onsemi FSB749

Overview

FSB749 from ON Semiconductor (formerly Fairchild) is a PNP low-saturation bipolar junction transistor optimized for high-current switching in compact power management circuits. It delivers V_CE(sat) = −600 mV at I_C = −3 A / I_B = −300 mA, h_FE ≥ 75 at I_C = −2 A, and operates across −55°C to +150°C. It is used in DC-DC converter synchronous rectification and automotive load switching.

For engineers reviewing the FSB749 datasheet, pinout, applications, or equivalent options, key selection criteria include its SuperSOT-3 package thermal resistance (R_θJA = 250°C/W), −25 V V_CEO rating, and verified low-V_CE(sat) performance under sustained 3 A conduction.

Technical Context

The FSB749 is fabricated using Fairchild's PC process to achieve high current gain and low saturation voltage simultaneously. Its design targets continuous collector currents up to −3 A with guaranteed h_FE ≥ 70 at −50 mA and ≥ 75 at −2 A, enabling efficient operation in medium-power linear and switching regulators.

It features a maximum total device dissipation of 500 mW and a breakdown voltage profile defined by V_CBO = −35 V, V_CEO = −25 V, and V_EBO = −5 V - establishing safe operating limits for PNP-based high-side switch and current mirror configurations.

Key Specifications

Pinout & Package

FSB749 is housed in the SuperSOT-3 (SSOT-3L) surface-mount package - a 3-lead, single-row, gull-wing leaded plastic package measuring 2.9 mm × 1.3 mm × 0.9 mm (L × W × H), optimized for high power density and automated assembly.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PNP low-saturation transistor applications.

Compared with MBT3906DW1T1G and PN3565, the FSB749 uniquely supports −3 A continuous conduction in a thermally optimized SuperSOT-3 package, making it the only option among the three qualified for medium-power switching where both current capacity and surface-mount manufacturability are required.

Availability

FSB749 is available at Aetrix Electronics and suitable for automotive body control modules, industrial DC-DC converters, and medical diagnostic equipment requiring stable component supply despite its discontinued status.

Supply support for FSB749 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 supplier of energy-efficient semiconductor solutions for automotive, industrial, cloud computing, and medical applications.

The FSB749 belongs to Fairchild's SuperSOT-3 PNP transistor family, engineered specifically for high-current, low-saturation switching in space-constrained power systems where thermal performance and reliability are critical.

FAQ

What is the maximum continuous collector current rating for the FSB749?

The FSB749 is rated for −3 A continuous collector current at T_A = 25°C with appropriate PCB copper area. This rating assumes operation within its absolute maximum junction temperature limit of +150°C and accounts for its 250°C/W thermal resistance. Derating is required above 25°C ambient per the datasheet thermal curves. The FSB749 must be operated below this limit to maintain reliability and avoid thermal runaway.

Is the FSB749 pin-compatible with other SuperSOT-3 PNP transistors?

Yes - the FSB749 uses the standard SuperSOT-3 (SSOT-3L) pinout: Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector. This matches industry-standard SuperSOT-3 PNP devices including the MMBT3906 series and ON Semiconductor's NSS20201MR6T1G. Layout reuse is possible, but electrical validation is required due to differences in h_FE, V_CE(sat), and thermal performance. Always verify drive conditions for the FSB749 specifically.

Why does the FSB749 datasheet state "DISCONTINUED"?

The FSB749 was officially discontinued by ON Semiconductor (which acquired Fairchild in 2016). Its discontinuation notice appears in all official documentation, including Rev. 2 of the FSB749 datasheet dated January 2026. However, Aetrix Electronics maintains active inventory and offers lifecycle management support, including traceable sourcing and long-term supply planning for legacy designs still dependent on the FSB749.

What is the typical V_CE(sat) of the FSB749 at 1 A collector current?

The FSB749 exhibits a typical V_CE(sat) of −300 mV at I_C = −1 A and I_B = −100 mA, as specified in its Electrical Characteristics table. This value is measured under pulsed conditions (≤ 300 μs, ≤ 2% duty cycle) at T_A = 25°C. For steady-state operation, thermal effects may raise V_CE(sat) slightly; design margins should account for worst-case −600 mV at −3 A.

Can the FSB749 be used in linear amplifier applications?

The FSB749 is not optimized for linear amplification. Its datasheet specifies parameters only for switching operation - notably V_CE(sat), h_FE at high current, and breakdown voltages - with no small-signal gain (h_fe) or noise figure data. While functional as a Class-A emitter follower at low frequencies, its high f_T (100 MHz) and lack of linearity characterization make it unsuitable for precision analog or RF amplification. Use dedicated amplifier transistors instead of the FSB749 for such roles.