onsemi MC7918CT

Overview

MC7918CT from onsemi is a fixed-output negative linear voltage regulator delivering −18 V at up to 1.0 A, featuring internal thermal shutdown, short-circuit current limiting, and safe-area compensation for rugged operation in industrial power supplies. It requires no external components for basic regulation and maintains stable output under load currents from 5 mA to 1.0 A with ≤180 mV load regulation (1.0 A range) and ≤360 mV line regulation across −21 V to −33 V input.

For engineers reviewing the MC7918CT datasheet, pinout, applications, or equivalent options, key selection considerations include its TO−220 package with heatsink-connected Pin 2, −18 V nominal output with ±0.7 V tolerance at +25°C, 1.3 V dropout at 1.0 A, 59 dB ripple rejection at 120 Hz, and suitability for dual-rail op-amp supplies, MECL logic, and industrial analog systems requiring robust negative bias.

Technical Context

The MC7918CT implements a monolithic bipolar pass transistor with integrated protection circuitry: thermal overload shutdown triggers at +150°C junction temperature, short-circuit limiting caps output current, and safe-area compensation dynamically reduces short-circuit current as V_CE increases. Its internal bias network draws 4.5 mA typical input current and exhibits −1.0 mV/°C average temperature coefficient over 0°C to +125°C.

Designed as a complement to the MC7800 series, it operates with a minimum input-to-output differential of 2.0 V (typical), supports input voltages up to −35 V (for −18 V output), and delivers 110 µV_RMS output noise (10 Hz–100 kHz). Power dissipation is internally limited, with θ_JA = 65°C/W (TO−220, case 221AB) and θ_JC = 5.0°C/W.

Key Specifications

Pinout & Package

MC7918CT is housed in a TO−220 package (Case 221AB), with the metal tab (heatsink surface) electrically connected to Pin 2. The device uses standard three-terminal configuration optimized for negative voltage regulation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar negative voltage regulation applications.

Compared with LM337T and MC7918CD2TR4G, the MC7918CT provides superior ease of use in fixed −18 V applications due to zero external components, proven thermal robustness in TO−220 form factor, and direct compatibility with legacy through-hole power supply layouts - avoiding resistor matching errors and SMT thermal design complexity.

Availability

MC7918CT is available at Aetrix Electronics and suitable for industrial control systems, analog sensor interfaces, and legacy telecom equipment requiring stable component supply with long lifecycle support and traceable sourcing.

Supply support for MC7918CT includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

onsemi (formerly ON Semiconductor) is a global semiconductor supplier specializing in energy-efficient power management, analog, sensing, and connectivity solutions for automotive, industrial, and cloud infrastructure markets.

The MC7900 Series was designed specifically to provide rugged, fixed-output negative voltage regulation as a complementary counterpart to the MC7800 positive regulator family - targeting cost-sensitive, high-reliability industrial and legacy system power supplies.

FAQ

What is the maximum input voltage rating for the MC7918CT?

The MC7918CT supports a maximum input voltage of −35 V DC when regulating −18 V output, as specified in the Absolute Maximum Ratings table. Exceeding this voltage risks permanent device damage. For reliable operation, the recommended input range is −21 V to −33 V, ensuring sufficient headroom while maintaining thermal safety margins under full 1.0 A load.

Does the MC7918CT require input and output capacitors for stable operation?

Yes - while the MC7918CT functions without external capacitors in minimal-load scenarios, onsemi recommends a 0.33 µF input bypass capacitor (tantalum or low-ESR film) placed close to Pins 1 and 2, and a 1.0 µF output capacitor (electrolytic or tantalum) between Pins 1 and 3. These improve high-frequency stability, transient response, and ripple rejection, especially when input wiring is long or output capacitance is large.

How does thermal shutdown work in the MC7918CT?

The MC7918CT incorporates an internal thermal protection circuit that disables the output pass transistor when junction temperature reaches +150°C. This shutdown is non-latching: once the die cools below the hysteresis threshold (~135°C), regulation automatically resumes. The feature prevents destruction during overload, poor heatsinking, or ambient temperature excursions - no external intervention is needed.

Can the MC7918CT be used in parallel to increase output current?

No - the MC7918CT is not designed for parallel operation. Its output voltage tolerance (±0.7 V) and lack of current-sharing circuitry mean paralleling units will cause unequal current distribution, thermal runaway, and potential failure. To achieve >1.0 A, use a current-boost configuration with an external pass transistor as shown in Figure 9 of the datasheet, or select a higher-current regulator.

What is the purpose of the metal tab connection on the MC7918CT TO−220 package?

The metal tab on the MC7918CT TO−220 package is electrically connected to Pin 2 (Input) and serves as the primary thermal conduction path to an external heatsink. Because it is tied to the negative input rail, the heatsink must be electrically isolated from chassis ground unless the system design intentionally references the heatsink to the negative supply - improper mounting can cause short circuits or ground loops.