REV. B

TMP12

–6–

Figure 4 shows the TMP12’s hysteresis profile. The hysteresis is

programmed by the user by setting a specific load current on the

TMP12, as shown in the Functional Block Diagram, and is

fed to a buffer with an analog switch.

HYSTERESIS

LOW

HYSTERESIS

HIGH

HYSTERESIS HIGH =

HYSTERESIS LOW

TEMPERATURE

HI

LO

OUTPUT

VOLTAGE

OVER, UNDER

Figure 4. Hysteresis Profile

After a temperature setpoint has been exceeded and a compara-

tor tripped, the hysteresis buffer output is enabled. The result is

a current of the appropriate polarity, which generates a hyster-

esis offset voltage across an internal 1 k

Ω resistor at the

comparator input. The comparator output remains on until the

voltage at the comparator input, now equal to the temperature

sensor voltage VPTAT summed with the hysteresis effect,

returns to the programmed setpoint voltage. The comparator

then returns LOW, deactivating the open-collector output and

disabling the hysteresis current buffer output. The scale factor

for the programmed hysteresis current is

II

A

C

A

VREF

==

° +

57

µµ

/

Thus, since VREF = 2.5 V, a reference load resistance of 357 k

Ω

or greater (output current of 7

µA or less) will produce a tempera-

ture setpoint hysteresis of zero degrees. For more details, see the

temperature programming discussion that follows. Larger

values of load resistance will only decrease the output current

below but will have no effect on the operation of the device. The

amount of hysteresis is determined by selecting an appropriate

value of load resistance for VREF, as shown below.

Programming the TMP12

The basic thermal monitoring application only requires a simple

three-resistor ladder voltage divider to set the high and low

setpoints and the hysteresis. These resistors are programmed in

the following sequence:

1. Select the desired hysteresis temperature.

3. Select the desired setpoint temperatures.

4. Calculate the individual resistor divider ladder values needed

to develop the desired comparator setpoint voltages at the

Set High and Set Low inputs.

The hysteresis current is readily calculated, as shown above. For

set to 17

are determined using the VPTAT scale factor of

55

273 15

mV K

mV

C

//

.

=° +

(

)

which is 1.49 V for 25

°C. Finally, the divider resistors are calcu-

lated, based on the setpoint voltages.

The setpoint voltages are calculated from the equation

VT

mV C

SET

SET

=+

(

)

°

(

)

273 15 5

./

Figure 5, determines the setpoints and the hysteresis value.

The equations used to calculate the resistors are

R1 k

V

– V

/ I

V– V

/ I

REF

SETHIGH

VREF

SETHIGH

VREF

Ω

()

=

()

2.5

R2 k

V

– V

/I

SETHIGH

SETLOW

VREF

Ω

()

R3 k

V

/I

SETLOW

VREF

Ω

V+

TMP12

OVER

UNDER

GND

HEATER

18

27

36

45

Figure 5. Setpoint Programming

For example, setting the high setpoint for 80

°C, the low setpoint

for 55

°C, and hysteresis for 3°C produces the following values:

II

C

A C

A

AA

A

HYS

VREF

== ° ×

°

(

+=

35

7

15

7

22

µµ

µµ

µ

/

VT

mV

C

CmV

C

V

SETHIGH

SETHIGH

=+

(

)

°

(

°+

(

)

°

(

273 15 5

80

273 15 5

1 766

./

./

.

VT

mV

C

CmV

C

V

SETLOW

SETLOW

=+

(

)

°

(

°+

(

)

°

(

273 15 5

55

273 15 5

1 641

./

./

.

R

VREF

V

I

VV

A

SETHIGH

VREF

1k

k

Ω

Ω

()

=

()

=

–/

.– .

/

.

25

1766

22

33 36

µ

RV

V

I

VV

A

SETHIGH

SETLOW

VREF

2k

k

Ω

Ω

()

=

()

=

–/

.– .

/

.

1 766

1 641

22

5 682

µ

RV

I

V

A

SETLOW

VREF

3k

k

ΩΩ

()

=

/.

/

.

1 641

22

74 59

µ

The total of R1 + R2 + R3 is equal to the load resistance needed to

The nomograph of Figure 6 provides an easy method of determin-

ing the correct VPTAT voltage for any temperature. Simply locate

the desired temperature on the appropriate scale (K,

°C, or °F)

and read the corresponding VPTAT value from the bottom scale.