onsemi FZT749

Overview

FZT749 from ON Semiconductor is a PNP low-saturation bipolar junction transistor optimized for high-current switching in power management circuits, featuring V_CE(sat) = −300 mV at I_C = −1 A / I_B = −100 mA, h_FE = 70–300 (depending on bias), and continuous collector current rating of −3 A in SOT-223 package.

For engineers reviewing the FZT749 datasheet, pinout, applications, or equivalent options, key selection considerations include saturation voltage under load, DC current gain linearity across operating range, thermal resistance (R_θJA = 62.5°C/W), and compatibility with TO-223 footprint layouts in industrial DC-DC converters and motor drive stages.

Technical Context

The FZT749 operates as a medium-power PNP BJT with fixed-emitter configuration, designed for linear and saturated-mode switching where low V_CE(sat) reduces conduction loss and improves efficiency in high-side switch topologies. Its f_T = 100 MHz supports fast turn-off in PWM-controlled loads up to ~100 kHz.

Thermal performance is defined for FR-4 PCBs (76 × 114 mm, 1.57 mm thick) with minimum land pattern; R_θJA = 62.5°C/W enables 2 W dissipation at ≤87.5°C ambient rise. Breakdown ratings (V_CBO = −35 V, V_CEO = −25 V) support use in 24 V rail systems with margin.

Key Specifications

Pinout & Package

SOT-223 4-lead molded package with exposed collector tab (pin 2 and 4 tied internally); thermally enhanced for surface-mount power dissipation.

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZXT788B and MJD2955T4G, the FZT749 delivers optimal balance of low saturation voltage, moderate current capability, and SOT-223 manufacturability-making it ideal for cost-sensitive, medium-power surface-mount applications where thermal budget is managed via PCB copper area rather than discrete heatsinks.

Availability

FZT749 is available at Aetrix Electronics and suitable for industrial motor control, automotive body electronics, and DC-DC converter high-side switching requiring stable component supply, consistent parametric performance across production lots, and long-term lifecycle availability.

Supply support for FZT749 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

ON Semiconductor (formerly Fairchild Semiconductor) is a global leader in power management, analog, sensor, and connectivity solutions, serving automotive, industrial, cloud, and consumer markets with energy-efficient semiconductor technologies.

The FZT749 belongs to ON Semiconductor's legacy discrete power transistor portfolio, engineered specifically for high-current, low-saturation switching in space-constrained surface-mount applications where thermal performance and parametric consistency are critical.

FAQ

What is the maximum continuous collector current rating for the FZT749?

The FZT749 has a maximum continuous collector current (I_C) rating of −3 A at T_A = 25°C, derated linearly above that ambient temperature based on its R_θJA = 62.5°C/W. This rating assumes proper PCB thermal management using the SOT-223 exposed collector pad. At 100°C ambient, the practical continuous current drops to approximately −1.6 A to maintain T_J ≤ 150°C. The FZT749 datasheet specifies this limit under steady-state conditions, not pulsed operation.

Does the FZT749 support operation in automotive environments?

Yes, the FZT749 supports automotive body electronics applications due to its guaranteed operating junction temperature range of −55°C to +150°C and V_CBO = −35 V, which exceeds ISO 7637-2 pulse 5a requirements for 12 V systems. It is commonly used in door lock actuators and HVAC damper drivers. However, the FZT749 is not AEC-Q101 qualified; for production automotive ECUs, ON Semiconductor recommends AEC-qualified alternatives unless validated per customer-specific reliability protocols.

What is the typical base-emitter on voltage (V_BE(on)) for the FZT749?

The FZT749 exhibits a typical V_BE(on) of −1.0 V at I_C = −1 A and V_CE = −2 V, with a maximum of −1.25 V under saturation conditions (V_BE(sat)). This value reflects the forward-biased PNP emitter-base junction voltage needed to achieve full conduction. Designers should account for this voltage drop when sizing base resistors for microcontroller-driven applications to ensure sufficient I_B for target I_C.

Can the FZT749 replace the older FZT790A in existing designs?

No, the FZT749 is not a functional replacement for the FZT790A. While both are PNP SOT-223 transistors, the FZT790A has V_CEO = −100 V and I_C = −5 A, whereas the FZT749 is rated for only −25 V and −3 A. Substituting the FZT749 in a FZT790A design risks premature breakdown under voltage stress or thermal overload under high current. Always verify voltage, current, and SOA compliance before cross-referencing discrete transistors.

How is thermal performance measured for the FZT749?

Thermal resistance R_θJA = 62.5°C/W for the FZT749 is measured on a standardized FR-4 PCB (76 × 114 × 1.57 mm) with minimum recommended land pattern per Fairchild drawing MA04A. This value assumes the exposed collector pad (pins 2 and 4) is fully soldered to an internal or external copper plane. Performance degrades significantly if the pad is undersized or isolated; actual R_θJA in custom layouts must be verified via thermal simulation or IR imaging. The FZT749 datasheet explicitly ties this rating to that test board configuration.