ADM1024ARU datasheet(15/28 Pages) AD

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ADM1024

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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 e.m.f 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 capability

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.

The temperature measured by either of the remote sensors exceeds

the limit programmed into address 14h.

The temperature measured by the on-chip sensor exceeds 70

°C,

which is hardware programmed into a read-only register at

address 17h.

The temperature measured by either of the remote sensors exceeds

°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 only return to normal operation after three consecutive

measurements 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 effect. 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 limits.

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 capacitors

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

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

maximum 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 17a to 17d show circuits for most common fan tach

outputs.

If the fan tach output has a resistive pull-up to VCC it can be directly

connected to the fan input, as shown in Figure 17a.

12V

PULL-UP

4.7k

TYP

TACHO

OUTPUT

FAN1 OR

FAN2

VCC

FAN SPEED

COUNTER

160k

Figure 17a. Fan with Tach. Pull-Up to +VCC

If the fan output has a resistive pull-up to 12 V (or other voltage

greater than 6.5 V), the fan output can be clamped with a Zener

diode, as shown in Figure 17b. The Zener voltage should be

chosen so it is greater than VIH but less than 6.5 V, allowing

for the voltage tolerance of the Zener. A value of between 3 V

and 5 V is suitable.

ZD1*

ZENER

*CHOOSE ZD1 VOLTAGE APPROX. 0.8

VCC

12V

PULL-UP

4.7k

TYP

TACHO

OUTPUT

FAN1 OR

FAN2

VCC

FAN SPEED

COUNTER

160k

Figure 17b. Fan with Tach. Pull-Up to Voltage >6.5 V

(e.g., 12 V ) Clamped with Zener Diode

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