AD7416-7418 Analog Devices, AD7416-7418 Datasheet - Page 13

• Manufacturer Part Number: AD7416-7418
• Description: 10-Bit Digital Temperature Sensor (AD7416) and Single/Four-Channel ADC (AD7417/AD7418)
• Manufacturer: Analog Devices

REV. B
OTI OUTPUT
The OTI output has two operating modes, which are selected
by Bit D1 of the Configuration Register. In the comparator
mode, (D1 = 0), the OTI output becomes active when the tem-
perature exceeds T
falls below T
AD7418 to be used as a thermostat, for example to control the
operation of a cooling fan.
The open-drain configuration of OTI allows the OTI outputs of
several AD7416/AD7417/AD7418s to be wire-ANDed together
when in active low mode.
The OTI output is used to indicate that an out-of-limit tem-
perature 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 T
the temperature falls below T
tion. Once OTI has become active by the temperature exceeding
T
ture remains, or subsequently rises again, above T
become active again until the temperature falls below T
will then remain active until reset by a read operation. Once
OTI has become active by the temperature falling below T
then been reset, it will remain inactive even if the temperature
remains, or subsequently falls again, below T
OTI is also reset when the AD7416/AD7417/AD7418 is placed
in shutdown mode, by setting bit D0 of the Configuration Regis-
ter to 1.
The OTI output requires an external pull-up resistor. This can
be connected to a voltage different from +V
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 tempera-
ture reading. The maximum value of pull-up resistor that will
meet the output high current specification of the OTI output is
30 k , but higher values may be used if a lower output current
is required. For most applications a value of 10 k will prove
suitable.
COMPARATOR
Figure 12. Operation of OTI Output (Shown Active Low)
OTI
OTI OUTPUT
OTI OUTPUT
INTERRUPT
*IN INTERRUPT MODE, A READ OPERATION OR SHUTDOWN RESETS THE OTI
OUTPUT, OTHERWISE THE OTI OUTPUT REMAINS ACTIVE INDEFINITLY, ONCE
TRIGGERED.
, then been reset, it will remain inactive even if the tempera-
MODE
MODE
T
HYST
T
OTI
HYST
READ* READ* READ* READ* READ* READ* READ*
. This mode allows the AD7416/AD7417/
OTI
, and remains active until the temperature
HYST
, until it is reset by a read opera-
OTI
, and remains active even if
HYST
S
(for example, to
.
OTI
. It will not
HYST
HYST
. It
,
–13–
FAULT QUEUE
To avoid false triggering of the AD7416/AD7417/AD7418 in
noisy environments, a fault queue counter is provided, which
can be programmed by Bits D3 and D4 of the Configuration
Register (see Table V) to count 1, 2, 4 or 6 fault events before
OTI becomes active. In order 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
T
the sequence will reset the fault queue counter, so if there are
three readings greater than T
T
POWER-ON DEFAULTS
The AD7416/AD7417/AD7418 always powers up with the
following defaults:
Address pointer pointing to Temperature Value Register
Comparator mode:
T
T
OTI Active LOW
Fault Queue = 1
These default settings allow the AD7416/AD7417/AD7418 to
be used as a stand-alone thermostat without any connection to a
serial bus.
OPERATING MODES
The AD7416/AD7417/AD7418 has two possible modes of
operation depending on the value of D0 in the Configuration
Register.
Mode 1
Normal operation of the AD7416/AD7417/AD7418 occurs when
D0 = 0. In this active mode, a conversion takes place every
400 s. Once the conversion has taken place, the part partially
powers down, consuming typically 350 A of current until the
next conversion occurs.
Two situations can arise in this mode on the request of a tem-
perature read. If a read occurs during a conversion, the conver-
sion aborts and a new one starts on the Stop/Repeat start
condition. The temperature value that is read is that of the
previous completed conversion. The next conversion will typi-
cally occur 400 s 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 s.
With a V
40 s (or 10% of the time) in conversion mode. It spends 360 s
(or 90% of time) in partial power-down mode. Thus the average
power dissipated by the AD7416/AD7417/AD7418 is:
Mode 2
For applications where temperature measurements are required
at a slower rate e.g., every second, power consumption of the
part can be reduced by writing to the part to go to a full power-
down between reads. The current consumption in full power-
down is typically 0.2 A and full power-down is initiated when
D0 = 1 in the configuration register. When a measurement is
required, a write operation can be performed to power-up the
OTI
OTI
OTI
HYST
, the fault queue counter will be reset without triggering OTI.
(or less than T
= 80 C
= 75 C
DD
= 3 V, for each 400 s cycle, the AD7416 spends
3 mW
HYST
AD7416/AD7417/AD7418
0.1 + 1 mW
) must occur. Any reading that breaks
OTI
followed by a reading less than
0.9 = 1.2 mW