2-MHz dual channel step-down regulator offers power-on reset and watchdog timer

August 17, 2011 // By Paul Buckley
Linear Technology Corporation has introduced a dual channel, current mode step-down switching regulator with a power-on reset and watchdog timer. The LT3641's 4 V to 42 V input voltage range with 55 V transient capability makes it ideal for load dump and cold crank conditions commonly found in automotive applications which require constant output regulation even with input transients as high as 55 V.

The LT3641 uses a unique dual channel design with a high input voltage (4 V IN to 42 V IN , 55 V IN transient capability) nonsynchronous channel, which delivers 1.3 A of output current combined with a lower input voltage (2.5 V IN to 5.5 V IN ) synchronous channel, delivering up to 1.1 A of continuous output current. Although the part allows separate inputs for each channel, most applications will use the output of the high voltage channel to power the lower voltage channel, offering dual outputs while optimizing efficiency and switching frequency. For example, using a nominal 12 V input and a 2 MHz switching frequency, the LT3641 can deliver a 3.3 V output at 600 mA with 85% efficiency via its high voltage channel and a 1.8 V output at 800 mA with 90% secondary conversion efficiency via its lower voltage channel.

The integrated microprocessor supervisor functions support high reliability applications such as automotive electronic control units. The LT3641 includes one power-on reset timer for each channel and one common watchdog timer. The reset and watchdog timeout periods are independently adjustable using external capacitors. Tight accuracy specifications and glitch immunity ensure reliable reset operation of a system without false triggering. The LT3641 implements a windowed watchdog timer monitoring for WDI falling edges grouped too close together or too far apart.

The LT3641’s switching frequency is user programmable from 350 kHz to 2.5 MHz, enabling the designer to optimize efficiency while avoiding critical noise-sensitive frequency bands. Similarly, its low minimum on-time enables it to offer high switching frequencies even with high step-down ratios. For example, even with inputs as high as 35 V, it can deliver 3.3 V and 1.1 V outputs using a 2 MHz switching frequency, allowing it to avoid critical frequency bands such as AM radio while minimizing the size of external components.

The LT3641 can deliver output voltages as low as 1.26 V