Configuration Register

The 8-bit Configuration register sets the fault queue, OS

polarity, shutdown control, and whether the OS output

functions in comparator or interrupt mode. When writing

to the Configuration register, set bits D7, D6, and D5 to

zero (see Table 5).

Bits D4 and D3, the fault queue bits, determine the

number of faults necessary to trigger an OS condition

(see Table 6). The number of faults set in the queue

must occur consecutively to trip the OS output. The

fault queue prevents OS false tripping in noisy environ-

ments.

Set bit D2, the OS polarity bit, to zero to force the OS

output polarity to active low. Set bit D2 to 1 to set the

OS output polarity to active high. OS is an open-drain

output under all conditions and requires a pullup resis-

tor to output a high voltage (see Figure 1).

Set bit D1, the comparator/interrupt bit to zero to oper-

ate OS in comparator mode. In comparator mode, OS

value. OS is deasserted when the temperature drops

operate OS in interrupt mode. OS is asserted in inter-

only after performing a read operation.

Set bit D0, the shutdown bit, to zero for normal opera-

tion. Set bit D0 to 1 to shutdown the LM75’s internal

registers can still be written to and read from while in

shutdown.

Applications Information

LM75 measures the temperature of its own die. The

thermal path between the die and the outside world

determines the accuracy of temperature measure-

ments. Most of the heat flows in to or out of the die

through the leads. Because of this, the LM75 most easi-

ly measures the PCB temperature. For ambient temper-

ature measurements, mount the LM75 on a separate

PCB away from high power sources. Temperature

errors due to self heating of the LM75 die is minimal

due to the low supply current.

Digital Noise Issues

The lowpass filters in the SCL and SDA digital lines miti-

gate the effects of bus noise, and make communica-

tions in noisy environments more robust. Good layout

practices also help. Keep switching power supplies

away from digital lines, and arrange for high-speed dig-

ital traces to cross SCL and SDA at right angles.

Properly terminate long PCB traces and bus traces

connected to multiple slaves.

Serial bus no-acknowledge (which causes unnecessary

bus traffic) is the most common symptom of excessive

noise coupling into the SDA and SCL lines. Noise with

undershoot more than 300mV below GND may prevent

successful serial communication.

Resistance can be added in series with the SDA and

SCL lines to help filter noise and ringing. A 5k

Ω resistor

placed in series with the SCL line and as close as pos-

sible to the SCL pin, with the 5pF to 10pF stray capaci-

tance of the device, provides a 6MHz to 12MHz

lowpass filter, which is sufficient filtering in many cases.

Digital Temperature Sensor and Thermal

Watchdog with 2-Wire Interface

10

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D7

D6

D5

D4

D3

D2

D1

D0

0

0

0

Fault Queue

Fault Queue

OS Polarity

Comparator/

Interrupt

Shutdown

Table 5. Configuration Register Definition

D4

D3

NUMBER OF FAULTS

0

0

1 (POR state)

01

2

10

4

11

6

Table 6. Configuration Register Fault

Queue Bits