TMP421 datasheet(22/30 Pages) TI

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SHUTDOWN MODE (SD)

SENSOR FAULT

REMOTE SENSING

UNDERVOLTAGE LOCKOUT

ERR

n * 1.008

1.008

273.15 ) T °C

(4)

FILTERING

TMP421

TMP422

TMP423

SBOS398B – JULY 2007 – REVISED MARCH 2008

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 3

µA; 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

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 3k

Ω. If filtering is needed, suggested

conversion state.

component values are 100pF and 50

Ω on each input;

exact values are application-specific.

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

open

circuit.

either discrete transistors or substrate transistors built

Short-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

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

transistor-

detected, the OPEN bit (bit 0) in the temperature

diode-connected (see Figure 11, Figure 12, and

result register is set to '1' and the rest of the register

Figure 13).

bits should be ignored.

Errors in remote temperature sensor readings are

When not using the remote sensor with the TMP421,

typically the consequence of the ideality factor and

the DXP and DXN inputs must be connected together

current excitation used by the TMP421/22/23 versus

to prevent meaningless fault warnings. When not

the manufacturer-specified operating current for a

using a remote sensor with the TMP422, the DX pins

given transistor. Some manufacturers specify a

should be connected (refer to Table 10) such that

high-level

and

low-level

current

for

the

DXP connections are grounded and DXN connections

temperature-sensing

substrate

transistors.

The

are left open (unconnected). Unused TMP423 DXP

TMP421/22/23 use 6

µA for I

LOW and 120µA for IHIGH.

pins can be left open or connected to GND.

The ideality factor (n) is a measured characteristic of

a remote temperature sensor diode as compared to

an ideal diode. The TMP421/22/23 allow for different

The TMP421/22/23 sense when the power-supply

n-factor values; see the N-Factor Correction Register

voltage has reached a minimum voltage level for the

section.

ADC to function. The detection circuitry consists of a

The ideality factor for the TMP421/22/23 is trimmed

voltage comparator that enables the ADC after the

to be 1.008. For transistors that have an ideality

power supply (V+) exceeds 2.45V (typical). The

factor that does not match the TMP421/22/23,

comparator output is continuously checked during a

Equation 4 can be used to calculate the temperature

conversion. The TMP421/22/23 do not perform a

error. Note that for the equation to be used correctly,

temperature conversion if the power supply is not

actual temperature (

°C) must be converted to kelvins

valid. The PVLD bit (bit 1, see Table 3) of the

(K).

individual Local/Remote Temperature Register is set

to '1' and the temperature result may be incorrect.

Where:

Remote junction temperature sensors are usually

n = ideality factor of remote temperature sensor

implemented in a noisy environment. Noise is most

°C) = actual temperature

often created by fast digital signals, and it can corrupt

measurements. The TMP421/22/23 have a built-in

TERR = error in TMP421/22/23 because n ≠ 1.008

65kHz filter

the

inputs

DXP

and

DXN

Degree delta is the same for

°C and K

(TMP421/TMP423), or on the inputs of DX1 through

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