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LTC2995

2995f

Temperature Measurements

The LTC2995 continuously measures the sensor diode at

different test currents and generates a voltage proportional

to the absolute temperature of the sensor at the VPTAT pin.

The voltage at VPTAT is updated every 3.5ms.

The gain of VPTAT is calibrated to 4mV/K for the measure-

ment of the internal diode as well as for remote diodes

with an ideality factor of 1.004.

TKELVIN =

VPTAT

4mV/K

(

η= 1.004)

If an external sensor with an ideality factor different from

1.004 is used, the gain of VPTAT will be scaled by the ratio

of the actual ideality factor (

ηACT) to 1.004. In these cases,

the temperature of the external sensor can be calculated

from VPAT by:

TKELVIN =

VPTAT

4mV/K

•

1.004

ηACT

Temperature in degrees Celsius can be deduced from

degrees Kelvin by:

TCELSIUS = TKELVIN – 273.15

The three-state diode select pin (DS) determines whether

the temperature of the external or the internal diode is

measured and displayed at VPTAT as described in Table 1.

Table 1. Diode Selection

DIODE LOCATION

DS PIN

Internal

VCC

External

GND

Both

Open

If the DS pin is left open, the LTC2995 measures both

diodes alternately and VPTAT changes every 30ms from the

voltage corresponding to the temperature of the internal

sensor to the voltage corresponding to the temperature

of the external sensor. If D+ is tied to VCC, the LTC2995

measures the internal diode regardless of the state of

the DS pin.

APPLICATIONS INFORMATION

Choosing an External Sensor

The LTC2995 is factory calibrated for an ideality factor of

1.004, which is typical of the popular MMBT3904 NPN

transistor. Semiconductor purity and wafer level process-

ing intrinsically limit device-to-device variation, making

these devices interchangeable between manufacturers

with a temperature error of typically less than 0.5°C. Some

recommended sources are listed in Table 2:

Table 2 Recommended Transistors for Use As Temperature

Sensors

MANUFACTURER

PART NUMBER

PACKAGE

Fairchild

Semiconductor

MMBT3904

SOT-23

Central

Semiconductor

CMBT3904

SOT-23

Diodes Inc.

MMBT3904

SOT-23

On Semiconductor

MMBT3904LT1

SOT-23

NXP

MMBT3904

SOT-23

Infineon

MMBT3904

SOT-23

Rohm

UMT3904

SC-70

Discrete two terminal diodes are not recommended as

remote sensing devices as their ideality factor is typically

much higher than 1.004. Also MOS transistors are not

suitable as they don’t exhibit the required current to tem-

perature relationship. Furthermore gold doped transistors

(low beta), high frequency and high voltage transistors

should be avoided as remote sensing devices.

Connecting an External Sensor

The change in sensor voltage per °C is hundreds of

microvolts, so electrical noise must be kept to a mini-

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

the LTC2995 to suppress external noise. Recommended

shielding and PCB trace considerations for best noise

immunity are illustrated in Figure 1.

Figure 1. Recommended PCB Layout

D+

D–

LTC2995

2995 F01

GND

470pF

GND SHIELD TRACE

NPN SENSOR

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