onsemi PN3645

Overview

PN3645 from Fairchild Semiconductor is a PNP general-purpose bipolar junction transistor (BJT) designed for amplification and switching applications up to 500 mA collector current. It features V_CEO = 60 V, V_CBO = 60 V, V_EBO = 5.0 V, h_FE = 40–300 (at IC = 0.1–150 mA), and TO-92 package. It is commonly used in low-voltage DC power control circuits, relay drivers, and signal inversion stages.

For engineers reviewing the PN3645 datasheet, pinout, applications, or equivalent options, key selection considerations include its PNP polarity, 625 mW power dissipation at 25°C, thermal resistance R_θJA = 200 °C/W, saturation voltage V_CE(sat) ≤ 0.4 V at I_C = 150 mA, and switching times (t_on = 40 ns, t_off = 100 ns) under specified test conditions.

Technical Context

The PN3645 operates as a silicon PNP BJT fabricated on Fairchild's Process 63, optimized for general-purpose linear amplification and saturated switching. Its DC current gain (h_FE) varies significantly with operating point-ranging from 40 at I_C = 0.1 mA to 20 at I_C = 300 mA-indicating strong bias-dependence. The device supports continuous collector currents up to 800 mA but is rated for 500 mA in typical amplifier/switch use cases per design guidance.

Thermal performance is defined by R_θJC = 83.3 °C/W and R_θJA = 200 °C/W, requiring careful PCB layout for heatsinking when operating near maximum power dissipation (625 mW). Switching behavior is characterized under pulsed conditions (≤300 µs, ≤2% duty cycle), with storage time (t_s = 70 ns) dominating turn-off delay.

Key Specifications

Pinout & Package

PN3645 is housed in a standard through-hole TO-92 plastic package with 3 leads. Lead configuration follows JEDEC TO-92 outline: Emitter (E), Base (B), Collector (C) arranged left-to-right when flat side faces viewer and leads point downward.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PNP general-purpose amplifier and switching applications.

Compared with MPSA92 and PN2907A, PN3645 delivers optimal cost-performance balance for ≤60 V PNP switching: it matches PN2907A's exact specs while offering better availability than obsolete variants, and avoids MPSA92's over-spec'd voltage rating that adds unnecessary cost and capacitance in low-voltage designs.

Availability

PN3645 is available at Aetrix Electronics and suitable for relay driver circuits, DC motor control modules, linear regulator pass elements, and signal inversion stages requiring stable component supply and long-term industrial availability.

Supply support for PN3645 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 portfolio emphasized reliability and manufacturability for industrial and automotive applications.

PN3645 belongs to Fairchild's general-purpose bipolar transistor product line, designed specifically for cost-sensitive, medium-current amplification and switching tasks in consumer, industrial, and power-supply subsystems where TO-92 form factor and proven process consistency are critical.

FAQ

What is the maximum continuous collector current rating for PN3645?

The PN3645 has an absolute maximum continuous collector current (I_C) of 800 mA at T_A = 25°C. However, the datasheet explicitly states it is "designed for use as general purpose amplifiers and switches requiring collector currents to 500 mA," indicating 500 mA as the recommended operational limit for sustained reliability. Derating applies at higher ambient temperatures per the 5.0 mW/°C thermal specification.

Is PN3645 pin-compatible with PN2907A?

Yes, PN3645 is fully pin-compatible and electrically identical to PN2907A. Both share the same TO-92 package, identical absolute maximum ratings (V_CEO, I_C, P_D, etc.), and matching electrical characteristics across all test conditions. The PN3645 designation reflects Fairchild's updated part numbering system for the same Process 63 device, with no functional or mechanical differences from PN2907A.

What is the typical V_CE(sat) of PN3645 at I_C = 150 mA?

The typical V_CE(sat) of PN3645 at I_C = 150 mA and I_B = 15 mA is 0.4 V, with a maximum guaranteed value of 0.4 V under those conditions. This low saturation voltage ensures efficient conduction in switching applications, minimizing power loss and self-heating. At lower currents (e.g., I_C = 50 mA), V_CE(sat) drops to 0.25 V typical, further improving efficiency in lightly loaded stages.

Does PN3645 support high-frequency amplification?

PN3645 is not optimized for RF or high-frequency amplification. Its small-signal current gain (h_fe) is specified only at f = 100 MHz and drops to 2.0 under I_C = 20 mA, V_CE = 20 V conditions-indicating limited bandwidth. The device is intended for audio-frequency amplification and low-MHz switching. For >10 MHz applications, dedicated RF transistors such as MMBT5551 or BFR92A are more appropriate than PN3645.

What thermal resistance values apply to PN3645 in TO-92 package?

PN3645 exhibits R_θJC = 83.3 °C/W (junction-to-case) and R_θJA = 200 °C/W (junction-to-ambient) at T_A = 25°C. These values assume standard PCB mounting with minimal copper area. To maintain junction temperature below 150°C at full 625 mW dissipation, ambient must stay below 25°C unless additional heatsinking (e.g., solder pad thermal relief, copper pour) is applied-otherwise, power must be derated per the 5.0 mW/°C slope.