ADM1024 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1024

System Hardware Monitor with Remote Diode Thermal Sensing

Analog Devices 

ADM1024

ANALOG OUTPUT

The ADM1024 has a single analog output from an unsigned 8-bit

DAC that produces 0 V to 2.5 V. The analog output register

defaults to FF during power-on reset, which produces maximum

fan speed. The analog output may be amplified and buffered

with external circuitry such as an op amp and transistor to

provide fan speed control.

Suitable fan drive circuits are given in Figures 9a to 9f. When

using any of these circuits, the following points should be noted:

1. All of these circuits will provide an output range from 0 V to

almost 12 V, apart from Figure 9a which loses the base-

emitter voltage drop of Q1 due to the emitter-follower

configuration.

2. To amplify the 2.5 V range of the analog output up to 12 V,

the gain of these circuits needs to be around 4.8.

3. Care must be taken when choosing the op amp to ensure

that its input common-mode range and output voltage swing

are suitable.

4. The op amp may be powered from the 12 V rail alone or

from ± 12 V. If it is powered from 12 V, then the input com-

mon-mode range should include ground to accommodate the

minimum output voltage of the DAC, and the output voltage

should swing below 0.6 V to ensure that the transistor can be

turned fully off.

5. If the op amp is powered from –12 V, precautions such as a

clamp diode to ground may be needed to prevent the base-

emitter junction of the output transistor being reverse-biased

in the unlikely event that the output of the op amp should

swing negative for any reason.

6. In all these circuits, the output transistor must have an ICMAX

greater than the maximum fan current, and be capable of

dissipating power due to the voltage dropped across it when

the fan is not operating at full speed.

7. If the fan motor produces a large back EMF when switched

off, it may be necessary to add clamp diodes to protect the

output transistors in the event that the output goes very

quickly from full scale to zero.

FAULT-TOLERANT FAN CONTROL

The ADM1024 incorporates a fault-tolerant fan control capabil-

ity that can override the setting of the analog output and force it

to maximum to give full fan speed in the event of a critical over-

temperature problem even if, for some reason, this has not been

handled by the system software.

There are four temperature set points that will force the analog

output to FFh if any one of them is exceeded for three or more

consecutive measurements. Two of these limits are programmable

by the user and two are hardware limits intended as must-not-

exceed limits that cannot be changed.

The analog output will be forced to FFh if:

The temperature measured by the on-chip sensor exceeds the

limit programmed into register address 13h.

or

The temperature measured by either of the remote sensors exceeds

the limit programmed into address 14h.

or

The temperature measured by the on-chip sensor exceeds 70°C,

which is hardware programmed into a read-only register at

address 17h.

or

The temperature measured by either of the remote sensors exceeds

85°C, which is hardware programmed into a read-only register

at address 18h.

Once the hardware override of the analog output is triggered, it will

return to normal operation only after three consecutive measure-

ments that are 5 degrees lower than each of the above limits.

The analog output can also be forced to FFh by pulling the

THERM pin (Pin 2) low.

The limits in Registers 13h and 14h can be programmed by the

user. Obviously, these limits should not exceed the hardware

values in Registers 17h and 18h, as they would have no ef-

fect. The power-on default values of these registers are the same

as the two hardware registers, 70°C and 85°C respectively, so

there is no need to program them if these limits are acceptable.

Once these registers have been programmed, or if the defaults

are acceptable, the values in these registers can be locked by

writing a 1 to Bits 1 and 2 of Configuration Register 2 (address

4Ah). This prevents any unauthorized tampering with the lim-

its. These lock bits can only be written to 1 and can only be

cleared by power-on reset or by taking the RESET pin low, so

registers 13h and 14h cannot be written to again unless the

device is powered off, then on.

LAYOUT AND GROUNDING

Analog inputs will provide best accuracy when referred to a

clean ground. A separate, low impedance ground plane for

analog ground, which provides a ground point for the voltage

dividers and analog components, will provide best performance

but is not mandatory.

The power supply bypass, the parallel combination of 10 µF

(electrolytic or tantalum) and 0.1 µF (ceramic) bypass capaci-

tors connected between Pin 9 and ground, should also be

located as close as possible to the ADM1024.

FAN INPUTS

Pins 5 and 6 may be configured as analog inputs or fan speed

inputs by programming Bits 0 and 1 of the Channel Mode

Register. The power-on default for these bits is all zeros, which

makes Pins 5 and 6 fan inputs.

Signal conditioning in the ADM1024 accommodates the slow

rise and fall times typical of fan tachometer outputs. The maxi-

mum input signal range is 0 to VCC. In the event that these

inputs are supplied from fan outputs that exceed 0 V to 6.5 V,

either resistive attenuation of the fan signal or diode clamping

must be included to keep inputs within an acceptable range.

Figures 10a to 10d show circuits for most common fan ta-

chometer outputs.

If the fan tachometer output has a resistive pull-up to VCC, it

can be directly connected to the fan input, as shown in Figure 10a.

–16–

REV. B

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