Rainbow Electronics MAX1618 - Manual (Page 12)

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 100
Ω. 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
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
12 ______________________________________________________________________________________
0011 001 High-Z GND
0011 000
ADDRESS
0101 001 GND High-Z
0011 010 V CC GND
0101 011 V CC High-Z
0101 010
1001 101 High-Z V CC
1001 100
GND GND
GND V CC
High-Z High-Z
1001 110 V CC V CC
ADD0 ADD1
Table 6. Slave Address Decoding
(ADD0 and ADD1)
Note: High-Z means the pin is left unconnected and floating.
Note: Transistors must be diode-connected (short the base to
the collector).
SMBT3904 Siemens (Germany)
CMPT3904 Central Semiconductor (USA)
MMBT3904 Fairchild Semiconductor (USA)
SST3904 Rohm Semiconductor (Japan)
FMMT3904CT-ND Zetex (England)
MANUFACTURER MODEL NUMBER
Table 7. SOT23 Type Remote-Sensor
Transistor Manufacturers
MMBT3904 Motorola (USA)
Pentium II is a registered trademark of Intel Corp.