Datasheet

V
TEMP
SM72480
V
DD
GND
OPTIONAL
BYPASS
CAPACITANCE
C
LOAD
d 1100 pF
SM72480
www.ti.com SNIS156C –NOVEMBER 2010–REVISED APRIL 2013
1. We see that for V
OL
of 0.2 V the electrical specification for OVERTEMP shows a maximim i
sink
of 385 µA.
2. Let i
L
= 1 µA, then i
T
is about 386 µA max. If we select 35 µA as the current limit then i
T
for the calculation
becomes 35 µA
3. We notice that V
DD(Max)
is 3.3V + 0.3V = 3.6V and then calculate the pull-up resistor as R
Pull-up
= (3.6 −
0.2)/35 µA = 97k
4. Based on this calculated value, we select the closest resistor value in the tolerance family we are using.
In our example, if we are using 5% resistor values, then the next closest value is 100 kΩ.
NOISE IMMUNITY
The SM72480 is virtually immune from false triggers on the OVERTEMP and OVERTEMP digital outputs due to
noise on the power supply. Test have been conducted showing that, with the die temperature within 0.5°C of the
temperature trip point, and the severe test of a 3 Vpp square wave "noise" signal injected on the V
DD
line, over
the V
DD
range of 2V to 5V, there were no false triggers.
TRIP TEST Digital Input
The TRIP TEST pin simply provides a means to test the OVERTEMP and OVERTEMP digital outputs
electronically by causing them to assert, at any operating temperature, as a result of forcing the TRIP TEST pin
high.
When the TRIP TEST pin is pulled high the V
TEMP
pin will be at the V
TRIP
voltage.
If not used, the TRIP TEST pin may either be left open or grounded.
V
TEMP
Analog Temperature Sensor Output
The V
TEMP
push-pull output provides the ability to sink and source significant current. This is beneficial when, for
example, driving dynamic loads like an input stage on an analog-to-digital converter (ADC). In these applications
the source current is required to quickly charge the input capacitor of the ADC. See the Applications Circuits
section for more discussion of this topic. The SM72480 is ideal for this and other applications which require
strong source or sink current.
NOISE CONSIDERATIONS
The SM72480's supply-noise rejection (the ratio of the AC signal on V
TEMP
to the AC signal on V
DD
) was
measured during bench tests. It's typical attenuation is shown in the Typical Performance Characteristics section.
A load capacitor on the output can help to filter noise.
For operation in very noisy environments, some bypass capacitance should be present on the supply within
approximately 2 inches of the SM72480.
CAPACITIVE LOADS
The V
TEMP
Output handles capacitive loading well. In an extremely noisy environment, or when driving a switched
sampling input on an ADC, it may be necessary to add some filtering to minimize noise coupling. Without any
precautions, the V
TEMP
can drive a capacitive load less than or equal to 1100 pF as shown in Figure 10. For
capacitive loads greater than 1100 pF, a series resistor is required on the output, as shown in Figure 11, to
maintain stable conditions.
Figure 10. SM72480 No Decoupling Required for Capacitive Loads Less than 1100 pF.
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