Voltage regulator module

Voltage regulator module for an IBM Netfinity 7000 M10 running an Intel Xeon 500 MHz processor

A voltage regulator module or VRM, sometimes called PPM (processor power module), is a buck converter that provides a microprocessor the appropriate supply voltage, converting +5 V or +12 V to a much lower voltage required by the CPU. Some are soldered to the motherboard while others are installed in an open slot. It allows processors with different supply voltage to be mounted on the same motherboard. Most modern CPUs require less than 1.5 volts. CPU designers tend to design to smaller CPU core voltages; lower voltages help reduce CPU power dissipation, often referred to as TDP or Thermal Design Power

Some voltage regulators provide a fixed supply voltage to the processor, but most of them sense the required supply voltage from the processor, essentially acting as a continuously-variable adjustable regulator. In particular, VRMs that are soldered to the motherboard are supposed to do the sensing, according to the Intel specification.

Modern Graphics processing units (GPU)s also use VRM due to a need of more power and high current.

Voltage identification

The correct supply voltage is communicated by the microprocessor to the VRM at startup via a number of bits called VID (voltage identification). In particular, the VRM initially provides a standard supply voltage to the VID logic, which is the part of the processor whose only aim is to then send the VID to the VRM. When the VRM has received the VID identifying the required supply voltage, it starts acting as a voltage regulator, providing the required constant voltage supply to the processor.

Instead of having a power supply unit generate some fixed voltage, the CPU uses a small set of digital signals, the VID lines, to instruct an on-board power converter of the desired voltage level. The switch-mode buck converter then adjusts its output accordingly. The flexibility so obtained makes it possible to use the same power supply unit for CPUs with somewhat different nominal supply voltages and to reduce power consumption during idle periods by lowering the supply voltage.

For example, a unit with 5-bit VID would output one of at most 32 (2⁵) distinct output voltages. These voltages are usually (but not always) evenly spaced within a given range. Some of the code words may be reserved for special functions such as shutting down the unit, hence a 5-bit VID unit may have fewer than 32 output voltage levels. How the numerical codes map to supply voltages is typically specified in tables provided by component manufacturers. As of 2008 VID comes in 5-, 6- and 8-bit varieties and is mostly applied to power modules outputting between 0.5V and 3.5V.

External links

• "Microprocessor Power Management"