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ADM1029
(Rev.:2012)

Dual PWM Fan Controller and Temperature Monitor for High Availability Systems

ON Semiconductor 

ADM1029

provided for temperature monitoring on some

microprocessors, but it could equally well be a discrete

transistor.

If a discrete transistor is used, the collector will not be

grounded, and should be linked to the base. If a PNP

I

NI

IBIAS

transistor is used, the base is connected to the D– input and

the emitter to the D+ input. If an NPN transistor is used, the

emitter is connected to the D– input and the base to the D+

input.

VDD

D+

REMOTE

SENSING

TRANSISTOR

D−

BIAS

DIODE

LOW-PASS FILTER

fC = 65 kHz

VOUT+

To ADC

VOUT−

Figure 21. Signal Conditioning for Remote Diode Temperature Sensors

To prevent ground noise interfering with the

measurement, the more negative terminal of the sensor is not

referenced to ground, but biased above ground by an internal

diode at the D– input. If the sensor is used in a noisy

environment, a capacitor of value up to 1000 pF may be

placed between the D+/D– pins.

To measure DVBE, the sensor is switched between

operating currents of I and N  I. The resulting waveform is

passed through a 65 kHz low-pass filter to remove noise, and

to a chopper-stabilized amplifier that performs the functions

of amplification and rectification of the waveform to

produce a dc voltage proportional to DVBE. This voltage is

measured by the ADC to give a temperature output in 8-bit

two’s complement format. To further reduce the effects of

noise, digital filtering is performed by averaging the results

of 16 measurement cycles. An external temperature

measurement takes nominally 9.6 ms.

The results of external temperature measurements are

stored in 8-bit, two’s complement format, as illustrated in

Table 6.

Offset Registers

Digital noise and other error sources can cause offset

errors in the temperature measurement, particularly on the

remote sensors. The ADM1029 offers a way to minimize

these effects. The offsets on the three temperature channels

can be measured during system characterization and stored

as two’s complement values in three offset registers at

addresses 30h to 32h. The offset values are automatically

added to, or subtracted from, the temperature values,

depending on whether the two’s complement number

corresponds to a positive or negative offset. Offset values

from –15C to +15C are allowed.

The default value in the offset registers is zero, so if no

offsets are programmed, the temperature measurements are

unaltered.

Temperature Limits

The contents of the Local and Remote Temperature Value

Registers (addresses A0h to A2h) are compared to the

contents of the High and Low Limit Registers at addresses

90h to 92h and 98h to 9Ah. How the ADM1029 responds to

overtemperature/undertemperature conditions depends on

the status of the Temperature Fault Action Registers

(addresses 40h to 42h). The response of CFAULT, INT, and

fan-speed-to-temperature events depends on the setting of

these registers, as explained later.

Table 6. TEMPERATURE DATA FORMAT

Temperature

Digital Output

−128C

−125C

−100C

−75C

−50C

−25C

0C

+10C

+25C

+50C

+75C

+100C

+125C

+127C

1000 0000

1000 0011

1001 1100

1011 0101

1100 1110

1110 0111

0000 0000

0000 1010

0001 1001

0011 0010

0100 1011

0110 0100

0111 1101

0111 1111

Layout Considerations

Digital boards can be electrically noisy environments, and

care must be taken to protect the analog inputs from noise,

particularly when measuring the very small voltages from a

remote diode sensor. The following precautions should be

taken:

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