Five-Channel Precision Temperature Monitor

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15

Discrete Remote Diodes

When the remote-sensing diode is a discrete transistor,

its collector and base must be connected together.

Table 10 lists examples of discrete transistors that are

appropriate for use with the MAX6602. The transistor

must be a small-signal type with a relatively high for-

ward voltage; otherwise, the A/D input voltage range

can be violated. The forward voltage at the highest

expected temperature must be greater than 0.25V at

10µA, and at the lowest expected temperature, the for-

ward voltage must be less than 0.95V at 100µA. Large

power transistors must not be used. Also, ensure that

the base resistance is less than 100

Ω. Tight specifica-

tions for forward current gain (50 < ß < 150, for exam-

ple) indicate that the manufacturer has good process

acteristics. Manufacturers of discrete transistors do not

normally specify or guarantee ideality factor. This is

normally not a problem since good-quality discrete

transistors tend to have ideality factors that fall within a

relatively narrow range. We have observed variations in

remote temperature readings of less than ±2°C with a

variety of discrete transistors. Still, it is good design

practice to verify good consistency of temperature

readings with several discrete transistors from any

manufacturer under consideration.

Unused Diode Channels

If one or more of the remote-diode channels is not

needed, short the DXP and DXN pins for that channel

together. The status register indicates a diode “fault”

for this channel and the channel is ignored during the

temperature-measurement sequence.

Thermal Mass and Self-Heating

When sensing local temperature, the MAX6602 mea-

sures the temperature of the PC board to which it is sol-

dered. The leads provide a good thermal path between

the PC board traces and the die. As with all IC temper-

ature sensors, thermal conductivity between the die

and the ambient air is poor by comparison, making air

temperature measurements impractical. Because the

thermal mass of the PC board is far greater than that of

the MAX6602, the device follows temperature changes

on the PC board with little or no perceivable delay.

When measuring the temperature of a CPU or other IC

with an on-chip sense junction, thermal mass has virtu-

ally no effect; the measured temperature of the junction

tracks the actual temperature within a conversion cycle.

When measuring temperature with discrete remote

transistors, the best thermal response times are

obtained with transistors in small packages (i.e., SOT23

or SC70). 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.

Remote-sensor self-heating due to the diode current

source is negligible.

ADC Noise Filtering

The integrating ADC has good noise rejection for low-

frequency signals, such as power-supply hum. In envi-

ronments with significant high-frequency EMI, connect

an external 2200pF capacitor between DXP_ and

DXN_. Larger capacitor values can be used for added

filtering, but do not exceed 3300pF because it can

introduce errors due to the rise time of the switched

current source. High-frequency noise reduction is

needed for high-accuracy remote measurements.

Noise can be reduced with careful PC board layout as

discussed in the PC Board Layout section.

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3904

Rohm Semiconductor (USA)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Zetex (England)

FMMT3904CT-ND

Table 10. Remote-Sensors Transistor

Manufacturers

Note: Discrete transistors must be diode connected (base

shorted to collector).