CPUs That Require More Than One Voltage

Newer CPUs will require more than one voltage. Even though all CPUs from AMD have
an integrated memory controller, only socket AM3 CPUs require a separated voltage
for this circuit. So on socket AM3 motherboards the voltage regulator circuit will
generate two separated voltages for the CPU, one for the "main" part of the CPU
("Vcore") and another for the integrated memory controller. That is why we knew, in
Figure 15, that the extra phase was for feeding the CPU integrated memory
controller: because that was a socket AM3 board.

With Intel CPUs, only socket LGA1156 and socket LGA1366 CPUs have an integrated
memory controller. So on these motherboards the voltage regulator circuit will
generate two voltages, one for the "main" part of the CPU ("Vcore") and another for
the integrated memory controller ("VTT"). On socket LGA1156 motherboards supporting
CPUs with integrated video controller (e.g., the ones based on H55 and H57
chipsets) the voltage regulator circuit will generate a third voltage for the CPU,
to be used by the integrated video controller ("VAXG").

On motherboards where the voltage regulator circuit provides more than one voltage
to the CPU, the manufacturer will refer to it like "x+y" or "x+y+z", where "x" is
the number of phases for the CPU main voltage ("Vcore"), "y" is the number of
phases for the CPU integrated memory controller and "z" is the number of phases for
the CPU integrated video controller. The motherboard shown on Figures 14 and 15 had
a "3+1" configuration, for example.

Below we summarize what kind of motherboard feeds the CPU socket with more than one
voltage.
Socket Voltages for the CPU
754, 939, 940, AM2, AM2+, 775 and older One
AM3, 1156, 1366 Two
1156 with H55, H57 and Q57 chipsets Three

Although in this tutorial we focused on the voltages required by the CPU, all
motherboards will have at least one phase for feeding the memories and one phase
for feeding the chipset. If you look around you will be able to spot these phases
(see Figure 18), unless when the memory phase is placed close to the CPU phases,
like it happened on the example from Figure 12.

Other Phases

Everything You Need to Know About The Motherboard Voltage Regulator Circuit
By
Gabriel Torres -
February 10, 2010 10525
SHARE
Facebook
Twitter
How it Works

The voltage regulator circuit gets the +12 V voltage present on the ATX12V or
EPS12V connector found on the motherboard and converts it to the voltage required
by the component that the voltage regulator is connected to (CPU, memory, chipset,
etc). This conversion is done using a DC-DC converter, also known as switching-mode
power supply (SMPS), the same system used inside the PC main power supply.

The heart of this converter is the PWM (Pulse Width Modulation) controller. This
circuit generates a square-wave signal that will drive each phase, with the duty
cycle from this signal varying depending on the voltage that the circuit wants to
produce (duty cycle is the amount of time the signal stays on its higher value; for
example, a signal with 50% duty cycle will spend half the time on its lower value –
usually zero volt – and the other 50% of the time on its higher value — which means
+12 V on the case of the voltage regulator circuit.

The value of the output voltage the voltage regulator circuit must produce is read
from the CPU “voltage ID” (VID) pins, which provide a binary code with the exact
voltage that must be supplied. Some motherboards allow you to manually change the
CPU voltage inside the motherboard setup program. What the setup does is to change
the code that is read by the PWM controller, so the controller will change the CPU
voltage according to what you’ve configured. Even though we are talking about the
CPU, the same idea applies for the memory and the chipset.

The DC-DC converter is a closed loop system. This means that the PWM controller is
constantly monitoring the outputs of the voltage regulator. If the voltage on the
output increases or decreases the circuit will readjust itself (changing the
frequency of the PWM signal) in order to correct the voltage. This is done through
a current sensor, since when current consumption increases the output voltage tends
to decrease and vice-versa.

In Figure 17 we have the block diagram of a PWM controller usually found on the CPU
voltage regulator circuit (NCP5392 from On Semiconductor). On this block diagram
you can easily identify the voltage ID pins (VID0 through VID7), the loopback pins
(CS, Current Sensor pins, located on the left side) and the outputs to drive each
phase (G pins, located on the right side). As you can see, this integrated circuit
can control up to four phases.

PWM ControllerFigure 17: PWM controller.

Each phase uses two transistors and one choke. The PWM controller does not provide
enough current to switch these transistors, so a MOSFET driver is required for each
phase. Usually this driver is made with a small integrated circuit. In other to cut
costs some manufacturers use a discrete driver using an additional transistor on
very low-end motherboards.

In Figure 18, you can see the basic schematics of one phase from a motherboard (the
loopback connection is missing on this diagram) driven by an NCP5359 MOSFET driver.
The driver and the MOSFET transistors will be fed by the +12 V voltage provided on
the ATX12V or EPS12V connector (where it is written “10 V to 13.2 V” and “4 V to 15
V”). You can see on this diagram the two MOSFETs (the top one is the "high side"
and the bottom one is the "low side"), the choke and the capacitors. The loopback
signal will be provided by linking two wires connected in parallel to the choke to
the PWM controller CS+ (CSP) and CS- (CSN) pins. The PWM pin is connected to the
PWM output provided by the PWM controller and the EN pin is the “enable” pin, which
activated the circuit.

MOSFET driverFigure 18: Phase simplified schematics.

As you can see in Figure 17, there is one PWM output for each phase. As explained,
the PWM signal is a square waveform where its width (duty cycle) changes depending
on the voltage you want (that is why this technique is called Pulse Width
Modulation). Assuming that the output voltage is stable, all PWM signals will have
the same duty cycle, i.e., the size of each “square” on the signal will be the
same. These signals will, however, have a delay between them. This delay is also
known as phase-shift.

For example, on a circuit with just two phases, the two PWM signals will be
mirrored. So while phase 1 is turned on, phase 2 will be turned off and vice-versa.
This will ensure that each phase will work 50% of the time. On a circuit with four
phases, the PWM signals will be delayed in such way that phases will be activated
in sequence: first phase 1 is activated, then phase 2, then phase 3 and then phase
4. While one phase is turned on all others are turned off. In this case, each phase
will be working 25% of the time.

The more phases you have, less time each phase will be turned on. As explained
earlier, this makes each phase to dissipate less heat and each transistor to work
less, which provides a higher life-span to this component.
Back to: Introduction