AO3401A - P-Channel MOSFET (Load Switch)

P-Channel MOSFET for USB-PD power path control, used as load switch with STUSB4500.

View on JLCPCB: C347476

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.