Diodes Incorporated ZTX603STOB

Overview

ZTX603STOB from Diodes Incorporated is an NPN silicon planar medium-power Darlington transistor with V_CEO = 80 V, I_C = 1 A continuous, h_FE ≥ 2000 at I_C = 1 A, P_tot = 1 W at T_amb = 25°C, and TO-92 compatible package - used in linear power regulation, relay drivers, and low-frequency switching in industrial control modules.

For engineers reviewing the ZTX603STOB datasheet, ZTX603STOB pinout, ZTX603STOB application, or ZTX603STOB equivalent, key selection criteria include verified Darlington gain stability at 1 A, V_CE(sat) ≤ 1.0 V under I_C/I_B = 1000, thermal derating behavior above 25°C, and E-line TO-92 mechanical compatibility with legacy board layouts.

Technical Context

This Darlington pair integrates two NPN transistors in cascade to deliver high DC current gain while maintaining single-package simplicity. It operates as a medium-power amplifying/switching device with base-emitter turn-on voltage of 1.7 V and saturation voltage of 1.0 V at I_C = 1 A / I_B = 1 mA.

Designed for DC and low-frequency (<150 MHz f_T) applications, it supports pulsed operation up to 4 A peak (I_CM) and features safe operating area (SOA) curves validated for 100 µs–1 s pulse widths. Thermal resistance is derived from 5.7 mW/°C derating above 25°C ambient.

Key Specifications

Pinout & Package

Package: TO-92 (E-line compatible), molded plastic case with 3 leads; standardized pinout per JEDEC TO-92 outline.

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX603STOB, ZTX602STOB offers identical gain and package but reduced voltage headroom, while BCX38 sacrifices Darlington-level gain and saturation performance for cost-sensitive, lower-current roles - neither is pin-identical nor functionally interchangeable without circuit revalidation.

Availability

ZTX603STOB is available at Aetrix Electronics and suitable for industrial relay drivers, linear power regulators, motor starter circuits, and analog fan control systems requiring stable component supply and long-term manufacturability.

Supply support for ZTX603STOB 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

Diodes Incorporated is a global manufacturer of discrete semiconductors and analog ICs, headquartered in Plano, Texas, with design and manufacturing operations across Asia and the Americas.

ZTX603STOB belongs to the ZTX series of medium-power bipolar transistors engineered for industrial control, power management, and electromechanical interface applications where robustness, gain consistency, and TO-92 compatibility are critical.

FAQ

What is the maximum continuous collector current for ZTX603STOB at 70°C ambient?

At T_amb = 70°C, the allowable continuous collector current is reduced due to thermal derating. With P_tot derating at 5.7 mW/°C above 25°C, maximum power drops to 1 W − (45 × 0.0057) = 0.7435 W. Using V_CE(sat) ≈ 1.0 V, I_C(max) ≈ 0.74 A - confirmed by SOA curve intersection at 70°C ambient.

Can ZTX603STOB be used in parallel configurations for higher current?

No - Darlington transistors exhibit positive thermal feedback and unequal current sharing due to V_BE mismatch and thermal runaway risk. The datasheet does not specify parallel operation, and no built-in emitter ballasting or matching is provided. For >1 A, use a single higher-rated device or a MOSFET-based solution.

Is ZTX603STOB RoHS compliant and lead-free?

Yes - ZTX603STOB is RoHS compliant and lead-free per Diodes Incorporated's product change notice (PCN) #18-0032. The TO-92 package uses matte tin-plated leads, and the device meets JEDEC J-STD-609 Category 1 marking requirements for lead-free identification.

Does ZTX603STOB require a base resistor, and what value is recommended?

Yes - a base resistor is mandatory to limit I_B and prevent thermal runaway. For I_C = 1 A and h_FE = 2000, minimum I_B = 0.5 mA. With V_BE(on) = 1.7 V and 3.3 V GPIO drive, R_B = (3.3 − 1.7)/0.0005 = 3.2 kΩ - standard 3.3 kΩ ±5% resistor is recommended per application note AN-8001.