Datasheet

TMP01
Rev. E | Page 11 of 20
SWITCHING LOADS WITH THE OPEN-COLLECTOR
OUTPUTS
In many temperature sensing and control applications, some
type of switching is required. Whether it be to turn on a heater
when the temperature goes below a minimum value or to turn
off a motor that is overheating, the open-collector outputs
OVER
and
UNDER
can be used. For the majority of
applications, the switches used need to handle large currents on
the order of 1 A and above. Because the TMP01 is accurately
measuring temperature, the open-collector outputs should
handle less than 20 mA of current to minimize self-heating.
The
OVER
and
UNDER
outputs should not drive the equip-
ment directly. Instead, an external switching device is required
to handle the large currents. Some examples of these are relays,
power MOSFETs, thyristors, IGBTs, and Darlingtons.
Figure 17 through Figure 21 show a variety of circuits where the
TMP01 controls a switch. The main consideration in these
circuits, such as the relay in Figure 17, is the current required to
activate the switch.
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
TMP01
R1
R2
R3
MOTOR
SHUTDOWN
2604-12-311
COTO
IN4001
OR EQUIV.
12V
0
0333-017
Figure 17. Reed Relay Drive
It is important to check the particular relay to ensure that the
current needed to activate the coil does not exceed the TMP01’s
recommended output current of 20 mA. This is easily deter-
mined by dividing the relay coil voltage by the specified coil
resistance. Keep in mind that the inductance of the relay creates
large voltage spikes that can damage the TMP01 output unless
protected by a commutation diode across the coil, as shown.
The relay shown has a contact rating of 10 W maximum. If
a relay capable of handling more power is desired, the larger
contacts probably require a commensurately larger coil, with
lower coil resistance and thus higher trigger current. As the
contact power handling capability increases, so does the current
needed for the coil. In some cases, an external driving transistor
should be used to remove the current load on the TMP01.
Power FETs are popular for handling a variety of high current
dc loads. Figure 18 shows the TMP01 driving a p-channel
MOSFET transistor for a simple heater circuit. When the out-
put transistor turns on, the gate of the MOSFET is pulled down
to approximately 0.6 V, turning it on. For most MOSFETs, a
gate-to-source voltage, or Vgs, on the order of −2 V to −5 V
is sufficient to turn the device on.
Figure 19 shows a similar circuit for turning on an n-channel
MOSFET, except that now the gate to source voltage is positive.
For this reason, an external transistor must be used as an
inverter so that the MOSFET turns on when the
UNDER
output pulls down.
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
NC = NO CONNECT
TMP01
R1
R2
R3
NC
NC
IRFR9024
OR EQUIV.
HEATING
ELEMENT
2.4kΩ (12V)
1.2kΩ (6V)
5%
V+
+
00333-018
Figure 18. Driving a P-Channel MOSFET
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
NC = NO CONNECT
TMP01
R1
R2
R3
NC
NC
IRF130
2N1711
4.7kΩ
V+
4.7kΩ
HEATING
ELEMENT
00333-019
Figure 19. Driving an N-Channel MOSFET
Isolated gate bipolar transistors (IGBT) combine many of the
benefits of power MOSFETs with bipolar transistors, and are
used for a variety of high power applications. Because IGBTs
have a gate similar to MOSFETs, turning on and off the devices
is relatively simple as shown in Figure 20.
The turn-on voltage for the IGBT shown (IRGBC40S) is
between 3.0 V and 5.5 V. This part has a continuous collector
current rating of 50 A and a maximum collector-to-emitter
voltage of 600 V, enabling it to work in very demanding
applications.
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
NC = NO CONNECT
TMP01
R1
R2
R3
NC
NC
IRGBC40S
2N1711
4.7kΩ
V+
4.7kΩ
MOTOR
CONTROL
00333-020
Figure 20. Driving an IGBT