NXP BZX84-B68: Key Specifications and Application Circuit Design Considerations
The BZX84-B68 from NXP Semiconductors is a popular 68V Zener diode housed in a compact SOT23 surface-mount package. As a critical component for voltage regulation and protection in modern electronic circuits, understanding its key parameters and optimal implementation is essential for designers.
Key Specifications
The primary function of the BZX84-B68 is to maintain a constant voltage across its terminals when operated in its reverse breakdown region. Its defining specification is its Zener voltage (Vz) of 68V, measured at a defined test current (Izt) of 5 mA. Designers must note that this voltage has a tolerance, typically ±5%.
The device's ability to handle power is dictated by its power dissipation rating. For the SOT23 package, this is a maximum of 350 mW at an ambient temperature of 25°C. This value decreases as the operating temperature increases, a factor that must be carefully considered in the thermal design of the application.
Another crucial parameter is the Zener impedance (Zzt), which is typically 60 Ohms at the 5 mA test current. A lower impedance indicates better voltage regulation. The device's leakage current (Ir) at voltages below the breakdown point is minimal, typically just 100 nA at 51.6V, ensuring minimal power loss when the diode is in its "off" state.
Application Circuit Design Considerations
1. Voltage Regulation: The most straightforward application is a simple shunt regulator. The Zener diode is placed in parallel with the load it is regulating. The series current-limiting resistor (Rs) is critical and must be calculated to ensure the Zener operates within its specified current range under all input voltage (Vin) and load (Iload) conditions.
Rs Calculation: The value is chosen using the formula: Rs = (Vin(min) - Vz) / (Izt + Iload(max)).
It must also be checked for power dissipation: P_Rs = (Vin(max) - Vz)² / Rs.
The Zener current must never exceed the maximum allowable, calculated from the power rating: Iz(max) = Ptot / Vz.
2. Overvoltage Protection: The BZX84-B68 is frequently used to protect sensitive IC input pins from voltage transients (ESD, surges). In this configuration, it is connected between the signal line and ground. Its fast response time clamps any voltage spike above 68V, shunting the harmful current away from the protected component. A small series resistor is often used to limit the peak current through the diode during a transient event.
3. Thermal Management: Despite its small size, thermal considerations are paramount. The 350 mW rating is only valid at 25°C. In a real-world environment where the ambient temperature (Ta) is higher, the maximum allowable power dissipation must be derated. Designers should consult the derating curve in the datasheet. Providing adequate copper pour on the PCB connected to the diode's pins acts as a heat sink, improving overall reliability.
4. Stability and Noise: Zener diodes can generate broadband noise in their breakdown region. For precision analog applications, bypassing the Zener with a small-value capacitor (e.g., 100 nF) across its terminals can significantly reduce this electrical noise and improve stability.
The NXP BZX84-B68 is a highly reliable and precise voltage reference component, ideal for space-constrained PCB designs requiring robust 68V regulation or protection. Its SOT23 package offers a perfect balance of size and performance. Successful implementation hinges on the meticulous selection of the series resistor and a thorough analysis of both electrical and thermal operating conditions to ensure long-term circuit stability and reliability.
Keywords: Zener Diode, Voltage Regulation, Overvoltage Protection, SOT23, Power Dissipation
