National Semiconductor Corporation introduced a pair of 3-in-1 power-supply voltage controllers fully compliant with the latest VRM DC-DC converter specification for Whitney or Camino-based PC motherboards.

By integrating a 5-bit synchronous step-down switching controller and two linear regulator controllers on a single IC, Nationalıs LM2637 and LM2638 each provide control and protection for three voltages. The devices are fully compliant with the latest VRM DC-DC converter specifications for motherboards in desktop PCs, workstations and servers.

ıAs the number of power supplies required on the motherboard continues to grow, PC designers face new challenges,ı said Venkatesh Shan, director of Nationalıs Power Management Product Group. ıThe LM2637 and LM2638 will provide a minimum of six different voltages as required for Whitney or Camino-based motherboards. The major differences between Nationalıs chips and other controllers include a suite of options in the LM2638 that allow it to support the implementation of the Instantly Available PC,ı he added.

A key addition in the LM2638 is a built-in charge pump that eliminates the need for a similar external device. A charge pump oscillator pin replaces the LM2637ıs over-voltage protection pin, while maximum PWM duty cycle is increased from 95% to 100% to reduce loading to the charge pump during standby mode.

Both chips offer two user-selectable over-current protection modes; one maximizes accuracy, the other minimizes cost. The switching section of the LM2637 and LM2638 is designed to supply the MPU core or other high-current loads with a programmable voltage from 3.3V to 1.3V. Operating frequency can also be adjusted between 50kHz and 1MHz through an external resistor, giving the power supply designer the flexibility to make tradeoffs between load transient response and efficiency.

A complete power solution for $3 for the Whitney or Camino motherboards is not to be sneezed at. Providing six different voltages for the motherboards, with three from each part, they use a step-down switching controller for the highest voltage in each case, and then provide two linear regulators for the remaining two voltages. While the latter regulators do not give the highest possible efficiency for those two voltages, the overall efficiency is determined more by that of the controller anyway. The addition of charge-pump to the LM2638 eliminates yet another external part.

The provision of a programmable operating frequency between 50 kHz and 1 MHz will allow the motherboard designer to trade the needs of the individual product: a portable product, for example, might well go for the higher frequency to maximize efficiency and minimize external components, while trading off transient response. The "power-good" signal may also eliminate the need for some supervisors on the motherboard as well.