LTC2997H データシートの表示（PDF） - Linear Technology

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LTC2997H

Remote/Internal Temperature Sensor

Linear Technology 

LTC2997

APPLICATIONS INFORMATION

Power Up and UVLO

The basic LTC2997 application using an external NPN

transistor is shown in Figure 2.

2.5V TO 5.5V

0.1μF

MMBT3904

D+

470pF

D–

VCC

LTC2997

VREF 1.8V

GND

VPTAT 4mV/K

2997 F02

Figure 2. Basic Application Circuit

The VCC pin must exceed the undervoltage threshold of

1.9V (typical) for normal operation. For VCC below UVLO

the LTC2997 enters power-on reset and VPTAT is pulled low.

Temperature Measurements

Before each conversion a voltage comparator connected

to D+ automatically sets the LTC2997 into external or in-

ternal mode. Tying D+ to VCC enables internal mode and

VPTAT represents the die temperature. The VPTAT gain, KT,

is 4mV/K. The temperature in Kelvin is easily calculated:

of up to 100Ω to an error smaller than 1°C (see Typical

Performance Characteristics). The LTC2997 continuously

measures the sensor diode at different test currents and

updates VPTAT every 3.5ms (typical).

Input Noise Filtering

The change in sensor voltage per °C is hundreds of mi-

crovolts, so electrical noise must be kept to a minimum.

Bypass D+ and D– with a 470pF capacitor close to the

LTC2997 to suppress external noise. Bypass capacitors

greater 1nF cause settling time errors of the different

measurement currents. See Typical Performance Char-

acteristics. Long wires connecting external sensors add

series resistance, mutual capacitance between D+ and

D–, and cause leakage currents. A 10m CAT6 cable has

~500pF of mutual capacitance and adds negligible series

resistance and leakage currents. Recommended shielding

and PCB trace considerations for best noise immunity are

illustrated in Figure 3.

GND SHIELD TRACE

www.DataSheet.net/

470pF

LTC2997

D+

D–

GND

NPN SENSOR

2997 F03

TKELVIN

=

VPTAT

KT

For VD+ more than 300mV below VCC (typical) the LTC2997

assumes that an external sensor is connected and will start

sending sensing currents to the remote sensor diode. The

anode of the external sensor must be connected to pin D+.

The cathode should be connected to D– for best external

noise immunity. For single wire measurements the sen-

sor cathode is connected to remote GND and D– must be

connected to local GND (see Figure 7). Small ground DC

voltages (<±200mV) between the two cathode potentials

do not impact the measurement accuracy. AC voltages at

odd multiples of 6kHz (±20%) cause temperature measure-

ment errors (see Typical Performance Characteristics).

The LTC2997 is calibrated to yield a VPTAT gain of 4mV/K

for a remote diode with an ideality factor of 1.004. A

built-in algorithm cancels errors due to series resistance

Figure 3. Recommended PCB Layout

Output Noise Filtering

The VPTAT output typically exhibits 1mV RMS (0.25°C RMS)

noise. For applications which require lower noise digital

or analog averaging can be applied to the output. Choose

the averaging time according to the following equation:

t AVG

=

⎛

⎝⎜

0.015 [°C /

TNOISE

Hz ]⎞ 2

⎠⎟

where tAVG is the averaging time and TNOISE the desired

temperature noise in °C RMS. For example, if the desired

noise performance is 0.015°C RMS, set the averaging time

to one second. See Typical Performance Characteristics.

2997fa

9

Datasheet pdf - http://www.DataSheet4U.co.kr/

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