By JEFF JULL,
Monolithic Power Systems (MPS)
www.monolithicpower.com

 

 

Audible noise in voltage regulator
(VR) systems has been a problem for a very long time. In the PC industry, the
issue became more pronounced as the CPU became responsible for significant and
repetitive voltage changes that induced noise through the VR. Those voltage
changes, along with the physical properties of ceramic capacitors and the
motherboard, produce an audible noise problem for the PC manufacturer with no
good solutions so far.

 

Ceramic capacitors are commonly used
in the decoupling of the VR input and output stages because of their low cost
and small size. The piezoelectric property of ceramic capacitors results in
movement within the component body when a voltage change is induced. With a
voltage change in one direction, the capacitor flexes one way, and then flexes
in the opposite direction when the voltage change is reversed. When the voltage
is changed repetitively within the audible frequency range, those ceramic
capacitors follow a repetitive flexing. However, this alone is not enough to
create noise. The flexing capacitors act like the voice coil in a speaker
system. The voice coil moves the cone, and the cone actually creates the sound.

 

The motherboard is the cone in our
speaker analogy. The motherboard is secured inside of a shell by several
points, but there is typically enough unsecured board area to flex. When enough
ceramic capacitors are flexing together, they can flex the motherboard quite
easily within the shell vertically enough to create audible noise.

 

The final component needed to create
audible noise in a VR is the repetitive voltage change. For many years now, the
CPU has managed its own performance, frequency, thermals, and power consumption
dynamically. A significant part of this management has been through adjusting
the input voltage to the CPU. In high performance needs, the voltage is
increased. When not needed for high performance, the voltage is reduced to reduce
the leakage current within the CPU, thus saving power. These voltage changes
are addressed to resolve audible noise in a PC.

 

Figure 1 shows
an example of voltage identification (VID) changes from the CPU and the voltage
response from the VR. A higher voltage is required for more performance, and
then the voltage is lowered to reduce leakage current.

 

MPS Smart-Ramp Tech_Fig1

 

Figure 1: VID Changes from CPU and the VR Vout
Voltage Response 2

 

 

The MPS Smart-Ramp audible noise
reduction technology is shown in Figure
2
. If the new VID from the CPU is both lower than the present VID and a
voltage step greater than a value defined in a register X, then the beginning
of the voltage ramp down is delayed for a duration defined in register Y.
Figure 2 shows an example of a short delay which may be enough of a change to
disrupt the flexing of the motherboard and thus reduce audible noise. The
operation of the CPU and the commands coming from the CPU are unchanged.

 

MPS Smart-Ramp Tech_Fig2

 

 

Figure 2: MPS Smart-Ramp Audible Noise
Reduction Technology

 

 

Another implementation of the MPS
solution is to extend the delay duration of the ramp down until the next VID
command to a higher level has been received. When the repetitive voltage change
has been removed completely, there is no more audible noise (see Figure 3).

 

 

MPS Smart-Ramp Tech_Fig3

 

Figure 3: Voltage-Induced Audible Noise Removed

 

 

As previously stated, the advantage
of voltage reduction to the CPU is a reduced leakage current, so the Smart-Ramp
solution impacts that power saving feature, albeit minimally. The greater power
savings are in the CPU operating in lower C-states. The Smart-Ramp technology
does not interfere with the CPU’s ability to enter its own power-saving
C-states. The only power impact is an increased leakage current into the CPU
during the short durations when the voltage would have been low if the voltage
output had followed every VID command without delay.  

 

However, changing the voltage also
has a power cost which would need to be subtracted from the increased leakage
power to understand the full power impact. When the VID is reduced, the charge
is discarded (wasted) by the system by forcing that charge to ground. Then,
additional power is required to re-charge the output when the VID is increased
again. In some systems, the power cost of discharging and re-charging the
output could be more than the leakage power seen when using the MPS Smart-Ramp
audible noise reduction solution.

 

Additionally, it is only the short
periods at the lower voltage during noise-causing events where the leakage current
is higher. Once the repetitive voltage changes stop, there is one last delay
before the MPS Smart-Ramp technology sets the voltage to the lower VID to save
that leakage current during the intended long durations of CPU power-saving
states.

 

The configurability of the MPS
Smart-Ramp technology means customers have the option to be as conservative or
aggressive as they desire. In one model, a customer may wait until a noise
issue is discovered and then re-program the MPS VR controller through a BIOS change
to activate and configure the feature to address their specific noise issue.
Another method would be to proactively configure the VR controller for all but
the smallest of VID changes coupled with a long delay setting. This would
remove all voltage change-induced noise from the VR, but would obviously come
with the potential for a larger power impact.

 

Conclusion

 

Voltage level changes to the
processor input power are a necessary power-saving feature that will likely be
used for many PC generations to come. When those changes result in audible
noise in the PC, manufacturers have had few and expensive options to make their
platforms viable for the marketplace. The Smart-Ramp audible noise reduction
technology provides PC manufacturers with an effective option that is easily
implemented.