MMUN2211LT1G

The MMUN2211LT1G is an NPN bias resistor transistor (BRT) from onsemi’s digital transistor series, integrating a single NPN switching transistor with a monolithic bias resistor network consisting of R1 = 10kΩ (series base resistor) and R2 = 10kΩ (base-emitter shunt resistor). This pre-biased configuration eliminates the need for external bias resistors, reducing component count from three (transistor + two resistors) to one, simplifying PCB layout and saving board space. The 10kΩ/10kΩ resistor values (R1/R2 ratio = 1) are optimized for direct drive from 5V logic outputs, providing appropriate base drive current while the shunt resistor ensures fast turn-off. The device is rated for 50V VCEO, 100mA collector current, and 246mW power dissipation, suitable for general-purpose switching, signal interfacing, and driver applications.

Bias Resistor Network

The MMUN2211LT1G integrates two resistors with the NPN transistor. R1 (10kΩ) is connected in series between the external base input pin and the internal transistor base, limiting base drive current. R2 (10kΩ) is connected between the internal base and emitter, providing a discharge path for stored base charge during turn-off and ensuring the transistor remains off when the input is floating or driven low.

Switching Operation

When a logic HIGH voltage (e.g., 5V) is applied to the input pin, current flows through R1 into the transistor base, turning it on and saturating the collector-emitter junction. The base current is approximately (VIN – VBE) / R1. When the input goes LOW, R2 provides a discharge path for the base stored charge, ensuring fast turn-off. The R1/R2 ratio of 1 provides balanced on/off switching performance for 5V logic drive.

Input Voltage Considerations

With R1 = 10kΩ, a 5V input produces approximately 430µA base current (assuming VBE = 0.7V), sufficient to drive the transistor into saturation at collector currents up to 15-20mA (hFE min = 35). For 3.3V logic, base current is reduced to approximately 260µA, supporting lower collector currents. Designers must verify saturation at their specific operating conditions.

• GPIO signal level conversion and digital interfacing where integrated bias resistors eliminate external components
• Relay and LED driver circuits where the pre-biased transistor directly connects to microcontroller outputs
• Digital logic signal buffering and inversion in control systems with minimal component count
• Microcontroller output expansion for driving multiple loads from limited I/O pins
• General-purpose low-side switching where the single-component BRT replaces transistor + two resistors