LM93 datasheet(28/92 Pages) NSC

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15.0 Using The LM93 (Continued)

Thermal Diode Temperature vs. LM93 Temperature

Reading

20068215

15.9.1 Accuracy Effects of Diode Non-Ideality Factor

The technique used in today’s remote temperature sensors

is to measure the change in V

BE at two different operating

points of a diode. For a bias current ratio of N:1, this differ-

ence is given as:

where:

η is the non-ideality factor of the process the diode is

manufactured on,

- q is the electron charge,

- k is the Boltzmann’s constant,

- N is the current ratio,

- T is the absolute temperature in ˚K.

The temperature sensor then measures

∆V

BE and converts

to digital data. In this equation, k and q are well defined

universal constants, and N is a parameter controlled by the

temperature sensor. The only other parameter is

η, which

depends on the diode that is used for measurement. Since

∆V

BE is proportional to both

η and T, the variations in η

cannot be distinguished from variations in temperature.

Since the non-ideality factor is not controlled by the tempera-

ture sensor, it directly adds to the inaccuracy of the sensor.

For example, assume a ±1% variation in

η from part to part

(Xeon processors targeted for the LM93 do not have pub-

lished thermal diode specifications at the time of this printing,

therefore this is probably a very conservative estimate).

Assume a temperature sensor has an accuracy specification

of ±3˚C at room temperature of 25˚C and the process used

to manufacture the diode has a non-ideality variation of

±1%. The resulting accuracy of the temperature sensor at

room temperature is:

TACC = ±3˚C+(±1% of 298˚K) = ±6˚C

The additional inaccuracy in the temperature measurement

caused by

η, can be eliminated if each temperature sensor is

calibrated with the remote diode that it is paired with. The

LM93 can be paired with an MMBT3904 when not being

used to monitor the thermal diode within an Intel Processor.

15.9.2 PCB Layout for Minimizing Noise

In the following guidelines, D+ and D− refer to the RE-

MOTE1+, REMOTE1−, REMOTE2+, REMOTE2− pins.

In a noisy environment, such as a power supply, layout

considerations are very critical. Noise induced on traces

running between the remote temperature diode sensor and

the LM93 can cause temperature conversion errors.

The following guidelines should be followed:

Place a 0.1 µF and 100 pF LM93 power bypass capaci-

tors as close as possible to the V

DD pin, with the 100pF

capacitor being the closest. Place 10 µF capacitor in the

near vicinity of the LM93 power pin.

Place 100 pF capacitor as close as possible to the LM93

thermal diode Remote+ and Remote− pins. Make sure

the traces to the 100 pF capacitor are matched and as

short as possible. This capacitor is required to minimize

high frequency noise error.

Ideally, the LM93 should be placed within 10 cm of the

thermal diode pins with the traces being as straight,

short and identical as possible. Trace resistance of 1

Ω

can cause as much as 1˚C of error.

Diode traces should be surrounded by a GND guard ring

to either side, above and below, if possible. This GND

guard should not be between the Remote+ and

Remote− lines. In the event that noise does couple to

the diode lines, it would be ideal if it is coupled to both

identically, i.e. common mode. That is, equally to the

Remote+ (D+) and Remote−(D-) lines. (See figure be-

low):

Recommended Diode Trace Layout

20068220

Avoid routing diode traces in close proximity to any

power supply switching or filtering inductors.

Avoid running diode traces close to or parallel to high

speed digital and bus lines. Diode traces should be kept

at least 2 cm apart from the high speed digital traces.

If it is necessary to cross high speed digital traces, the

diode traces and the high speed digital traces should

cross at a 90 degree angle.

Leakage current between Remote+ and GND should be

kept to a minimum. 1 nA of leakage can cause as much

as 1˚C of error in the diode temperature reading. Keep-

ing the printed circuit board as clean as possible mini-

mizes leakage current.

15.10 FAN CONTROL

15.10.1 Automatic Fan Control Algorithm

The LM93 fan speed control method is optimized for fan

power efficiency, fan reliability and minimum cost. The

PWMx outputs can be filtered using an external switching

regulator type output stage that provides 5V to 12V DC for

fan power. A high PWM frequency is required to minimize the

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