Quad 2-channel analog multiplexer / demultiplexer # Technical Documentation: BU4551B High-Voltage, High-Current Darlington Transistor Array
Manufacturer : ROHM Semiconductor
Document Version : 1.0
Date : October 26, 2023
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## 1. Application Scenarios
The BU4551B is a monolithic silicon epitaxial planar NPN Darlington transistor array, designed for applications requiring high-voltage switching and medium-current drive capability. Its integrated structure and robust performance make it suitable for a range of industrial and consumer electronic systems.
### Typical Use Cases
* Inductive Load Driving : The primary use case is driving inductive loads such as relays, solenoids, stepper motor windings, and small DC motors. The Darlington configuration provides the high current gain needed to switch these loads with a low-power control signal from a microcontroller or logic gate.
* Lamp and LED Array Driving : Used for switching incandescent lamps, halogen lamps, or high-power LED arrays where inrush currents can be significant. The device's high collector-emitter voltage rating (`VCEO`) is crucial for handling the back-EMF generated when the load is switched off.
* Display Drivers : Employed in vacuum fluorescent display (VFD) or electroluminescent (EL) display anode/grid drivers, where multiple high-voltage channels are required.
* Interface/Buffer Circuits : Acts as a robust buffer between sensitive logic circuits (e.g., CMOS, TTL) and higher-voltage, higher-current peripheral devices, providing isolation and protection.
### Industry Applications
* Industrial Automation : Control modules for programmable logic controllers (PLCs), actuator drivers in robotic arms, and sensor interface boards.
* Automotive Electronics : Body control modules (BCMs) for driving interior lighting, power window motors, and relay coils (in non-safety-critical circuits).
* Office Equipment : Printers, photocopiers, and scanners for driving paper feed motors, clutches, and high-voltage corona wires.
* Consumer Appliances : Control boards in washing machines, dishwashers, and HVAC systems for solenoid valve and pump motor control.
* Test & Measurement Equipment : As part of switching matrices and programmable load circuits.
### Practical Advantages and Limitations
Advantages:
* High Integration : Contains seven independent Darlington pairs with common emitters and integral suppression diodes in a single 18-pin package (DIP18/SOP18), saving board space and simplifying assembly.
* High Voltage & Current Capability : Each channel can typically switch up to 500V (`VCEO`) and a continuous collector current (`IC`) of 500mA per channel, suitable for a wide range of loads.
* High Current Gain : The Darlington pair offers a very high DC current gain (`hFE`), often exceeding 1000, allowing it to be driven directly from microcontrollers or CMOS logic with minimal base current.
* Built-in Protection : Each Darlington pair includes an integrated clamp diode between the collector and emitter, essential for suppressing voltage spikes caused by switching inductive loads, thereby protecting the transistor and the driving circuit.
Limitations:
* Saturation Voltage : The Darlington configuration results in a higher collector-emitter saturation voltage (`VCE(sat)`, typically ~1.6V at 500mA) compared to a single BJT or a MOSFET. This leads to higher power dissipation (`Pc`) and reduced efficiency, especially in high-current applications.
* Switching Speed : Not optimized for high-frequency switching (beyond a few tens of kHz). The storage time and fall time are relatively slow, making it unsuitable for switch-mode power supplies (SMPS) or high-speed PWM motor control.
* Thermal Management : When multiple channels are active simultaneously, the total power dissipation can be significant. Careful thermal design
