SS8018TR SSC [Silicon Standard Corp.], SS8018TR Datasheet - Page 5

• Manufacturer Part Number: SS8018TR
• Manufacturer: SSC [Silicon Standard Corp.]

Rev.2.01 6/06/2003
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The SS8018 is a temperature sensor designed to work
in conjunction with an external microcontroller (µC) or
other intelligence in thermostatic, process-control or
monitoring applications. The µC is typically a power-
management or keyboard controller, generating SMBus
serial commands by “bit-banging” general-purpose in-
put-output (GPIO) pins or via a dedicated SMBus inter-
face block.
Essentially a serial analog-to-digital converter (ADC)
with a sophisticated front end, the SS8018 contains a
switched current source, a multiplexer, an ADC, an
SMBus interface and associated control logic (Figure 1).
Temperature data from the ADC is loaded into two data
registers, where it is automatically compared with data
previously stored in several over/under-temperature
alarm registers.
ADC and Multiplexer
The ADC is an averaging type that integrates over a
60ms period (each channel, typical), with excellent noise
rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes, measures their
forward voltages, and computes their temperatures.
Both channels are automatically converted once the
conversion process has started, either in free-running or
single-shot mode. If one of the two channels is not used,
the device still performs both measurements, and the
user can simply ignore the results of the unused channel.
If the remote diode channel is unused, tie DXP to DXN
rather than leaving the pins open.
The worst-case DXP-DXN differential input voltage
range is 0.25V to 0.95V.
Excess resistance in series with the remote diode
causes about +0.6°C error per ohm. Likewise, 240µV of
offset voltage forced on DXP-DXN causes about 1°C
error.
A/D Conversion Sequence
If a Start command is written (or generated automatically
in the free-running auto-convert mode), both channels
are converted, and the results of both measurements
are available after the end of conversion. A BUSY status
bit in the status byte shows that the device is actually
performing a new conversion; however, even if the ADC
is busy, the results of the previous conversion are al-
ways available.
Remote Diode Selection
Temperature accuracy depends on having a good- qual-
ity,
SS8018 can also directly measure the die temperature
APPLICATIONS INFORMATION
diode-connected
small-signal
www.SiliconStandard.com
transistor.
The
of CPUs and other integrated circuits having on-board
temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively high forward voltage; otherwise, the A/D input volt-
age range can be violated. The forward voltage must be
greater than 0.25V at 10µA; check to ensure this is true
at the highest expected temperature. The forward
voltage must be less than 0.95V at 300µA; check to en-
sure this is true at the lowest expected temperature.
Large power transistors don’t work at all. Also, ensure
that the base resistance is less than 100 . Tight speci-
fications for forward-current gain (+50 to +150, for ex-
ample) indicate that the manufacturer has good process
controls and that the devices have consistent V
characteristics.
Table 1. Remote-Sensor Transistor Manufacturers
Philips
Motorola(USA)
National Semiconductor (USA)
Note: Transistors must be diode-connected (base
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the SS8018’s
effective accuracy. The thermal time constant of the
SOP package is about 140 seconds in still air. For the
SS8018 junction temperature to settle to within +1°C
after a sudden +100°C change requires about five time
constants or 12 minutes. The use of smaller packages
for remote sensors, such as SOT23s, improves the
situation. Take care to account for thermal gradients
between the heat source and the sensor, and ensure
that stray air currents across the sensor package do not
interfere with measurement accuracy. Self-heating does
not significantly affect measurement accuracy. Re-
mote-sensor self-heating due to the diode current source
is negligible. For the local diode, the worst-case error oc-
curs when auto-converting at the fastest rate and simul-
taneously sinking maximum current at the
For example, at an 8Hz rate and with
1mA, the typical power dissipation is VCC x 320µA plus
0.4V x 1mA. Package R(J-A) is about 120°C /W, so with
VCC = 3.3V and no copper PC board heat-sinking, the
resulting temperature rise is:
Even with these contrived circumstances, it is difficult to
introduce significant self-heating errors.
shorted to collector).
MANUFACTURER
dT = 1.45mW x 120°C /W = 0.17°C
PMBS3904
MMBT3904
MMBT3904
MODEL NUMBER
ALERT
ALERT
SS8018
sinking
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