Open-Drain Outputs: Understanding the PG Pin
How the CH224D's Power Good (PG) pin works and why the LED circuit is connected the way it is.
The Question
When designing the LED status indicator for the USB-PD circuit, I needed to understand:
"The PG pin connects to GND when power is stable at 15V? So the circuit becomes: +5V → LED → GND, and the LED lights up?"
The answer is YES ✅, and here's why.
What is an Open-Drain Output?
The CH224D's PG (Power Good) pin is an open-drain output. This is a common digital output type that works differently from regular logic outputs.
Regular Logic Output (Push-Pull)
A typical digital output can drive both HIGH and LOW:
Regular Output Pin:
├─ HIGH: Connects to VCC (e.g., 3.3V)
└─ LOW: Connects to GND (0V)
Open-Drain Output
An open-drain output can only pull LOW or float:
Open-Drain Output Pin:
├─ Active: Connects to GND (pulls LOW)
└─ Inactive: High-impedance (floating, not connected to anything)
Think of it as a switch to ground:
- Switch CLOSED → Pin = GND
- Switch OPEN → Pin = floating (not connected)
How the PG Pin Works
The CH224D uses the PG pin to indicate when USB-PD negotiation is successful and 15V is stable.
Internal Circuit Diagram
Inside CH224D:
┌─────────────┐
│ Control │
│ Logic │
└──────┬──────┘
│
┌──────▼──────┐
PG pin ────────────┤ Transistor ├──→ GND (inside chip)
(to LED cathode) │ (N-channel) │
└──────────────┘
When "power good":
→ Control logic turns transistor ON
→ PG pin connects to GND
Two States
State 1: Power NOT Good (negotiating or failed)
PG Pin = HIGH-Z (floating)
+5V ──[330Ω]──[LED]──→ PG pin (floating)
↑
No path to GND
→ No current flow
→ LED OFF ❌
State 2: Power Good (15V stable)
PG Pin = LOW (connected to GND)
+5V ──[330Ω]──[LED]──→ PG pin (= GND)
↓
GND
Complete circuit: +5V → Resistor → LED → GND
→ Current flows (~8mA)
→ LED ON ✅ (Green, indicates safe power)
Why Connect LED This Way?
The LED circuit in our design:
+5V Rail ──[R1: 330Ω]──[LED Anode]──[LED Cathode]──→ PG pin
Why LED Anode to +5V?
Because the PG pin can only sink current (pull to GND), not source current (provide voltage). The LED needs a voltage source (+5V) on one side and a current sink (PG → GND) on the other.
Why Not Connect Directly to 15V?
We could connect to 15V, but:
If using 15V:
I_LED = (15V - 2.2V) / 330Ω = 38.8mA ❌ (Too high! Max is 20mA)
If using +5V:
I_LED = (5V - 2.2V) / 330Ω = 8.5mA ✅ (Safe and bright)
The +5V rail provides the correct voltage for safe LED current with a standard 330Ω resistor.
Circuit Behavior Timeline
When you plug in a USB-C PD charger:
Time 0ms:
- USB-C connected, VBUS starts at 5V
- PG pin = HIGH-Z (floating)
- LED OFF (green)
- Status: "Negotiating..."
Time 0-500ms:
- CH224D negotiates 15V via CC pins
- PG pin = HIGH-Z (still floating)
- LED OFF (green)
- Status: "Negotiating..."
Time 500ms-1000ms:
- VBUS transitions 5V → 15V
- CH224D verifies voltage is stable
- PG pin = HIGH-Z (still floating)
- LED OFF (green)
- Status: "Verifying..."
Time >1000ms:
- 15V confirmed stable
- PG pin = LOW (pulls to GND) ✅
- LED ON (green) ✅
- Status: "Power Good - 15V Ready"
Key Takeaways
- Open-drain outputs can only pull LOW or float (they cannot drive HIGH)
- PG pin acts like a switch to GND that closes when power is good
- LED must be powered from +5V (not from the PG pin itself)
- When PG pulls LOW, it completes the circuit: +5V → LED → GND
- Green LED ON = Power is safe (15V negotiated successfully)
Common Mistake to Avoid
❌ WRONG - Trying to power LED from PG pin:
PG pin ──[Resistor]──[LED]──→ GND
Problem: PG pin cannot source voltage!
When PG = LOW, both sides of LED are at GND → No current → LED OFF
✅ CORRECT - Power LED from voltage rail:
+5V ──[Resistor]──[LED]──→ PG pin
When PG = LOW: +5V → LED → GND → Current flows → LED ON ✅
Related Circuits
This open-drain concept is used in many digital ICs:
- I²C bus (SDA and SCL are open-drain)
- 1-Wire protocol (Dallas/Maxim sensors)
- Interrupt outputs (many sensor ICs)
- Status indicators (like our PG pin)
Understanding open-drain outputs is essential for reading datasheets and designing digital circuits correctly!
See Also
- CH224D Documentation - Full CH224D specifications
- Circuit Diagrams - Diagram1 - Complete USB-PD circuit