MAX6654 View Datasheet(PDF) - Maxim Integrated

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MAX6654

1°C Accurate Remote/Local Temperature Sensor with SMBus Serial Interface

Maxim Integrated 

1°C Accurate Remote/Local Temperature

Sensor with SMBus Serial Interface

Detailed Description

The MAX6654 is a temperature sensor that communi-

cates through an SMBus/I2C-compatible interface with a

µP in thermal-management applications. Essentially an

11-bit serial analog-to-digital converter (ADC) with a

sophisticated front end, the MAX6654 measures the

change in diode voltage at different current levels to cal-

culate temperature. It contains a current source, a multi-

plexer, an ADC, an SMBus interface, and associated

control logic (Figure 1). Temperature data from the ADC

is loaded into data registers, where it is automatically

compared with data previously stored in four

over/undertemperature alarm registers.

ADC and Multiplexer

The ADC is an averaging type that integrates over a

60ms period (each channel, typically, in the 8-bit “lega-

cy” mode), with excellent noise rejection.

The multiplexer automatically steers bias currents

through the remote and local diodes. The ADC and

associated circuitry measure their forward voltages and

compute their temperatures. Both channels are auto-

matically 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 per-

forms both measurements, and the user can ignore the

results of the unused channel. If the remote-diode

channel is unused, connect DXP to DXN rather than

leave the pins open.

The DXN input is biased at 1VBE above ground by an

internal diode to set up the ADC inputs for a differential

measurement. The worst-case DXP-DXN differential

input voltage range is 0.28V to 0.9V.

Excess resistance in series with the remote diode caus-

es about +1/2°C error per ohm when the parasitic resis-

tance cancellation mode is not being used. When the

parasitic resistance cancellation mode is being used,

excess resistance of up to 100Ω will not cause any dis-

cernable error. A 200µV offset voltage forced on DXP-

DXN causes about 1°C error.

A/D Conversion Sequence

A conversion sequence consists of a local temperature

measurement and a remote temperature measurement.

Each time a conversion begins, whether initiated auto-

matically in the free-running autoconvert mode

(RUN/STOP = 0) or by writing a “One-Shot” command,

both channels are converted, and the results of both

measurements are available after the end of conver-

sion. A BUSY status bit in the status byte shows that the

device is actually performing a new conversion; howev-

er, even if the ADC is busy, the results of the previous

conversion are always available.

Table 1. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

Central Semiconductor (USA)

Fairchild Semiconductor (USA)

ON Semiconductor (USA)

Rohm Semiconductor (USA)

Samsung (Korea)

Siemens (Germany)

Zetex (England)

MODEL NUMBER

CMPT3904

2N3904, 2N3906

2N3904, 2N3906

SST3904

KST3904-TF

SMBT3904

FMMT3904CT-ND

Note: Transistors must be diode connected (base shorted to

collector).

Remote-Diode Selection

The MAX6654 can directly measure the die tempera-

ture of CPUs and other ICs having on-board tempera-

ture-sensing diodes as shown in the Typical Operating

Circuit, or it can measure the temperature of a discrete

diode-connected transistor. For best accuracy, the dis-

crete transistor should be a small-signal device with its

collector and base connected together. Accuracy has

been experimentally verified for all of the devices listed

in Table 1.

The transistor must be a small-signal type with a rela-

tively high forward voltage; otherwise, the A/D input

voltage range can be violated. The forward voltage

must be >0.28V at 10µA; check to ensure this is true at

the highest expected temperature. The forward voltage

must be <0.9V at 100µA; check to ensure this is true at

the lowest expected temperature. Large power transis-

tors don’t work at all. Also, ensure that the base resis-

tance is <100Ω. Tight specifications for forward-current

gain (+50 to +150, for example) indicate that the manu-

facturer has good process controls and that the

devices have consistent VBE characteristics.

For heat-sink 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).

Thermal Mass and Self-Heating

Thermal mass can significantly affect the time required

for a temperature sensor to respond to a sudden

change in temperature. The thermal time constant of

the 16-pin QSOP package is about 140s in still air. For

the junction temperature of a MAX6654 in still air to set-

tle to within +1°C after a sudden +100°C change in air

temperature, about five time constants or 12 minutes

are required. However, the MAX6654 is not intended to

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