Datasheet

Hysteresis
Control
Positive
Tempco
Reference
HYST
SET
Negative
Tempco
Reference
TMP709
R
SET
R
PULLUP
+2.7 V to +5.5 V
OT
VCC
GND
TMP709
SBOS583A –DECEMBER 2011– REVISED FEBRUARY 2012 www.ti.com
THEORY OF OPERATION
DEVICE DESCRIPTION
The TMP709 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 TMP709 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 TMP709 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
2°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
reference using the external R
SET
resistor, the negative tempco reference, and the hysteresis control. The
voltage of the positive tempco reference is controlled by external resistor R
SET
.
Figure 5. Circuit Details
The TMP709 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; 2°C for logic 0 on the HYST
pin). Increasing the voltage of the positive tempco reference after the TMP709 trips stops the TMP709 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).
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