onsemi MC34166TH

Overview

MC34166TH from onsemi is a fixed-frequency voltage-mode switching regulator IC designed for step-down, inverting, and step-up/down DC-DC converter topologies. It integrates a 3.0 A output switch, 72 kHz oscillator, precision 5.05 V ±2% reference, error amplifier, PWM latch, cycle-by-cycle current limiting, undervoltage lockout (5.9 V startup), and thermal shutdown. It operates from 7.5 V to 40 V input and supports stable 5.0 V/3.0 A output in step-down configurations.

For engineers reviewing the MC34166TH datasheet, pinout, applications, or equivalent options, this page delivers verified technical context, exact pin functions, real-world converter design constraints, thermal behavior in TO-220 packages, and validated alternative options for automotive, industrial, and embedded power supply designs.

Technical Context

The MC34166TH implements voltage-mode control with an internal 5.05 V reference applied to the non-inverting input of a high-gain error amplifier (80 dB typical gain, 600 kHz unity-gain bandwidth). Its fixed 72 kHz oscillator uses an internal trimmed current source and external timing capacitor CT to generate a ramp signal that drives a PWM comparator with 0–95% duty cycle capability.

Cycle-by-cycle current limiting monitors switch conduction via an internal sense resistor and comparator, triggering immediate latch reset at ≥4.3 A peak current. Undervoltage lockout provides 5.9 V startup threshold with 0.9 V hysteresis, while thermal shutdown activates at ~170°C junction temperature - independent of ambient rating (0°C to +70°C).

Key Specifications

Pinout & Package

The MC34166TH is housed in a 5-lead TO-220 package (Case 314B), with the metal tab thermally and electrically connected to Pin 3 (Ground). The package is vertical-mount, Pb-free, and MSL Level 1.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar switching regulator applications.

Compared with MC34166TH, MC33166D2TR4G offers wider temperature range and SMT compatibility but requires PCB redesign, while LM2576-5.0 simplifies design for fixed 5 V step-down but removes inverting functionality and alters loop compensation requirements.

Availability

MC34166TH is available at Aetrix Electronics and suitable for automotive body electronics, industrial PLC power supplies, and medical portable device DC-DC converters requiring stable component supply, long-lifecycle support, and qualified sourcing.

Supply support for MC34166TH 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 manufacturer specializing in energy-efficient power, analog, sensing, and connectivity solutions for automotive, industrial, and cloud infrastructure markets.

The MC34166TH belongs to onsemi's legacy monolithic switching regulator product line, engineered for cost-effective, robust DC-DC conversion in space-constrained and thermally demanding environments.

FAQ

What is the maximum input voltage rating for the MC34166TH?

The MC34166TH has an absolute maximum power supply input voltage (V_CC) of 40 V, as specified in the Maximum Ratings table. This allows safe operation across 12 V automotive systems including load-dump transients, and 24 V industrial rails with margin. Exceeding 40 V risks permanent damage to the internal NPN switch and oscillator circuitry. The device enters undervoltage lockout below 5.9 V and resumes operation only after hysteresis recovery.

Can the MC34166TH be used in voltage-inverting configurations?

Yes, the MC34166TH supports voltage-inverting topologies, as confirmed in the datasheet's Figure 23 and Section "Voltage−Inverting Converter". It achieves negative output by relocating the IC ground pin to the negative output rail, which reverses error amplifier polarity and keeps the switch emitter within its −1.5 V limit. This configuration delivers −12 V/1.0 A with 81.2% efficiency and 80 mVpp ripple, as validated in test data.

What is the purpose of Pin 5 (Feedback) on the MC34166TH?

Pin 5 is the inverting input of the internal error amplifier and serves as the voltage feedback node. It compares the scaled output voltage (via external R1/R2 divider) against the 5.05 V internal reference to regulate output. When left unconnected, the device defaults to 5.05 V output; adding R1 enables programmable output voltages above 5.05 V using V_OUT = 5.05 × (1 + R1/R2). Its input bias current is ≤1.0 µA, minimizing divider loading error.

Does the MC34166TH support standby or low-power mode?

Yes, the MC34166TH features a low-power standby mode that reduces supply current to 36 µA at 12 V. This is activated by clamping Pin 1 (Compensation) to <150 mV, which disables the oscillator and output switch while retaining reference and UVLO functionality. Figure 16 shows a simple resistor network to implement this, enabling system-level power gating without removing V_CC.

What thermal considerations apply to the MC34166TH in TO-220 package?

The MC34166TH in TO-220 (Case 314B) has θ_JA = 65°C/W and θ_JC = 5.0°C/W. With 3.0 A output and typical 82.8% efficiency, power dissipation reaches ~1.3 W - requiring a heatsink (e.g., AAVID 5903B) to maintain T_J <150°C. The metal tab is electrically connected to Pin 3 (Ground), so isolation hardware must be used if mounting to chassis ground. Thermal shutdown triggers at ~170°C junction temperature.