AD7416ARMZ datasheet(17/24 Pages) AD

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AD7416/AD7417/AD7418

Rev. I | Page 17 of 24

OTI OUTPUT

The OTI output has two operating modes that are selected by

Bit D1 of the configuration register. In the comparator mode,

(D1 = 0), the OTI output becomes active when the temperature

exceeds TOTI and remains active until the temperature falls

below THYST. This mode allows the AD7416/AD7417/AD7418 to

be used as a thermostat, for example, to control the operation of

a cooling fan.

TOTI

THYST

OTI OUTPUT

COMPARATOR

MODE

OTI OUTPUT

INTERRUPT

MODE

1IN INTERRUPT MODE, A READ OPERATION OR SHUTDOWN RESETS THE OTI

OUTPUT; OTHERWISE, THE OTI OUTPUT REMAINS ACTIVE INDEFINITELY,

ONCE TRIGGERED.

READ1 READ1 READ1 READ1 READ1 READ1 READ1

Figure 20. Operation of OTI Output (Shown Active Low)

The open-drain configuration of OTI allows the OTI outputs of

several AD7416/AD7417/AD7418 devices to be wire-AND’ed

together when in active low mode.

The OTI output is used to indicate that an out-of-limit tempera-

ture excursion has occurred. OTI is an open-drain output that

can be programmed to be active low by setting Bit D2 of the

configuration register to 0 or active high by setting Bit D2 of

the configuration register to 1.

In the interrupt mode (D1 = 1), the OTI output becomes active

when the temperature exceeds TOTI and remains active even if

the temperature falls below THYST, until it is reset by a read opera-

tion. Once OTI becomes active by the temperature exceeding

TOTI, and resets, it remains inactive even if the temperature

remains, or subsequently rises again, above TOTI. It does not

become active again until the temperature falls below THYST. It

then remains active until reset by a read operation. Once OTI

becomes active by the temperature falling below THYST and then

resets, it remains inactive even if the temperature remains, or

subsequently falls again, below THYST.

OTI is also reset when the AD7416/AD7417/AD7418 are placed

in shutdown mode by setting Bit D0 of the configuration

The OTI output requires an external pull-up resistor. This can

be connected to a voltage different from VDD (for example, to

allow interfacing between 5 V and 3.3 V systems) provided that

the maximum voltage rating of the OTI output is not exceeded.

The value of the pull-up resistor depends on the application but

should be as large as possible to avoid excessive sink currents at

the OTI output, which can heat the chip and affect the temperature

reading. The maximum value of the pull-up resistor that meets

the output high current specification of the OTI output is 30 kΩ,

but higher values can be used if a lower output current is

required. For most applications, a value of 10 kΩ is suitable.

FAULT QUEUE

To avoid false triggering of the AD7416/AD7417/AD7418 in

noisy environments, a fault queue counter is provided that can

be programmed by Bit D3 and Bit D4 of the configuration

OTI becomes active. To trigger OTI, the faults must occur

consecutively. For example, if the fault queue is set to 4, then

four consecutive temperature measurements greater than TOTI

(or less than THYST) must occur. Any reading that breaks the

sequence resets the fault queue counter, so if there are three

readings greater than TOTI followed by a reading less than TOTI,

the fault queue counter is reset without triggering OTI.

POWER-ON DEFAULTS

The AD7416/AD7417/AD7418 always power up with the

following defaults:

•

Address pointer pointing to temperature value register

comparator mode

•

TOTI = 80°C

•

THYST = 75°C

•

OTI active low

•

Fault queue = 1

These default settings allow the AD7416/AD7417/AD7418 to

be used as a standalone thermostat without any connection to a

serial bus.

OPERATING MODES

The AD7416/AD7417/AD7418 have two possible modes of

operation depending on the value of D0 in the configuration

Mode 1

Normal operation of the AD7416/AD7417/AD7418 occurs

when D0 = 0. In this active mode, a conversion takes place

every 400 μs. After the conversion has taken place, the part

partially powers down, consuming typically 350 μA of the

current until the next conversion occurs.

Two situations can arise in this mode on the request of a tempera-

ture read. If a read occurs during a conversion, the conversion

aborts and a new one starts on the stop/repeat start condition.

The temperature value that is read is that of the previous com-

pleted conversion. The next conversion typically occurs 400 μs

after the new conversion has begun.

If a read is called between conversions, a conversion is initiated

on the stop/repeat start condition. After this conversion, the

part returns to performing a conversion every 400 μs.

With VDD = 3 V for each 400 μs cycle, the AD7416/AD7417/

AD7418 spend 40 μs (or 10% of the time) in conversion mode.

The part spends 360 μs (or 90% of time) in partial power-down

mode. Thus, the average power dissipated by the AD7416/

AD7417/AD7418 is

3 mW × 0.1 + 1 mW × 0.9 = 1.2 mW

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