ADM1026JSTZ-R7 datasheet(21/55 Pages) ONSEMI

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ADM1026

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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 temperature measurement is stored in

As mentioned previously, the ADC performs a conversion

every 711

ms 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:

(eq. 11)

(18

0.711) ) (2

2.13) + 273 ms

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

tolerance 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

W) 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 reference voltage setting). The input

data for this DAC is contained in the DAC control register

(Address 04h). The DAC control register defaults to FFh

during a 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 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 35 through

Figure 39. 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 35, 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

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.

 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.

 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.

 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 from full scale to zero very quickly.

Figure 35. Fan Drive Circuit with Op Amp and

Emitter-follower

2N2219A

DAC

10k

1/4

LM324

12 V

Figure 36. Fan Drive Circuit with Op Amp and PNP

Transistor

BD136

2SA968

DAC

10k

1/4

LM324

12 V

1kW

39kW

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