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LM75AD Datasheet(PDF) 11 Page - NXP Semiconductors |
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LM75AD Datasheet(HTML) 11 Page - NXP Semiconductors |
11 / 18 page ![]() Philips Semiconductors Product data sheet LM75A Digital temperature sensor and thermal Watchdog ™ 2004 Oct 05 11 OS output and polarity The OS output is an open-drain output and its state represents results of the device Watchdog operation as described in the “General operation” section. In order to observe this output state, an external pull-up resistor is needed. The resistor should be as large as possible, up 200 k Ω, to minimize the temp reading error due to internal heating by the high OS sinking current. The OS output active state can be selected as HIGH or LOW by programming bit B2 of the Configuration register: setting B2 to 1 selects OS active HIGH and setting B2 to 0 sets OS active LOW. At power-up, this bit is equal to 0 and the OS active state is LOW. OS comparator and interrupt modes As described in the “General operation” section, the device OS output responds to the result of the comparison between the Temp data and the programmed limits, Tos and Thyst, in different ways depending on the selected OS mode: OS comparator or OS interrupt. The OS mode is selected by programming bit B1 of the configuration register: setting B1 to 1 selects the OS interrupt mode, and setting B1 to 0 selects the OS comparator mode. At power up, this bit is equal to 0 and the OS comparator is selected. The main difference between the two modes is that in OS comparator mode, the OS output becomes active when the Temp has exceeded the Tos and reset when the Temp has dropped below the Thyst, reading a register or putting the device into shut-down does not change the state of the OS output; while in OS interrupt mode, once it has been activated either by exceeding the Tos or dropping below the Thyst, the OS output will remain active indefinitely until reading a register or putting the device into shut-down occurs, then the OS output is reset. The Tos & Thyst limits must be selected so that Tos temp value > Thyst temp value. Otherwise, the OS output state will be undefined. OS fault queue Fault queue is defined as the number of faults that must occur consecutively to activate the OS output. It is provided to avoid false tripping due to noise. Because faults are determined at the end of data conversions, fault queue is also defined as the number of consecutive conversions returning a temperature trip. The value of fault queue is selectable by programming the two bits B4 and B3 of the configuration register. Notice that the programmed data and the fault queue value are not the same. The Fault queue table (Table 11) shows the one-to-one relationship between them. At power-up, fault queue data = 0 and fault queue value = 1. Table 11. Fault queue table Fault queue data Fault queue value B4 B3 Decimal 0 0 1 0 1 2 1 0 4 1 1 6 Shutdown mode The device operation mode is selected by programming bit B0 of the Configuration register: Setting B0 to 1 will put the device into shut-down mode. Resetting B0 to 0 for the device normal mode. In shut-down mode, the device draws a small current of about 3.5 µA and the power dissipation is minimized; the temperature conversion stops, but the I2C interface remains active and register write/read operation can be performed. If the OS output is in comparator mode, then it remains unchanged. Otherwise, the OS output is reset in interrupt mode. Power-up default and Power-on Reset The LM75A always powers-up in its default state with: – Normal operation mode – OS comparator mode – Tos = 80 °C – Thyst = 75 °C – OS output active state = LOW – Pointer value = 0. When the power supply voltage is dropped below the device power-on reset level of about 1.9 V (POR) and then rises up again, the device will be reset to its default condition as listed above. |
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