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NCT75DR2G Datasheet(PDF) 5 Page - ON Semiconductor |
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NCT75DR2G Datasheet(HTML) 5 Page - ON Semiconductor |
5 / 16 page NCT75 http://onsemi.com 5 APPLICATION INFORMATION Functional Description The NCT75 temperature sensor converts an analog temperature measurement to a digital representation by using an on-chip measurement transistor and a 12 bit Delta-Sigma ADC. The device includes an open drain ALERT output which can be used to signal that the programmed temperature limit has been exceeded. The two main modes of operation are normal and shutdown mode. In normal mode the NCT75 performs a new temperature conversion every 80 ms. This new value is then updated to the temperature value register (address 0x00) and also compared to the TOS register limit (default = 80 °C). If the temperature value register is read during the conversion sequence the value returned is the previously stored value. A bus read does not affect the conversion that is in progress. In shutdown mode temperature conversion is disabled and the temperature value register holds the last valid temperature reading. The NCT75 can still be communicated with in this mode as the interface is still active. The device mode is controlled via bit 0 of the configuration register. While in shutdown mode a conversion can be initiated by writing an arbitrary value to the one-shot register (0x04). This has the effect of powering up the NCT75, performing a conversion, comparing the new temperature with the programmed limit and then going back into shutdown mode. The OS/ALERT pin can be configured in many ways to allow it to be used in many different system configurations. The overtemperature output can be configured to operate as a comparator type output (which is self clearing once the temperature has returned below the hysteresis value) or an interrupt type output (which requires the master to read an internal register AND the temperature to return below the hysteresis value before going into an inactive state). The ALERT pin can also be configured as an active high or active low output. Temperature Measurement Results The results of the on chip temperature measurements are stored in the temperature value register and compared with the TOS and THYST limit register. The temperature value, TOS and THYST registers are 16 bits wide and have a resolution of 0.0625 °C. The data is stored as a 12 bit 2s complement word. The data is left justified, D15 is the MSB and is the sign bit. The four LSBs (D3 to D0) are always 0 as they are not part of the result. While the ADC of the NCT75 can theoretically measure temperatures in the range of −128 °C to 127°C, the NCT75 is guaranteed to measure from −55 °C to +125°C. Table 6 shows the relevant temperature bits for a 12 bit temperature reading. A 2-byte read is required to obtain the full 12 bit temperature reading. If an 8 bit (1 °C resolution) reading is required then a single byte read is sufficient. Table 6. 12-BIT TEMPERATURE DATA FORMAT Temperature Binary Value D15 to D4 Hex Value −55 °C 1100 1001 0000 0xC90 −25 °C 1110 0111 0000 0xE70 −0.0625 °C 1111 1111 1111 0xFFF 0 °C 0000 0000 0000 0x000 +0.0625 °C 0000 0000 0001 0x001 +25 °C 0001 1001 0000 0x190 +75.25 °C 0100 1011 0100 0x4B4 +100 °C 0110 0100 0000 0x640 +125 °C 0111 1101 0000 0x7D0 Temperature Data Conversion 12-bit Temperature Data Format Positive Temperature = ADC Code (decimal)/16 Example 190h = 400d/16 = +25 °C Negative Temperature = (ADC Code(decimal) − 4096)/16 Example E70h = (3696d – 4096)/16 = −25 °C One-shot Mode One of the features of the NCT75 is a One-shot Temperature Measurement Mode. This mode is useful if reduced power consumption is a design requirement. To enable one-shot mode bit 5 of the configuration register needs to be set. Once, enabled, the NCT75 goes immediately into shutdown mode. Here, the current consumption is reduced to a typical value of 3 mA. Writing address 0x04 to the address pointer register initiates a one-shot temperature measurement. This powers up the NCT75, carries out a temperature measurement, and then powers down again. The data written to this register is irrelevant and is not stored. It is the write operation that causes the one-shot conversion. |
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