MAX8688
Digital Power-Supply Controller/Monitor
with PMBus Interface
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Temperature Sensing
It is intended that the MAX8688 be placed in close prox-
imity to the POL. An on-chip temperature sensor on the
MAX8688 senses the temperature of the die, which is
related to the exposed pad temperature of the MAX8688
by the junction-to-case thermal resistance. The exposed
pad of the MAX8688 can be connected to the heat dis-
sipating ground plane of the POL, and the POL board
may be characterized to obtain the relationship between
the POL temperature and temperature as measured by
the MAX8688. This information may be used to set
overtemperature fault settings in the MAX8688.
External EEPROM Interface
The MAX8688 is capable of communicating with an
EEPROM attached to the A1/SCLE and A2/SDAE. The
MAX8688 communicates to the EEPROM with an
address byte of “1010 0000” for writing and “1010 0001”
for reading. For the data values of 2 bytes, the most sig-
nificant byte is stored in the lower offset, whereas the
least significant byte is stored in the higher offset.
Upon reset, the MAX8688 tests for the presence of a
configuration EEPROM. It searches for the SIGNATURE
bytes in the attached EEPROM. If the SIGNATURE
bytes are present, it concludes that it has a valid con-
figuration EEPROM and starts reading configuration
information from the attached EEPROM. If slave
address information is present, this overrides the slave
address information previously set by the address
A3:A1 pins.
Table 1 shows the contents and offsets of the configu-
ration information expected by the MAX8688. This infor-
mation is for reference only. It is recommended to use a
properly configured, working MAX8688 and to save its
state to the EEPROM and limit modifications to as few
fields as possible (such as the slave address).
Some ‘reserved’ fields may contain data other than 0
when the state is saved to the EEPROM. These locations
are ignored on restoration from the EEPROM or are fre-
quently recomputed. Some reserved fields need to be set
to greater than 0 to guarantee proper operation timing.
Temperature, voltage, and current values are stored in
internal representation, which is not identical to the for-
mat used by the corresponding PMBus command(s).
For details on EEPROM internal representation, see the
notes following Table 1.
For example, to store to the EEPROM, VOUT_COMMAND
= 3.0V, m = 19995, b = 0, R = -1. First calculate the
PMBus command value, which is 5998. If the voltage
range is 2V, no conversion is required. Hence write 17h
to offset 14 and 6Eh to offset 15. If the voltage range is
5.5V, the stored EEPROM value = 5998/2.75 = 2181. So
at offset 14, write 08h and offset 15, write 85h.
Note that the conversion is automatically handled by
the MAX8688 as it restores and stores configuration
information into the EEPROM.
