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SS8018

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,

diode-connected

small-signal

transistor.

The

SS8018 can also directly measure the die temperature

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

Ω. Tight speci-

fications for forward-current gain (+50 to +150, for ex-

ample) indicate that the manufacturer has good process

characteristics.

Table 1. Remote-Sensor Transistor Manufacturers

MANUFACTURER

MODEL NUMBER

Philips

PMBS3904

Motorola(USA)

MMBT3904

National Semiconductor (USA)

MMBT3904

Note: Transistors must be diode-connected (base

shorted to collector).

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-

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:

dT = 1.45mW x 120°C /W = 0.17°C

Even with these contrived circumstances, it is difficult to

introduce significant self-heating errors.

Rev.2.01 6/06/2003