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

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.