AO3401A - P-Channel MOSFET (Load Switch)
P-Channel MOSFET for USB-PD power path control, used as load switch with STUSB4500.
Overview
The AO3401A is a P-Channel enhancement mode MOSFET used as a high-side load switch in the USB-PD power path. It is controlled by the STUSB4500's VBUS_EN_SNK pin to enable power to downstream circuits only after successful PD negotiation.
Key Specifications
| Parameter | Value | Notes |
|---|---|---|
| JLCPCB Part Number | C347476 | |
| Manufacturer Part Number | AO3401A | Alpha & Omega Semi |
| Package | SOT-23 | 3-pin surface-mount |
| Stock Availability | 1,094,498 | Excellent availability |
| Unit Price | ~$0.02 | Very low cost |
| Drain-Source Voltage | -30V | Suitable for 20V PD |
| Continuous Drain Current | -4A | Sufficient for 3A load |
| Rds(on) @ Vgs=-4.5V | 44mΩ | Low power loss |
| Gate Threshold Voltage | -0.5V to -1.2V | Logic-level compatible |
| Power Dissipation | 1.4W | SOT-23 package limit |
Pin Configuration
AO3401A (SOT-23)
Top View
┌───────────────┐
│ ┌─────┐ │
│ │ │ │
G │1 │ │ 3│ S
│ │ │ │
│ └─────┘ │
│ 2 │
└───────┬───────┘
D
Pin 1: Gate (G)
Pin 2: Drain (D)
Pin 3: Source (S)
Application in STUSB4500 Circuit
Load Switch Circuit:
VBUS_IN (15V from USB-C)
│
├───────────────────────────────┐
│ │
R1 (100kΩ) Source (pin 3)
│ │
├─────────── Gate (pin 1) ─── AO3401A
│ │
R2 (56kΩ) Drain (pin 2)
│ │
VBUS_EN_SNK ──┘ │
(from STUSB4500) │
│ ▼
C5 (100nF) VBUS_OUT
│ (to DC-DC)
GND
Operation:
- VBUS_EN_SNK = LOW: Gate pulled HIGH by R1 → MOSFET OFF
- VBUS_EN_SNK = HIGH: Gate pulled LOW through R2 → MOSFET ON
- C5 provides soft-start (slew rate control)
Component Values
| Reference | Value | Purpose |
|---|---|---|
| R1 | 100kΩ | Gate pull-up (default OFF) |
| R2 | 56kΩ | Gate voltage divider |
| C5 | 100nF | Soft-start |
Soft-Start Calculation
Time constant: τ = R2 × C5 = 56kΩ × 100nF = 5.6ms
This limits the dV/dt during turn-on, reducing inrush current.
Power Dissipation
At maximum load (3A, 15V):
- Conduction loss: P = I² × Rds(on) = 3² × 0.044 = 0.4W
- Well within SOT-23 thermal limit (1.4W)
Why P-Channel for High-Side Switch?
| Switch Type | Position | Gate Drive | Complexity |
|---|---|---|---|
| P-Channel | High-side | Simple (ref to VBUS) | Low |
| N-Channel | High-side | Needs charge pump | High |
| N-Channel | Low-side | Simple | Breaks ground path |
P-Channel is ideal for high-side switching with simple gate drive.