Voltage regulators employ emulated-current mode, virtual-knob tool
By Paul Rako, Technical Editor -- EDN, January 30, 2007
National Semiconductor’s new 6 to 42V LM25574/5/6 and 6 to 75V LM5574/5/6 series of Simple Switcher voltage regulators outputs 0.5, 1.5 and 3A. National designed these current-mode parts with the same process as the LM5000 series. The new parts operate at 50 kHz to 1 MHz. Unlike competitors’ switchers, derating below 1 MHz occurs only with step-down ratios beyond 36 to 2V. The Simple Switchers also use a novel method to emulate the current in the inductor without inserting an efficiency-robbing sense resistor or using the dc resistance of the inductor, which varies from design to design, part to part, and over temperature. ECM (emulated-current mode) senses the current in the freewheeling diode during the leisurely off-time period, providing better transient response. This clever feature is one of the reasons the new line of parts needs 16 or 20 pins, rather than five, as its predecessor had. The anode of the freewheeling diode connects to the part, so the part can measure the diode current with an accurate and simple low-side sense circuit.
Using the Simple Switchers, you can optimize designs for operating frequency, efficiency, soft-start time, board size, and transient response. Designing with so many variables would be daunting except for the fact that National has made its Webench online-design tool do all the work. The designer needs only to enter the required parameters, and the Webench tool calculates a BOM (bill of materials), runs a Spice simulation, and does a thermal analysis of the design. Webench now has a large, intuitive knob that lets designers choose between efficiency and board size. If you want efficiency of more than 95%, you turn the virtual knob in one direction. If you want the smallest possible board area, you turn the knob in the other direction. Webench then sends you the BOM as well as a kit of those parts, including a small Built-It evaluation board.
The 0.5 and 1.5A parts come in TSSOP-16 packages, and the 3A parts come in TSSOP-16 packages. They are available and in distribution. Prices ranges from $1.35 to $2.90 (1000). Demo boards and Built-It boards for Webench are available.
