EVAL-ADM1026EB ON Semiconductor, EVAL-ADM1026EB Datasheet - Page 21

• Manufacturer Part Number: EVAL-ADM1026EB
• Description: BOARD EVAL FOR ADM1026
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of EVAL-ADM1026EB

• Contents: Evaluation Board
• For Use With/related Products: ADM1026
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

•
A
irrespective of which configuration is selected for these pins.
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
Register 29h, and there is no valid result in Register 27h.
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.
temperature inputs is therefore nominally:
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
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.
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
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.
Figure 39. When using any of these circuits, note the
following:
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( 18
IN8
Pins 28 and 27 are measured both as analog inputs
If Pins 28 and 27 are configured as A
As mentioned previously, the ADC performs a conversion
The total monitoring cycle time for voltage and
The ADC uses the internal 22.5 kHz clock, which has a
Scaling of the analog inputs is performed on−chip, so
Because the resistors form part of the input attenuators,
The ADM1026 has a single analog output from an
Suitable fan drive circuits are shown in Figure 35 through
Two remote temperature
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.
/A
IN9
16
and as remote temperature input D2+/D2−,
0.711 ) ) ( 2
16
2.13 ) + 273 ms
IN8
/A
IN9
http://onsemi.com
(eq. 11)
, the
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Figure 36. Fan Drive Circuit with Op Amp and PNP
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
I
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.
CMAX
Figure 35. Fan Drive Circuit with Op Amp and
greater than the maximum fan current, and be
DAC
DAC
Emitter−Follower
R1
10kΩ
R1
10kΩ
1/4
LM324
1/4
LM324
Transistor
R2
39kW
R3
1kW
R4
1kW
12 V
12 V
2N2219A
2SA968
BD136
Q1
Q1