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LTC2914 Datasheet(PDF) 10 Page - Linear Technology |
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LTC2914 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC2995 10 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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