VRM/VRD Power Management ICs

The Voltage Regulator Module (VRM) concept was developed by Intel to guide the design of dc-dc converters that supply the required voltage and current to a Pentium® microprocessor. The maximum voltage is determined by the five-bit VID (Voltage Identity) code provided to the VRM, as shown in Table 1. The five-bit, five-pin VID code connects the power supply controller to the corresponding pins on the microprocessor. Therefore, the internal coding in the microprocessor controls the dc voltage applied to processor. VRM guidelines are intended for a special module, usually a small circuit board, that plugs into the computer system board and supplies power for the microprocessor.

A later version of guidelines are for a similar circuit called the Voltage Regulator-Down (VRD) developed by Intel to guide the design of a voltage regulator integrated onto the computer system motherboard with a single processor. These guidelines are based on the six-bit VID code shown in Table 2.

Multiphase Converters
At the present time and in the near future the VRM and VRD circuits must provide 60A to 90A for the Intel microprocessors. At this time, the only practical circuit that can provide those current levels is the multiphase configuration. Multiphase converters employ two or more identical, interleaved converters connected so that their output is a summation of the outputs of the cells, as shown in Fig. 1. Each of the multiphase converters described includes either a five-bit or six-bit VID code, so they are applicable for VRM/VRD applications.

Multiphase cells operate at a common frequency, but are phase shifted so that conversion switching occurs at regular intervals controlled by a common control chip. The control chip staggers the switching time of each converter so that the phase angle between each converter switching is 360°/n, where n is the number of converter elements. The outputs of the converters are paralleled so that the effective output ripple frequency is n × f, where f is the operating frequency of each converter. This provides better dynamic performance and significantly less decoupling capacitance than a single phase system.

Current sharing among the cells is necessary so that one does not hog too much current. Ideally, each multiphase cell should consume the same amount of current. To achieve equal current sharing the output current for each cell must be monitored and controlled.

The multiphase approach also offers packaging advantages. Each converter delivers 1/n of the total output power, reducing the physical size and value of the magnetics employed in each phase. Also, the power semiconductors in each phase only need to handle 1/n of the total power. This spreads the internal power dissipation over multiple power devices, eliminating the concentrated heat sources and possibly the need for a heat sink. Even though this uses more components, its cost tradeoffs can be favorable.

As current requirements increase, so does the need for increasing the number of phases in the converter. ICs providing just two phases are not adequate because of their limited output current range. An optimum design requires tradeoffs between the number of phases, current per phase, switching frequency, cost, size and efficiency. Higher output current and lower voltage require tighter output voltage regulation.

1. This multiphase converter and gate driver IC work together to form an efficient synchronous buck switching regulator to meet Intel's VRD/VRM 10 power specifications. These devices have been optimized for converting a 12V main supply into the highly accurate core supply voltage required<