onsemi ZTX749

Overview

ZTX749 from Fairchild Semiconductor is a PNP bipolar junction transistor (BJT) designed for high-current switching applications with low saturation voltage, featuring V_CEO = −25 V, I_C = −2 A continuous, and V_CE(sat) = −300 mV at I_C = −1 A / I_B = −100 mA. It operates across −55°C to +150°C and is commonly used in power supply control, motor driver stages, and relay drivers.

For engineers reviewing the ZTX749 datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-226 package, PNP polarity, guaranteed h_FE ≥ 70 at I_C = −50 mA, low V_CE(sat), and thermal resistance of 125°C/W - all critical for thermally constrained linear or switching designs.

Technical Context

The ZTX749 is a silicon PNP BJT optimized for low-saturation operation in medium-power switching circuits. Its structure delivers high DC current gain (h_FE up to 300) at moderate collector currents while maintaining tight V_CE(sat) control under I_C ≤ −2 A and I_B ≤ −200 mA drive conditions.

Thermal performance is defined by R_θJA = 125°C/W and a maximum junction temperature of 150°C, requiring heatsinking or airflow for sustained operation above ~300 mW dissipation. Breakdown ratings (BV_CEO = −25 V, BV_CBO = −35 V) support use in grounded-emitter configurations with up to 20 V rail margins.

Key Specifications

Pinout & Package

Package: TO-226 (also known as TO-92 variant TO-226AE), epoxy-molded plastic case with 3 leads, intended for through-hole mounting. Dimensions conform to JEDEC TO-226AE standard (lead spacing 1.40 mm, body height 4.70 mm).

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX749, ZTX750 offers higher voltage and current headroom at the cost of marginally higher conduction loss, while BC557B provides superior gain and voltage rating but cannot handle the ZTX749's 2 A load - making ZTX749 the optimal balance for medium-power PNP switching where both current and saturation voltage matter.

Availability

ZTX749 is available at Aetrix Electronics and suitable for power supply enable switches, relay drivers, DC motor control circuits, and LED current sinks requiring stable component supply and long-term industrial availability.

Supply support for ZTX749 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

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in power management, analog, and discrete components before its acquisition by ON Semiconductor in 2016. Its discrete transistor portfolio emphasized ruggedness, manufacturability, and broad industrial qualification.

The ZTX749 belongs to Fairchild's ZTX series of general-purpose PNP transistors engineered for reliable medium-power switching in cost-sensitive industrial and consumer power systems.

FAQ

What is the maximum continuous collector current rating for the ZTX749?

The ZTX749 has a maximum continuous collector current (I_C) rating of −2 A at T_A = 25°C. This rating assumes adequate PCB copper area or heatsinking to maintain junction temperature below 150°C. At elevated ambient temperatures or with limited thermal relief, derating is required per the R_θJA = 125°C/W specification. The ZTX749 must not be operated beyond this limit in sustained DC or high-duty-cycle pulse applications without verification of thermal margin.

Is the ZTX749 suitable for use as a high-side switch in a 12 V automotive circuit?

Yes, the ZTX749 is suitable as a high-side PNP switch in 12 V automotive circuits, provided transient voltage clamping is implemented. Its BV_CEO = −25 V provides margin above nominal 12 V and typical load-dump spikes (up to ~20 V), but external TVS protection is recommended for ISO 7637-2 compliance. The ZTX749's −300 mV V_CE(sat) at −1 A ensures minimal voltage drop and heating during active operation, supporting robust performance in engine-control or body-electronics modules.

What is the pin configuration of the ZTX749 in its TO-226 package?

The ZTX749 uses a TO-226 (TO-92 variant) package with leads arranged in C-B-E order when viewed with flat side facing outward and leads pointing downward: Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector. This matches JEDEC TO-226AE mechanical outline and is consistent across Fairchild's ZTX series. Correct orientation is essential to avoid reverse-bias damage or circuit malfunction; always verify pinout with physical marking (notch or dot) before PCB layout or assembly.

Does the ZTX749 have a specified transition frequency (f_T)?

Yes, the ZTX749 has a specified transition frequency f_T of 100 MHz, measured at I_C = −100 mA, V_CE = −5 V, and f = 100 MHz. This indicates usable small-signal amplification bandwidth, though the device is primarily optimized for switching rather than RF amplification. In switching applications, f_T correlates with achievable turn-on/turn-off speed - the ZTX749 supports clean edges in PWM frequencies up to several hundred kHz when properly base-driven, making it appropriate for SMPS gate driving or digital logic interfacing.

How does the ZTX749's thermal resistance affect PCB layout requirements?

The ZTX749's R_θJA = 125°C/W means that in still air, each watt of dissipated power raises the junction temperature by 125°C above ambient. To keep T_J ≤ 150°C at 70°C ambient, maximum allowable power is ~0.64 W - requiring ≥1 cm² of 2-oz copper pour connected to the emitter pad for passive cooling. For higher-power operation, the ZTX749 should be mounted on a dedicated thermal pad with multiple vias to inner-layer planes. PCB layout directly impacts reliability; insufficient copper will cause premature thermal runaway even within I_C and V_CE limits.