Infineon BSS138IXTSA1: Key Specifications and Application Circuit Design Guide

The Infineon BSS138IXTSA1 is a popular N-channel logic-level enhancement mode MOSFET housed in a compact SOT-23-3 package. Renowned for its low threshold voltage and high efficiency, it is a cornerstone component in modern low-power electronic design, particularly in portable devices and interface applications.

This MOSFET is designed to be driven directly from logic-level signals (as low as 1.8V), making it an ideal choice for interfacing between microcontrollers and higher-voltage circuits. Its key specifications define its operational boundaries and strengths:

Drain-Source Voltage (V_DS): 50 V. This rating allows it to comfortably handle voltages found in 12V and 24V systems, providing a good safety margin.

Continuous Drain Current (I_D): 220 mA. Sufficient for driving small motors, LEDs, relays, and other peripheral components.

On-Resistance (R_DS(on)): 3.5 Ω (max) at V_GS = 4.5V. A low on-resistance is critical for minimizing power loss and voltage drop across the MOSFET when it is switched on, ensuring efficient operation.

Gate Threshold Voltage (V_GS(th)): 0.5V - 1.5V. This low threshold is what qualifies it as a logic-level device, ensuring full enhancement with GPIO pins from 3.3V or 5V microcontrollers.

Application Circuit Design Guide

A primary application for the BSS138IXTSA1 is as a low-side switch. This configuration is simple, effective, and widely used.

1. Basic Low-Side Switch Circuit:

The core circuit involves connecting the load (e.g., an LED, a small motor, or a relay coil) between the positive supply rail (V_DD, e.g., 12V) and the drain pin of the MOSFET. The source pin is connected directly to ground. The microcontroller GPIO pin is connected to the gate pin through a series resistor (R_G).

Gate Resistor (R_G): A resistor value between 100Ω and 1kΩ is typically used. It serves two main purposes: it dampens high-frequency oscillations that can occur due to the MOSFET's inherent gate capacitance and inductance in the driving circuit, and it limits inrush current into the gate during fast switching transitions.

Pull-Down Resistor (R_GS): A high-value resistor (e.g., 10kΩ to 100kΩ) connected between the gate and source (ground) is essential. It ensures the gate is pulled to a known low state (0V) when the microcontroller pin is in a high-impedance state (e.g., during boot-up or reset), preventing false triggering and keeping the MOSFET firmly off.

2. Driving Inductive Loads:

When driving inductive loads like relay coils or motors, a flyback diode is absolutely critical. The inductor will generate a high-voltage reverse spike when the current is suddenly switched off (by turning the MOSFET off). This spike can easily exceed the V_DS rating and destroy the MOSFET.

Protection Diode: A Schottky diode is connected in reverse bias across the inductive load (anode to drain, cathode to V_DD). This diode provides a safe path for the inductive kickback current to dissipate, protecting the MOSFET from voltage transients.

Design Considerations:

Gate Driving: While the BSS138 can be driven by 3.3V logic, for the absolute lowest R_DS(on) and minimal power loss, driving the gate with 4.5V or 5V is preferable.

Power Dissipation: Ensure the power dissipated (I_D² R_DS(on)) does not exceed the package limits, considering ambient temperature and any heatsinking (which is minimal for a SOT-23 package).

PCB Layout: Keep gate drive traces short and direct to minimize parasitic inductance.

ICGOOODFIND: The Infineon BSS138IXTSA1 is an exceptionally versatile and robust logic-level MOSFET. Its combination of a low threshold voltage, respectable current handling, and a 50V drain rating makes it a default choice for designers for power switching, level shifting, and peripheral driving in space-constrained, low-power applications. Proper circuit design, including gate resistors and protection diodes for inductive loads, is key to leveraging its full potential and ensuring long-term reliability.

Keywords: Logic-Level MOSFET, Low-Side Switch, Application Circuit, RDS(on), Inductive Load Protection