MAX1298/MAX1299
12-Bit Serial-Output Temperature Sensors
with 5-Channel ADC
16 ______________________________________________________________________________________
Place the MAX1298/MAX1299 in a low-current power-
down state between conversions to conserve power.
Select standby, standby-plus, or shutdown through bits
PM1 and PM0 of the initialization byte (Table 4).
The MAX1298/MAX1299 assume the shutdown power
mode when V
DD
is first applied.
Standby Mode
Standby mode turns off the MAX1298/MAX1299 ADC,
internal clock, and reference buffer amplifier. Special
circuitry for temperature conversions is also deactivat-
ed. Wake-up time is limited by the reference buffer
amplifier and the associated bypass capacitor (see
Internal Reference
). When an external reference is
used, wake-up time is 0.1ms.
Standby-Plus Mode
Standby-plus mode is similar to the standby mode, but
the internal reference output buffer remains active to
shorten the wake-up time to 0.1ms for internal refer-
ence mode. When using an external reference, stand-
by-plus mode is equivalent to standby mode.
Shutdown Mode
Shutdown mode turns off all functions other than start-
up circuitry, thereby reducing typical supply current to
2µA. Data registers are cleared. Use this power mode
when interconversion times are no less than 5ms.
Monitoring V
DD
This mode of operation samples and converts the sup-
ply voltage, V
DD
/4, which is internally generated. The
reference voltage must be larger than V
DD
/8 for the
operation to work properly. From the result of a conver-
sion (CODE), CODE = 256 V
DD
/ V
REF
.
Temperature Measurements
The MAX1298/MAX1299 perform temperature measure-
ments with internal or external diode-connected transis-
tors through a three-step process. First, the diode bias
current changes from 31.6µA to 10µA to produce a
temperature-dependent bias voltage difference, which
is amplified by a factor of 20 and converted to digital
format. Second, the bias current changes from 31.6µA
to 100µA, and the bias voltage difference is similarly
amplified by a factor of 20 and converted to digital for-
mat. Third, the intermediate results are subtracted to
achieve a digital output that is proportional to absolute
temperature in degrees Kelvin.
The reference voltage used in conjunction with tempera-
ture measurements is derived from the internal reference
source to ensure that 1LSB corresponds to 1/8 of a
degree. To convert to degrees Celsius, subtract 273.15
from the temperature inferred from the ADC output.
Temperature measurements require a conversion time
of 2.2ms.
Shield Output Buffer
The MAX1298/MAX1299 provide a shield output buffer
voltage at SHO that is approximately 0.6V (one diode
drop) above V
DD
/2. When performing temperature
measurements with an external diode, use this voltage
to suppress error-producing leakage currents (see
Remote Diode Shielding
). Figure 7 shows the SHO out-
put circuit.
Applications Information
Remote Diode Selection
Temperature accuracy depends on having a good-
quality, diode-connected small-signal transistor.
Accuracy has been experimentally verified for 2N3904
devices. CPUs and other ICs having on-board temper-
ature-sensing diodes can also be monitored if the
diode connections are uncommitted.
The transistor must be a small-signal type with a base
resistance less than 100Ω. Tight specifications for for-
ward current gain (+50 to +150, for example) indicate
that the manufacturer has good process controls and
that the devices have consistent V
be
characteristics.
(See Table 6 for recommended devices.)
For heatsink mounting, the 500-32BT02-000 thermal
sensor from Fenwal Electronics is a good choice. This
device consists of a diode-connected transistor, an alu-
minum plate with screw hole, and twisted-pair cable
(Fenwal Inc., Milford MA, 508-478-6000).
Table 4. Power-Mode Selection
