Datasheet

T =
ERR
h 1.008-
1.008
(273.15+T( C))°´
TMP421
TMP422
TMP423
www.ti.com SBOS398C –JULY 2007–REVISED MAY 2012
SHUTDOWN MODE (SD) DX4 (TMP422), to minimize the effects of noise.
However, a bypass capacitor placed differentially
The TMP421/22/23 Shutdown Mode allows the user
across the inputs of the remote temperature sensor is
to save maximum power by shutting down all device
recommended to make the application more robust
circuitry other than the serial interface, reducing
against unwanted coupled signals. The value of this
current consumption to typically less than 3μA; see
capacitor should be between 100pF and 1nF. Some
Figure 10, Shutdown Quiescent Current vs Supply
applications attain better overall accuracy with
Voltage. Shutdown Mode is enabled when the SD bit
additional series resistance; however, this increased
(bit 6) of Configuration Register 1 is high; the device
accuracy is application-specific. When series
shuts down once the current conversion is completed.
resistance is added, the total value should not be
When SD is low, the device maintains a continuous
greater than 3kΩ. If filtering is needed, suggested
conversion state.
component values are 100pF and 50Ω on each input;
exact values are application-specific.
SENSOR FAULT
REMOTE SENSING
The TMP421 can sense a fault at the DXP input
resulting from incorrect diode connection. The
The TMP421/22/23 are designed to be used with
TMP421/22/23 can all sense an open circuit. Short-
either discrete transistors or substrate transistors built
circuit conditions return a value of –64°C. The
into processor chips and ASICs. Either NPN or PNP
detection circuitry consists of a voltage comparator
transistors can be used, as long as the base-emitter
that trips when the voltage at DXP exceeds
junction is used as the remote temperature sense.
(V+) – 0.6V (typical). The comparator output is
NPN transistors must be diode-connected. PNP
continuously checked during a conversion. If a fault is
transistors can either be transistor- or diode-
detected, the OPEN bit (bit 0) in the temperature
connected (see Figure 11, Figure 12, and Figure 13).
result register is set to '1' and the rest of the register
bits should be ignored. Errors in remote temperature sensor readings are
typically the consequence of the ideality factor and
When not using the remote sensor with the TMP421,
current excitation used by the TMP421/22/23 versus
the DXP and DXN inputs must be connected together
the manufacturer-specified operating current for a
to prevent meaningless fault warnings. When not
given transistor. Some manufacturers specify a high-
using a remote sensor with the TMP422, the DX pins
level and low-level current for the temperature-
should be connected (refer to Table 10) such that
sensing substrate transistors. The TMP421/22/23 use
DXP connections are grounded and DXN connections
6μA for I
LOW
and 120μA for I
HIGH
.
are left open (unconnected). Unused TMP423 DXP
pins can be left open or connected to GND. The ideality factor (η) is a measured characteristic of
a remote temperature sensor diode as compared to
an ideal diode. The TMP421/22/23 allow for different
UNDERVOLTAGE LOCKOUT
η-factor values; see the N-Factor Correction Register
The TMP421/22/23 sense when the power-supply
section.
voltage has reached a minimum voltage level for the
The ideality factor for the TMP421/22/23 is trimmed
ADC to function. The detection circuitry consists of a
to be 1.008. For transistors that have an ideality
voltage comparator that enables the ADC after the
factor that does not match the TMP421/22/23,
power supply (V+) exceeds 2.45V (typical). The
Equation 4 can be used to calculate the temperature
comparator output is continuously checked during a
error. Note that for the equation to be used correctly,
conversion. The TMP421/22/23 do not perform a
actual temperature (°C) must be converted to kelvins
temperature conversion if the power supply is not
(K).
valid. The PVLD bit (bit 1, see Table 3) of the
individual Local/Remote Temperature Register is set
to '1' and the temperature result may be incorrect.
(4)
FILTERING
Where:
Remote junction temperature sensors are usually
η = ideality factor of remote temperature sensor
implemented in a noisy environment. Noise is most
T(°C) = actual temperature
often created by fast digital signals, and it can corrupt
T
ERR
= error in TMP421/22/23 because η ≠ 1.008
measurements. The TMP421/22/23 have a built-in
Degree delta is the same for °C and K
65kHz filter on the inputs of DXP and DXN
(TMP421/TMP423), or on the inputs of DX1 through
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