MAX8688
Digital Power-Supply Controller/Monitor
with PMBus Interface
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Performance Consideration
The MAX8688 can be viewed as a task scheduler
where it periodically goes through its task list and per-
forms the required tasks. While it may be tempting to
monitor the POL at the highest supported frequency,
MFR_SAMPLE_RATE, it must be noted that doing so
takes away resources from other tasks, such as com-
munication with the system controller or adjusting the
DACOUT voltage. The same is true with employing an
increasing number of MFR_SAMPLE_RATE and
MFR_FILTER_MODE parameters. Since each applica-
tion is unique in its own merit, it is impossible to pre-
scribe a solution that suffices in all conditions. System
designers are thus urged to derive optimum configura-
tion based on the particular system needs.
Graphic User Interface (GUI)
The MAX8688 evaluation kit comes with a free GUI that
eliminates the need for any software development and
provides a simple and user-friendly method for configur-
ing large systems in a short time. Once the configuration
is complete, the results can be saved to the external
EEPROM for MAX8688 configuration on power-up or
loaded at power-up onto the MAX8688 through the
PMBus by a master controller. The powerful MAX8688
feature set can be inferred from the GUI screenshot
shown in Figure 17, where the programmable parame-
ters are displayed. These parameter values set serve as
data bytes for PMBus and manufacturer-specific com-
mands supported by the MAX8688. For details on using
the GUI, refer to the MAX8688 evaluation kit.
PCB Considerations
PCB layout for the MAX8688 is simple. It is easy to
achieve accurate voltage and current measurement
and voltage regulation by following these guidelines for
good PCB layout:
1) Place IC decoupling and filter capacitors for AVDD,
DVDD, REFO, RS_C, and ISN_C as close to the IC
pins as possible. If using an external EEPROM, place
it close to the MAX8688 and use short direct traces for
interconnections.
2) Use Kelvin connections for the traces from the ISN+
and ISN- to the current-sense resistor for accurate
current sensing. In the case of DCR sensing (see the
Current Sensing
section), connect the RC filter
across the POL output inductor terminals using
Kelvin connections. Route differential pair traces from
the capacitor of the RC filter to ISN+ and ISN-.
3) Use Kelvin connections for the differential pair traces
from the desired remote-sense points on the POL
output-voltage power plane to RS+ and RS- of the
MAX8688 for accurate POL output-voltage sensing.
4) Connect the analog ground (AGND) and digital
ground (DGND) of the MAX8688 to a ground plane
right at the IC. Terminate all other ground connec-
tions to this ground plane. Connect this ground
plane to the quiet analog ground plane of the POL
so that the reference voltage to the POL is unaffect-
ed by switching noise. Use a single-point (Star)
grounding technique to connect the analog ground
plane of the POL to the heat dissipating power
ground plane of the POL. Place the MAX8688 as
close as possible to the POL for best temperature
measurement performance.
5) Refer to the MAX8688 evaluation kit for sample layout.
