Hysteresis

Control

Positive

Tempco

Reference

HYST

SET

Negative

Tempco

Reference

TMP708

R

SET

R

PULLUP

+2.7 V to +5.5 V

OT

VCC

GND

TMP708

SBOS585A

– DECEMBER 2011 – REVISED FEBRUARY 2012

www.ti.com

THEORY OF OPERATION

DEVICE DESCRIPTION

The

TMP708

is

a

fully-integrated,

resistor-programmable

temperature

switch

that

incorporates

two

temperature-dependent voltage references and one comparator. One voltage reference exhibits a positive

temperature coefficient (tempco), and the other voltage reference exhibits a negative tempco. The temperature at

which both voltage references are equal determines the temperature trip point.

The TMP708 temperature threshold is programmable from 0

°C to +125°C and is set by an external 1% resistor

from the SET pin to the GND pin. The TMP708 has an open-drain, active-low output structure that can easily

interface with a microprocessor.

HYSTERESIS INPUT

The HYST pin is a digital input that allows the input hysteresis to be set at either 10

°C (when HYST = VCC) or

30

°C (when HYST = GND). The hysteresis function keeps the OT pin from oscillating when the temperature is

near the threshold. Thus, the HYST pin should always be connected to either VCC or GND. Other input voltages

on this pin may cause abnormal supply currents and/or function.

CIRCUIT DETAILS

Figure 5 shows the comparator, the NFET open-drain device connected to the OT pin, the positive tempco

Figure 5. Circuit Details

The TMP708 reaches the temperature trip point when the voltage from the positive tempco reference is greater

than the voltage from the negative tempco reference. This situation causes the output of the comparator to

switch from logic 0 to logic 1. The comparator output drives the gate of the NFET open-drain device and pulls the

voltage on the OT pin from logic 1 to logic 0 under these conditions (output trips). Furthermore, the logic 1 output

from the comparator causes the hysteresis control to increase the voltage of the positive tempco reference by an

amount set by the logic setting on the HYST pin (10

°C for logic 1 on the HYST pin; 30°C for logic 0 on the HYST

pin). Increasing the voltage of the positive tempco reference after the TMP708 trips stops the TMP708 from

untripping (voltage on the OT pin changing from logic 0 to logic 1) until the local temperature has been reduced

by the amount set by the HYST pin. After the local temperature has been reduced and the voltage from the

positive tempco reference is less than the voltage from the negative tempco reference, the output of the

comparator switches from logic 1 to logic 0. This condition causes the voltage on the OT pin to change from logic

0 to logic 1 (device untrips).

6

Submit Documentation Feedback

Copyright

© 2011–2012, Texas Instruments Incorporated

Product Folder Link(s): TMP708