National Semiconductor’s High-Voltage Step-Down Controller Family

Well-suited for a variety of telecom, networking and industrial infrastructure applications, the highly integrated, easy-to-use buck controllers power sensitive, low-voltage circuitry from high input voltages. The new controllers feature 85 percent peak efficiency when powering low voltage loads from a high voltage supply, positioning them among National’s PowerWise® family of energy-efficient products.

The LM5085, LM25085, LM5088 and LM25088 buck controllers feature an input voltage range up to 75V and can regulate output voltages as low as 1.2V at load currents in excess of 5A. Each offers programmable switching frequency up to 1 MHz using a single resistor, enabling the designer to optimize the design based on varying size and efficiency constraints. The LM5085 and LM5088 operate from input voltages up to 75V, and the LM25085 and LM25088 have a maximum input voltage of 42V.

Offered in a 16-pin TSSOP package, the LM5088 and LM25088 are full-featured, low-noise controllers for high-voltage step-down applications. The N-channel MOSFET controllers feature an emulated current-mode (ECM) control PWM control topology to reliably regulate very low output voltages (as low as 1.2V) when operating from high input voltages. In addition, the controllers simplify the power supply design process by offering a unique frequency dithering feature that varies the switching frequency by ±5 percent around the nominal switching frequency, reducing EMI peak amplitude by 10 dB or more in typical applications. Synchronization to an external clock further controls system EMI.

The LM5088 and LM25088 can operate as a slave to an external master supply for applications requiring multiple supply rails that track each other during the start-up and power-down periods. Protection options include programmable cycle-by-cycle current limiting for transient overload conditions and a hiccup mode for reduced power dissipation during continuous load faults.

Offered in 8-pin mini SOIC and LLP packages, the LM5085 and LM25085 are tiny P-channel MOSFET controllers that support a wide range of high-voltage step-down applications. A constant on-time (COT) control method provides ultra-fast load transient response with no loop compensation required, minimizing the number of external components and simplifying the design. To ease EMI filtering, an input feed-forward scheme maintains nearly constant switching frequency over line and load variations. An integrated P-channel MOSFET driver minimizes external components and enables 100 percent duty cycle operation, allowing the output to remain in regulation under conditions where the input voltage is only slightly higher than the output voltage.