ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 10

• Manufacturer Part Number: ADT14GP
• Description: Quad Setpoint, Programmable Temperature Monitor and Controller
• Manufacturer: AD [Analog Devices]

ADT14
High Current Switching
As mentioned earlier, internal dissipation due to large loads on
the ADT14 outputs will cause some temperature error due to
self-heating. External transistors buffer the load from the ADT14,
so that virtually no power is dissipated in the internal transistors
and minimal self-heating occurs. Figures 19a–19c show a few
examples using external transistors. The simplest case uses a
single transistor on the output to invert the output signal as
shown in Figure 19a. When the open-collector of the ADT14
turns ON and pulls the output down, the external transistor Q1’s
base will be pulled low, turning off the transistor. Another transis-
tor can be added to reinvert the signal as shown in Figure 19b.
Figure 19a. An External Resistor Minimizes Self-Heating
NC
NC
2
1
3
4
5
6
7
8
NC = NO CONNECT
ADT14
NC
NC
2
1
3
4
5
6
7
8
NC = NO CONNECT
HYSTERESIS
GENERATOR
REFERENCE
& VOLTAGE
SENSOR
TEMP
ADT14
Figure 19c. Darlington Transistor Can Handle Large Currents
16
15
14
13
12
10
11
9
V
REF
HYSTERESIS
GENERATOR
REFERENCE
& VOLTAGE
SENSOR
TEMP
V+
4.7k
Q1
I
L
16
15
14
13
12
11
10
9
V
REF
–10–
4.7k
Now, when the output of the ADT14 is pulled down, the first
transistor, Q1, turns off and its collector goes high, which turns
on Q2, pulling its collector low. Thus, the output taken from the
collector of Q2 is identical to the output of the ADT14. By pick-
ing a transistor that can accommodate large amounts of current,
many high power devices can be switched.
An example of a higher power transistor is a standard Darlington
configuration as shown in Figure 19c. The part chosen, TIP-110,
can handle 2 A continuous, which is more than enough to control
many high power relays. In fact the Darlington itself can be used
as the switch, similar to MOSFETs and IGBTs.
NC
NC
V+
Figure 19b. Second Transistor Maintains Polarity of
ADT14 Output
2N1711
1
2
4
6
8
3
5
7
NC = NO CONNECT
4.7k
ADT14
TIP110
+12V
HYSTERESIS
GENERATOR
REFERENCE
& VOLTAGE
SENSOR
TEMP
I
L
RELAY
16
15
14
13
12
11
10
9
V
REF
V+
4.7k
4.7k
Q1
2N1711
REV. 0
Q2
2N1711
I
L