UA78M15HC

The UA78M15HC is a member of Texas Instruments’ 78Mxx family of fixed-output three-terminal positive voltage regulators. Designed as a medium-current variant between the 1.5A 78xx family and the 100mA 78Lxx family, the 78Mxx series delivers up to 500mA of output current in a compact package, making it ideal for applications requiring moderate current capability in a space-constrained design.

The UA78M15HC provides a fixed 15V output with tight voltage regulation of ±2% across the full operating temperature range of 0°C to +125°C (H-grade). The device features excellent line regulation of 11mV typical across the full 17.5V to 30V input range, and load regulation of 12mV typical from 5mA to 500mA output current.

A key feature of the 78Mxx family is its robust protection circuitry. The device includes internal thermal shutdown that activates when the junction temperature exceeds approximately 125°C, preventing damage from overheating. Current limiting protects the regulator and load from short-circuit conditions, with the current limit set at approximately 550mA typical. Output safe-area compensation ensures the power transistor operates within its safe operating area under all load conditions.

The UA78M15HC is packaged in the TO-252 (DPAK) surface-mount package (indicated by the ‘C’ suffix), which provides a good balance of thermal performance and board area. The package includes a tab for heatsinking to the PCB ground plane. Only two external capacitors are typically required: an input capacitor for stability and an output capacitor for transient response, making the UA78M15HC a simple and cost-effective solution for 15V regulation needs.

The UA78M15HC is a classic series-pass linear regulator that operates as follows:

1. Reference Voltage: A precision bandgap reference generates a temperature-stable reference voltage of approximately 1.25V. This reference is the foundation for the output voltage accuracy and temperature stability.

2. Error Amplifier: The error amplifier compares a fraction of the output voltage (sampled by an internal resistive voltage divider) against the bandgap reference. The amplifier’s output drives the series pass transistor to adjust the output voltage, maintaining regulation.

3. Series Pass Transistor: An NPN bipolar transistor serves as the series pass element. The transistor operates in its linear (active) region, varying its collector-emitter voltage to dissipate excess power between input and output. The dropout voltage (VIN – VOUT minimum for regulation) is approximately 2V at full load current.

4. Current Limiting: A current sense resistor in series with the pass transistor monitors output current. When the current exceeds the limit threshold (nominally 550mA), the current limit circuit reduces the base drive to the pass transistor, limiting the output current to a safe value.

5. Thermal Shutdown: An on-chip temperature sensor monitors the junction temperature. If Tj exceeds approximately 125°C, the thermal shutdown circuit disables the regulator, allowing the device to cool down. Hysteresis prevents oscillation near the threshold.

6. Safe-Area Compensation: This circuit monitors both the output current and the voltage across the pass transistor, reducing the current limit as the voltage drop increases. This ensures the pass transistor always operates within its safe operating area (SOA), preventing second-breakdown failure.

7. Startup and Stability: On power-up, the output voltage rises smoothly to its regulated value. The regulator is designed to be stable with a wide range of output capacitor values and ESR characteristics, simplifying application design.

Package: TO-252-3 (DPAK / DDPAK). Viewed from top (pin side): Pin 1 = VIN (left), Pin 2 = GND (center tab), Pin 3 = VOUT (right). The large center tab provides both ground connection and thermal dissipation path.

• Post-Regulation for Switching Power Supplies: Second-stage linear regulation after a DC-DC converter, providing low-noise 15V for sensitive analog circuits such as op-amps, sensors, and reference circuits.
• Industrial Control Systems: Power supply for 15V analog circuits, 4-20mA current loop transmitters, PLC analog modules, and industrial sensors requiring stable, noise-free voltage.
• Automotive Electronics: Post-regulation from the 12V vehicle battery system (with appropriate transient protection), providing clean 15V for audio amplifiers, sensor interfaces, and control circuits.
• Telecommunications Equipment: Distributed power systems in telecom line cards and networking gear, where the 78M15 provides local 15V regulation from a higher-voltage intermediate bus.
• Test and Measurement Instruments: Benchtop and portable test equipment requiring clean, stable 15V analog supplies with good ripple rejection for accurate measurements.
• Consumer Electronics: Audio/video equipment, appliances, and consumer devices where a simple, reliable 15V supply is needed with minimal external components.
• General-Purpose Regulation: Breadboards, prototype boards, and hobbyist projects where a simple three-terminal regulator provides a quick, reliable 15V power solution.