MAX8524/MAX8525
Traces from the load point back to RS+ and RS- should
be routed close to each other and as far away as possi-
ble from noise sources (such as inductors and high
di/dt traces). Use a ground plane to shield the remote-
sense traces from noise sources. To filter out common-
mode noise, RC filtering is recommended for these pins
as shown in Figure 3. For VRD applications, a 100Ω
resistor with a 470pF capacitor should be used. For
VRM applications, additional 50Ωresistors should be
connected from these pins to the local outputs of the
converter before the VRM connector. This avoids
excessive voltage at the CPU in case the remote-sense
connections get disconnected.
Loop Compensation (COMP)
During a load transient, the output voltage instantly
changes due to the ESR of the output capacitors by an
amount equal to their ESR times the change in load
current (ΔVOUT = -RESR_CO x ΔILOAD). The voltage-
positioning method allows better utilization of the output
regulation window, resulting in fewer output capacitors.
The MAX8524/MAX8525 employ rapid-active average
scheme, a proprietary current-mode architecture that
adjusts the output current based on instantaneous out-
put voltage, resulting in fast voltage positioning.
The voltage-error amplifier consists of a high bandwidth
and high-accuracy transconductance amplifier (GMV).
See the Functional Diagram. The negative input of the
transconductance amplifier is connected to the output
of the remote-voltage differential amplifier, and the pos-
itive input is connected to the output of an internal DAC
controlled by VID inputs. The DC gain of the transcon-
ductance amplifier is set to a finite value to achieve fast
output-voltage positioning by connecting an equivalent
resistor, RE, from the COMP pin to GND (RE= RU//RB).
The value of REis determined by the amount of droop
required at full load, which is specified as the output
impedance or the load line in Intel VRM specifications.
According to the Intel VRM specifications, the output
voltage at no load cannot exceed the voltage specified
by the VID code, including the initial set tolerance, rip-
ple voltage, and other errors. Therefore, the actual out-
put voltage should be biased lower to compensate for
these errors. Connect a resistor-divider, RUand RB,
from REF to GND, with the tap connected to COMP, to
set the offset voltage.
For 6- or 8-phase operations, connect COMP pins of
the two controllers together for active current sharing.
Dynamic VID Change (MAX8525 Only)
The MAX8525 offers the ability to dynamically change
the VID inputs while the controller is operating (on-the-
fly, or OTF). This feature allows the processor to adjust
its core voltage in a 250mV window. The MAX8525 out-
put voltage changes in 12.5mV steps when a VID
change is detected.
The VID inputs of the MAX8525 comply with Intel’s 400ns
logic-skew timing specifications to prevent false code
changes. Once the timer expires, the controller starts to
change the DAC output. Figure 4 shows the output volt-
age step during a VID OTF event. The MAX8525 con-
troller accepts both step-by-step changes of VID inputs
or all-at-once VID inputs changes. For all-at-once VID
input changes, the output-voltage slew rate is the same
as 12.5mV per step and 2µs duration.
Paralleling Operation (CLKI and CLKO)
Two MAX8524/MAX8525s can be connected together
to generate 6-phase or 8-phase core supplies. In this
configuration, one MAX8524/MAX8525 serves as a
master and the other serves as a slave. Connect the
CLKI pin of the slave controller to the CLKO pin of the
master controller. Interleaved operation is achieved by
synchronizing the master controller to the CLKO rising
edge and the slave controller to the CLKO falling edge.
Figure 5 shows the clock timing between the phases of
both master and slave controllers.
2-Phase and 3-Phase Operation Selection
(PWM3 and PWM4)
The MAX8524/MAX8525 can operate in 2-, 3-, and
4-phase operation. Connect PWM4 to VCC for 2-, 3-, or
6-phase operation. Also connect PWM2 to VCC for
2-phase operation. All PWM outputs are held low dur-
ing shutdown.
14 ______________________________________________________________________________________
2- to 8-Phase VRM 10/9.1 PWM Controllers
with Precise Current Sharing and Fast Voltage
Positioning