Switch-Mode Power Supply

National Semiconductor has been designing and manufacturing integrated circuits (ICs) for switch-mode power supplies for many years. The application of these devices is typically straightforward, supported by comprehensive documentation. A common example of a switch-mode power supply is based on the LM2671 or LM2674 ICs. Components are available for output voltages of 3.3 V, 5 V, and 12 V, with a version that provides a presettable output voltage. Within the specified application, these supplies can deliver currents of up to 500 mA. Notably, the high switching frequency of 260 kHz allows for the use of low-value inductors and capacitors, resulting in excellent efficiency and compact dimensions. Under normal conditions, the efficiency is around 90%, potentially reaching up to 96%. Both ICs include protection against current and temperature overloads. The LM2671 features additional capabilities such as soft start and the option to operate with an external clock, enabling synchronization of multiple supplies for improved electromagnetic compatibility (EMC) control. The application depicted in the diagram provides an output voltage of 5 V and an output current of up to 500 mA. Diode D1 is a Schottky type with a voltage rating of at least 45 V and a maximum current rating of at least 3 A.

The LM2671 and LM2674 integrated circuits are designed for efficient voltage regulation in switch-mode power supply applications. These ICs are capable of stepping down higher input voltages to stable output voltages, making them suitable for various electronic devices. The choice of output voltage—3.3 V, 5 V, or 12 V—allows flexibility in powering different components. The capability to handle up to 500 mA of output current ensures that these power supplies can adequately support a wide range of applications, from microcontrollers to sensors.

The high switching frequency of 260 kHz is a significant feature, as it reduces the size of passive components such as inductors and capacitors. Smaller components not only save board space but also contribute to the overall efficiency of the power supply. The efficiency ratings of 90% to 96% indicate that minimal energy is wasted as heat, which is critical for thermal management in compact designs.

Protection features are essential in power supply design to prevent damage to the circuit and connected devices. The current and temperature overload protection integrated into the LM2671 and LM2674 ensures that the power supply operates safely under varying load conditions. The soft start feature of the LM2671 gradually ramps up the output voltage, reducing inrush current and minimizing stress on the components during startup.

The ability to synchronize multiple LM2671 ICs using an external clock is particularly advantageous in applications where electromagnetic interference must be minimized. By coordinating the switching of multiple power supplies, it is possible to reduce the overall noise generated, enhancing the performance of sensitive electronic systems.

In the specific application example mentioned, the power supply provides a stable 5 V output at a maximum current of 500 mA. The inclusion of a Schottky diode (D1) with a voltage rating of at least 45 V and a current rating of at least 3 A ensures efficient rectification of the output voltage while maintaining low forward voltage drop, further improving the overall efficiency of the power supply circuit.National Semiconductor has been producing and designing ICs for use in switch-mode power supplies for many years. The application of these devices is normally straightforward, helped by the excellent documentation that is available.

A typical example of a switch-mode power supply is that based on the LM2671 or LM2674. The components for it are ava ilable for outputs of 3. 3 V, 5 V and 12 V. There is also a version providing a presettable output voltage. Within the specified application, the supplies can deliver currents of up to 500 mA. Note-worthy is the high switching frequency of 260 kHz. This has the advantage that only low-value inductor and capacitors are needed, and this results in excellent efficiency and small dimensions. In normal circumstances, the efficiency is 90% and may even go up to 96%. Both ICs provide protection against current and temperature overloads. The LM2671 has a number of additional facilities such as soft start and the option to work with an external clock.

The latter enables several supplies to be synchronized so as to give better control of any EMC (ElectroMagnetic Compatibility). The application shown in the diagram provides an output voltage of 5 V and an output current of up to 500 mA.

Diode D1 is a Schottky type (Uco‰¥ 45 V and Imax‰¥ 3 A). 🔗 External reference