EVAL-ADM1026EB datasheet(22/56 Pages) AD

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ADM1026

Rev. A | Page 22 of 56

Limit Values

Limit values for analog measurements are stored in the appropri

ate limit registers. In the case of voltage measurements, high and

low limits can be stored so that an interrupt request is generated

if the measured value goes above or below acceptable values. In

the case of temperature, a hot temperature or high limit can be

programmed, and a hot temperature hysteresis or low limit can

be programmed, which is usually some degrees lower. This can

be useful because it allows the system to be shut down when the

hot limit is exceeded, and restarted automatically when it has

cooled down to a safe temperature.

Analog Monitoring Cycle Time

The analog monitoring cycle begins when a 1 is written to the

start bit (Bit 0), and a 0 to the INT_Clear bit (Bit 2) of the con-

figuration register. INT_Enable (Bit 1) should be set to 1 to

enable the INToutput. The ADC measures each analog input in

turn, starting with Remote Temperature Channel 1 and ending

with local temperature. As each measurement is completed, the

result is automatically stored in the appropriate value register.

This round-robin monitoring cycle continues until it is disabled

by writing a 0 to Bit 0 of the configuration register. Because the

ADC is typically left to free-run in this way, the most recently

measured value of any input can be read out at any time.

For applications where the monitoring cycle time is important,

it can easily be calculated.

The total number of channels measured is

•

Five dedicated supply voltage inputs

•

Ten general-purpose analog inputs

•

3.3 V MAIN

•

3.3 V STBY

•

Local temperature

•

Two remote temperature

Pins 28 and 27 are measured both as analog inputs AIN8/AIN9 and

as remote temperature input D2+/D2−, irrespective of which

configuration is selected for these pins.

If Pins 28 and 27 are configured as AIN8/AIN9, the measurements

for these channels are stored in Registers 27h and 29h, and the

invalid temperature measurement is discarded. On the other

hand, if Pins 28 and 27 are configured as D2+/D2−, the temper-

ature measurement is stored in Register 29h, and there is no

valid result in Register 27h.

As mentioned previously, the ADC performs a conversion every

711 µs on the analog and local temperature inputs and every

2.13 ms on the remote temperature inputs. Each input is

measured 16 times and averaged to reduce noise.

The total monitoring cycle time for voltage and temperature

inputs is therefore nominally

(18 × 16 × 0.711) + (2 × 16 × 2.13) = 273 ms

The ADC uses the internal 22.5 kHz clock, which has a toler-

ance of ±6%, so the worst-case monitoring cycle time is 290 ms.

The fan speed measurement uses a completely separate

monitoring loop, as described later.

Input Safety

Scaling of the analog inputs is performed on-chip, so external

attenuators are typically not required. However, because the

power supply voltages appear directly at the pins, it is advisable

to add small external resistors (that is, 500 Ω) in series with the

supply traces to the chip to prevent damaging the traces or

power supplies should an accidental short such as a probe

connect two power supplies together.

Because the resistors form part of the input attenuators, they

affect the accuracy of the analog measurement if their value

is too high. The worst such accident would be connecting

−12 V to +12 V where there is a total of 24 V difference. With

the series resistors, this would draw a maximum current of

approximately 24 mA.

ANALOG OUTPUT

The ADM1026 has a single analog output from an unsigned

8-bit DAC that produces 0 V to 2.5 V (independent of the refer-

ence voltage setting). The input data for this DAC is contained

in the DAC control register (Address 04h). The DAC control

duces maximum fan speed. The analog output may be amplified

and buffered with external circuitry such as an op amp and a

transistor to provide fan speed control. During automatic fan

speed control, described later, the four MSBs of this register set

the minimum fan speed.

Suitable fan drive circuits are shown in Figure 36 through

Figure 40. When using any of these circuits, note the following:

•

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

almost +12 V, apart from Figure 36, which loses the base-

emitter voltage drop of Q1 due to the emitter-follower

configuration.

•

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

the gain of these circuits needs to be about 4.8.

•

Take care when choosing the op amp to ensure that its

input common-mode range and output voltage swing are

suitable.

•

The op amp may be powered from the +12 V rail alone

or from ±12 V. If it is powered from +12 V, the input

common-mode range should include ground to accom-

modate 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.

•

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.

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