TMP421
TMP422
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SBOS398A–JULY 2007–REVISED SEPTEMBER 2007
HIGH-SPEED MODE
When not using the remote sensor with the TMP421,
the DXP and DXN inputs must be connected together
to prevent meaningless fault warnings. When not
using a remote sensor with the TMP422, the DX pins
should be connected using Table 10 such that DXP
connections are grounded and DXN connections are
left open (unconnected).
In order for the Two-Wire bus to operate at
frequencies above 400kHz, the master device must
issue a High-Speed mode (Hs-mode) master code
(0000 1xxx) as the first byte after a START condition
to switch the bus to high-speed operation. The
TMP421/22 does acknowledge this byte, but switches
the input filters on SDA and SCL and the output filter
on SDA to operate in Hs-mode, allowing transfers at
up to 3.4MHz. After the Hs-mode master code has
been issued, the master transmits a Two-Wire slave
address to initiate a data transfer operation. The bus
continues to operate in Hs-mode until a STOP
condition occurs on the bus. Upon receiving the
STOP condition, the TMP421/22 switches the input
and output filters back to fast mode operation.
UNDERVOLTAGE LOCKOUT
The TMP421/22 sense when the power-supply
voltage has reached a minimum voltage level for the
ADC to function. The detection circuitry consists of a
voltage comparator that enables the ADC after the
power supply (V+) exceeds 2.45V (typical). The
comparator output is continuously checked during a
conversion. The TMP421/22 does not perform a
temperature conversion if the power supply is not
valid. The PVLD bit (bit 1, see Table 3) of the
Local/Remote Temperature Register is set to '1' and
the temperature result may be incorrect.
TIMEOUT FUNCTION
The TMP421/22 reset the serial interface if either
SCL or SDA are held low for 30ms (typical) between
a START and STOP condition. If the TMP421/22 are
holding the bus low, it releases the bus and waits for
a START condition. To avoid activating the timeout
function, it is necessary to maintain a communication
speed of at least 1kHz for the SCL operating
frequency.
GENERAL CALL RESET
The TMP421/22 support reset via the Two-Wire
General Call address 00h (0000 0000b). The
TMP421/22 acknowledge the General Call address
and respond to the second byte. If the second byte is
06h (0000 0110b), the TMP421/22 execute
a
SHUTDOWN MODE (SD)
software reset. This software reset restores the
power-on reset state to all TMP421/22 registers, and
aborts any conversion in progress. The TMP421/22
take no action in response to other values in the
second byte.
The TMP421/22 Shutdown Mode allows the user to
save maximum power by shutting down all device
circuitry other than the serial interface, reducing
current consumption to typically less than 3μA; see
Figure 10, Shutdown Quiescent Current vs Supply
Voltage. Shutdown Mode is enabled when the SD bit
of the Configuration Register is high; the device shuts
down once the current conversion is completed.
When SD is low, the device maintains a continuous
conversion state.
FILTERING
Remote junction temperature sensors are usually
implemented in a noisy environment. Noise is most
often created by fast digital signals, and it can corrupt
measurements. The TMP421/22 have
a built-in
65kHz filter on the inputs of DXP and DXN (TMP421),
or on the inputs of DX1 through DX4 (TMP422), to
minimize the effects of noise. However, a bypass
capacitor placed differentially across the inputs of the
remote temperature sensor is recommended to make
the application more robust against unwanted
coupled signals. The value of this capacitor should be
between 100pF and 1nF. Some applications attain
better overall accuracy with additional series
resistance; however, this increased accuracy is
application-specific. When series resistance is added,
the total value should not be greater than 3kΩ. If
filtering is needed, suggested component values are
100pF and 50Ω on each input; exact values are
application-specific.
SENSOR FAULT
The TMP421 can sense a fault at the DXP input
resulting from incorrect diode connection. Both the
TMP421 and the TMP422 can sense an open circuit.
Short-circuit conditions return a value of –64h. The
detection circuitry consists of a voltage comparator
that trips when the voltage at DXP exceeds
(V+) – 0.6V (typical). The comparator output is
continuously checked during a conversion. If a fault is
detected, the OPEN bit (bit 0) in the temperature
result register is set to '1' and the rest of the register
bits should be ignored.
Copyright © 2007, Texas Instruments Incorporated
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