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ADM1034

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The user can also configure the THERM pin to be pulled

low as an output whenever the Remote 1 temperature

exceeds the Remote 1 THERM limit. Set the Enable

Remote 1 THERM events bit (Bit 1) of Configuration

The last option is to configure the THERM pin to be pulled

low as an output whenever the Remote 2 temperature

exceeds the Remote 2 THERM limit. Set the Enable

Remote 2 THERM events bit (Bit 2) of Configuration

THERM % Limit Register

The THERM % limit is programmed to Register 0x19. An

ALERT is generated, if THERM is asserted for longer than

the programmed percentage limit. The limit is programmed

as a percentage of the chosen time window.

THERM % limit register is an 8−bit register.

0x00 = 0%

0xFF = 100%

Therefore, 1 LSB = 0.39%.

Example:

If a time window of 8 seconds is chosen, and an ALERT

is to be generated if THERM is asserted for more than 1

second, program the following value to the limit register:

% Limit = 1/8 x 100 = 12.5%

12.5% / 0.39% = 32d = 0x20 = 0010 0000

An ALERT is generated if the THERM limit is exceeded

after the time window has elapsed, assuming it is not

masked.

Fan Drive Signal

The ADM1034 contols the speed of up to two cooling

fans. Varying the duty cycle (on/off time) of a square wave

applied to the fan varies the speed of the fan. The ADM1034

uses a control method called synchronous speed control, in

which the PWM drive signal applied to the fan is

synchronized with the fan’s TACH signal. See the

Synchronous Speed Control section for more information.

The external circuitry required to drive the fan is very

simple. A single N−channel MOSFET is the only drive

device required. The specifications of the MOSFET depend

on the maximum current required by the fan and the gate

voltage drive (VGS < 3.0 V for direct interfacing to the drive

pin). VGS can be greater than 3.0 V, as long as the pullup on

the gate is tied to 5.0 V. The MOSFET should also have a low

on−resistance to ensure that there is no significant voltage

drop across the FET. A high on−resistance reduces the

voltage applied across the fan and therefore the maximum

operating speed of the fan. Figure 33 shows a scheme for

driving a 3−wire fan.

Figure 33. Interfacing a 3−Wire Fan to the ADM1034

by Using an N−Channel MOSFET

12V

FAN

1N4148

NDT3055L

ADM1034

DRIVE

TACH

3.3V

100kΩ

10kΩ

4.7kΩ

12V

Figure 33 uses a 10 k

W pullup resistor for the TACH

signal. This assumes that the TACH signal is an open

collector from the fan. In all cases, the fan’s TACH signal

must be kept below 5.0 V maximum to prevent damaging the

ADM1034.

If in doubt as to whether a fan has an open−collector or

totem pole TACH output, use one of the input signal

conditioning circuits shown in the Fan Inputs section.

When designing drive circuits with transistors and FETs,

make sure that the drive pins are not required to source

current and that they sink less than the maximum current

specified here.

Synchronous Speed Control

The ADM1034 drives the fan by using a control scheme

called synchronous speed control. In this scheme, the PWM

drive signal applied to the fan is synchronized with the

TACH signal. Accurate and repeatable fan speed

measurements are the main benefits. The fan is allowed to

run reliably at speeds as low as 30 percent of the full

capability.

The drive signal applied to the fan is synchronized with

the TACH signal. The ADM1034 switches on the drive

signal and waits for a transition on the TACH signal. When

a transition takes place on the TACH signal, the PWM drive

is switched off for a period of time called toff. The drive

signal is then switched on again. The toff time is varied in

order to vary the fan speed. If the fan is running too fast, the

toff time is increased. If the fan is running too slow, the toff

time is decreased.

Since the drive signal is synchronized with the TACH

signal, the frequency with which the fan is driven depends

on the current speed of the fan and the number of poles in it.

Figure 34 shows how the synchronous speed drive signal

works. The ideal TACH signal is the TACH signal that

would be output from the fan if power were applied 100

percent of the time. It is representative of the actual speed of

the fan. The actual TACH signal is the signal the user would

see on the TACH output from the fan if the user were to put

a scope on it. In effect, the actual TACH signal is the ideal

TACH signal chopped with the drive signal.

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