Open-Drain Outputs: Understanding the PG Pin

Historical reference

This page is retained from the previous design documentation. Parts, geometry, status and numerical claims may be superseded. Start with thecurrent architecture and verify exact components in thecatalog. See the archive guide for context.

How the CH224D's Power Good (PG) pin works and why the LED circuit is connected the way it is.

Historical: CH224D was replaced by the STUSB4500 (v1.1)

This page is written around the CH224D, the deprecated v1.0 USB-PD controller (see CH224D USB-PD Controller). The current controller, STUSB4500, has no dedicated PG pin — it exposesATT (ATTACH) and PDOK (POWER_OK2) as open-drain flags instead, wired the same way electrically (pull to GND when asserted, no on-board pull-up). SeeBoard A: USB-PD Corefor the current pin map. The open-drain concept taught below is still correct and still applies — only the specific pin name and IC have changed.

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 ✅

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!