MAX1618 Datasheet, PDF (12/20 Pages) Maxim Integrated Products – Remote Temperature Sensor with SMBus Serial Interface

Remote Temperature Sensor
with SMBus Serial Interface
Table 6. Slave Address Decoding
(ADD0 and ADD1)
Table 7. SOT23 Type Remote-Sensor
Transistor Manufacturers
ADD0
ADD1
ADDRESS
GND
GND
0011 000
GND
High-Z
0011 001
GND
V_CC
0011 010
High-Z
GND
0101 001
High-Z
High-Z
0101 010
High-Z
V_CC
0101 011
V_CC
GND
1001 100
V_CC
High-Z
1001 101
V_CC
V_CC
1001 110
Note: High-Z means the pin is left unconnected and floating.
MANUFACTURER
MODEL NUMBER
Central Semiconductor (USA)
CMPT3904
Fairchild Semiconductor (USA)
MMBT3904
Motorola (USA)
MMBT3904
Rohm Semiconductor (Japan)
SST3904
Siemens (Germany)
SMBT3904
Zetex (England)
FMMT3904CT-ND
Note: Transistors must be diode-connected (short the base to
the collector).
Slave Addresses
The device address can be set to one of nine different
values by pin-strapping ADD0 and ADD1 so more than
one MAX1618 can reside on the same bus without
address conflicts (Table 6).
The address pin states are checked at POR only, and
the address data stays latched to reduce quiescent
supply current due to the bias current needed for high-
impedance (high-Z) state detection.
The MAX1618 also responds to the SMBus Alert
Response slave address (see the Alert Response
Address section).
POR and UVLO
The MAX1618 has a volatile memory. To prevent ambigu-
ous power-supply conditions from corrupting the data in
the memory and causing erratic behavior, a POR voltage
detector monitors V_CC and clears the memory if V_CC falls
below 1.7V (typical, see the Electrical Characteristics
table). When power is first applied and V_CC rises above
1.75V (typ), the logic blocks begin operating, although
reads and writes at V_CC levels below 3V are not recom-
mended. A second V_CC comparator, the ADC UVLO com-
parator, prevents the ADC from converting until there is
sufficient headroom (V_CC = 2.8V typ).
Power-Up Defaults:
• Interrupt latch is cleared.
• Address select pins are sampled.
• Command byte is set to 01h to facilitate quick
remote Receive Byte queries.
• T_HIGH and T_LOW registers are set to max and min
limits, respectively.
• Device is in normal mode. (ALERT acts as a latched
interrupt output.)
Applications Information
Remote Diode Selection
Temperature accuracy depends on having a good-
quality, diode-connected, small-signal transistor.
Accuracy has been experimentally verified for all of the
devices listed in Table 7. The MAX1618 can also direct-
ly measure the die temperature of CPUs and other inte-
grated circuits with on-board temperature sensing
diodes, such as the Intel Pentium II^®.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the A/D input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process controls and that the devices
have consistent V_BE characteristics. Do not use power
transistors.
ADC Noise Filtering
The integrating ADC has inherently good noise rejec-
tion, especially of low-frequency signals such as
60Hz/120Hz power-supply hum. Micropower operation
places constraints on high-frequency noise rejection.
Lay out the PCB carefully with proper external noise fil-
tering for high-accuracy remote measurements in elec-
trically noisy environments.
Filter high-frequency electromagnetic interference
(EMI) at DXP and DXN with an external 2200pF capaci-
tor connected between the two inputs. This capacitor
can be increased to about 3300pF (max), including
cable capacitance. A capacitance higher than 3300pF
Pentium II is a registered trademark of Intel Corp.
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