onsemi ZTX749A

Overview

ZTX749A from Fairchild Semiconductor is a PNP bipolar junction transistor (BJT) designed for high-current switching applications with low saturation voltage, featuring V_CEO = −35 V, I_C = −2 A continuous, and V_CE(sat) = −300 mV at I_C = −1 A / I_B = −100 mA. It is commonly used in industrial power control, relay drivers, and DC-DC converter output stages.

For engineers reviewing the ZTX749A datasheet, pinout, applications, or equivalent options, key selection criteria include its TO-226 package compatibility, guaranteed h_FE ≥ 70 at I_C = −50 mA, low thermal resistance (R_θJA = 125 °C/W), and −55 °C to +150 °C operating temperature range.

Technical Context

The ZTX749A operates as a medium-power PNP BJT optimized for saturated switching, not linear amplification. Its design emphasizes low V_CE(sat) and high DC current gain across collector currents from −50 mA to −2 A, with validated performance under pulsed conditions (≤300 µs, ≤2% duty cycle).

Thermal management relies on its TO-226 package's 125 °C/W junction-to-ambient resistance and maximum 1 W total dissipation. Breakdown ratings (BVCBO = −45 V, BVCEO = −35 V) define safe operating area boundaries for inductive load switching.

Key Specifications

Pinout & Package

Package: TO-226AE (also known as TO-92 variant with leadform per Fairchild drawing S0.51–0.36 etc.), epoxy-molded, through-hole compatible, 3-pin plastic case.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJD2955 and ZTX750, the ZTX749A delivers optimal balance of low saturation loss, TO-226 footprint, and 24 V system compatibility - making it preferred for space-constrained, thermally moderate industrial controls where efficiency below 2 A matters most.

Availability

ZTX749A is available at Aetrix Electronics and suitable for industrial relay drivers, DC-DC converter output stages, linear voltage regulator pass elements, and motor H-bridge high-side drivers requiring stable component supply and long-term manufacturability.

Supply support for ZTX749A 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.

The ZTX749A belongs to Fairchild's ZTX series of general-purpose PNP transistors, engineered specifically for high-efficiency, medium-current switching in industrial and power conversion applications.

FAQ

What is the maximum continuous collector current rating for the ZTX749A?

The ZTX749A 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 minimal heatsinking; derating is required above 25 °C ambient per the R_θJA = 125 °C/W specification. At 70 °C ambient, the practical continuous I_C drops to approximately −1.2 A to maintain T_J ≤ 150 °C. The ZTX749A datasheet specifies this limit under steady-state conditions, not pulsed operation.

Is the ZTX749A pin-compatible with the ZTX750?

Yes, the ZTX749A is pin-compatible with the ZTX750 - both use the TO-226AE (TO-92 variant) package with identical CBE pinout (pin 1 = E, pin 2 = B, pin 3 = C). No PCB layout change is needed when substituting ZTX750 for ZTX749A, though design verification is required due to ZTX750's higher V_CEO (−60 V) and higher V_CE(sat) (−500 mV at −2 A). The ZTX749A remains preferable for 24 V systems where lower saturation loss is critical.

What is the typical DC current gain (h_FE) of the ZTX749A at I_C = −1 A?

The ZTX749A exhibits a minimum h_FE of 100 at I_C = −1 A, V_CE = −2 V, per its datasheet Electrical Characteristics table. Typical values range between 150 and 250 under those conditions, supporting robust base drive design with safety margin. This gain level ensures reliable saturation even with ±20% variation in base resistor tolerance or temperature drift. The ZTX749A maintains usable h_FE down to −2 A, where min. h_FE = 75 is guaranteed.

Can the ZTX749A be used in linear amplifier applications?

The ZTX749A is not optimized for linear amplification; its datasheet emphasizes low V_CE(sat) and high-current switching performance, with no small-signal parameters (e.g., h_fe, f_T linearity, distortion specs) characterized for analog use. While functional as a Class-A amplifier at low currents, thermal instability and gain nonlinearity above 100 mA make it unsuitable for precision audio or RF amplification. The ZTX749A is intended for saturated switching, not linear operation.

What is the thermal resistance (R_θJA) of the ZTX749A, and how does it affect PCB layout?

The ZTX749A has a specified R_θJA of 125 °C/W under standard JEDEC test conditions (single-layer 1 in² copper pad). In practice, doubling the copper area reduces R_θJA by ~20–30%; adding internal ground planes or thermal vias can improve it further. For 1 W dissipation, a 70 °C ambient requires ~75 °C rise - achievable with ≥2 in² of 2-oz copper. The ZTX749A's TO-226 package relies entirely on PCB conduction, not case mounting, so layout directly governs thermal performance.