onsemi PN3644

Overview

PN3644 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 = 45 V, V_CE(sat) = 0.25 V at I_C = 50 mA / I_B = 2.5 mA, h_FE = 100 at I_C = 10 mA, and TO-92 package. It is commonly used in low-voltage DC power control circuits and signal inversion stages.

For engineers reviewing the PN3644 datasheet, pinout, applications, or equivalent options, key selection considerations include its PNP polarity, 45 V breakdown rating, 800 mA absolute maximum collector current, thermal resistance of 200 °C/W (junction-to-ambient), and suitability for linear amplification or saturated-switching operation in industrial control and consumer electronics.

Technical Context

The PN3644 operates as a silicon PNP BJT fabricated using Fairchild's Process 63. Its DC current gain (h_FE) ranges from 40 to 300 depending on bias conditions, with minimum 100 at 10 mA collector current and 10 V V_CE. Saturation voltages are specified at two operating points, confirming robust low-loss switching capability.

Small-signal parameters include h_fe = 2.0 at 100 MHz (I_C = 20 mA, V_CE = 20 V), C_ob = 8.0 pF, and C_ib = 35 pF - indicating moderate high-frequency performance suitable for audio and low-MHz switching. Switching times (t_on = 40 ns, t_off = 100 ns) reflect optimized charge storage for general-purpose use.

Key Specifications

Pinout & Package

PN3644 is housed in a standard through-hole TO-92 package with straight leads and no lead clip. The device uses the conventional B-E-C pinout configuration when viewed with flat side facing outward and leads pointing downward.

Key Features

Applications

Equivalent & Alternatives

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

Compared with PN2907A and MMBT3906, the PN3644 offers identical electrical behavior to PN2907A in through-hole form but provides higher voltage margin than MMBT3906 - making it optimal for legacy-compatible 45 V industrial switching where leaded assembly is required.

Availability

PN3644 is available at Aetrix Electronics and suitable for DC power switching, signal inversion, linear amplification, and current mirror reference applications requiring stable component supply and long-term obsolescence management.

Supply support for PN3644 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 devices before its acquisition by ON Semiconductor in 2016. Its legacy products remain widely deployed in industrial and automotive systems.

The PN3644 belongs to Fairchild's general-purpose bipolar transistor product line, engineered for cost-effective, reliable amplification and switching in non-critical industrial, consumer, and instrumentation applications.

FAQ

What is the pin configuration of the PN3644?

The PN3644 uses the standard TO-92 pinout: Pin 1 is Emitter, Pin 2 is Base, and Pin 3 is Collector when viewed with the flat side facing outward and leads pointing downward. This matches the mechanical layout shown in Fairchild's TO-92 Package Dimensions document (Rev. B, Jan 2000) and is consistent with PN2907A.

Is the PN3644 suitable for switching 24 V relay coils?

Yes, the PN3644 is suitable for switching 24 V relay coils. With V_CEO = 45 V and I_C = 500 mA typical usage, it safely handles inductive kickback and steady-state coil currents. Use a base resistor to ensure I_B ≥ I_C/h_FE(min), and add a flyback diode across the coil for protection. The PN3644's V_CE(sat) ≤ 0.4 V minimizes power loss during conduction.

How does the PN3644 compare to the PN2907A?

The PN3644 and PN2907A share identical absolute maximum ratings, thermal characteristics, and electrical specifications per Fairchild documentation. Both are PNP transistors in TO-92 packages with matched pinout and performance. The PN3644 datasheet explicitly references PN2907A for characteristics, confirming functional equivalence and interchangeability in existing designs.

What is the maximum operating temperature for the PN3644?

The PN3644 has a specified operating junction temperature range of –55 °C to +150 °C. At ambient temperatures above 25 °C, power dissipation must be derated at 5.0 mW/°C due to its R_θJA = 200 °C/W. For sustained 625 mW operation, ambient must remain ≤25 °C; at 70 °C ambient, maximum allowable power drops to ~400 mW.

Does the PN3644 support high-frequency signal amplification?

The PN3644 is not characterized for RF or high-speed amplification. Its small-signal h_fe is specified only at 100 MHz (value = 2.0), and no f_T or S-parameter data is provided. It is intended for audio-frequency and low-MHz switching applications - not broadband RF. For frequencies above 10 MHz, consider purpose-built RF transistors such as the MMBT918 or BF199 instead of the PN3644.