CC ESD (opt)"]

  %% ---------------- USB2 PROGRAMMING ----------------
  USBC -->|"A6/B6 D+ → D+ IN"| ESDUSB
  USBC -->|"A7/B7 D− → D− IN"| ESDUSB
  ESDUSB -->|"D+ OUT → UD+"| CH340
  ESDUSB -->|"D− OUT → UD−"| CH340
  CH340 -->|"TXD → ESP GPIO3 U0RXD"| ESP
  CH340 -->|"RXD → ESP GPIO1 U0TXD"| ESP
  CH340 -->|"DTR/RTS → EN/IO0 auto-prog"| ESP

  %% ---------------- USB-C PD NEGOTIATION ----------------
  USBC -->|"A5 CC1 → CC1 IN"| ESDCC
  USBC -->|"B5 CC2 → CC2 IN"| ESDCC

7. Next Steps

Before PCB layout:

• Confirm servo count and stall current
• Decide PD voltage request (9V vs 12V)
• Lock in buck inductor + capacitor values per datasheet

Once confirmed, this design is PCB-ready for a low-cost, reliable USB-C powered robot arm controller.