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TI 施密特触发器介绍&TI 施密特触发器介绍
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TI 施密特触发器介绍&TI 施密特触发器介绍
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Application Brief
Understanding Schmitt Triggers
Most CMOS, BiCMOS and TTL devices require fast
edges on the high and low transitions of their inputs.
If the edges are too slow, they can cause excessive
current, oscillation, or may damage the device.
Slow or Noisy Edges
Slow edges are sometimes hard to avoid at power-
up or when using push-button or manual switches
with the large capacitors needed for filtering. Heavily
loaded outputs can also cause input rise and fall time
to be out of specification for the next part down the
line. On a normal (non-Schmitt trigger) input, the part
will switch at the same point on the rising edge and
falling edge. With a slow rising edge the part will
switch at the threshold. When the switch occurs, it will
require current from Vcc.
When current is forced from VCC, the VCC level
can drop and cause the threshold to shift. When the
threshold shifts it will cross the input again causing
the part to switch again. This pattern can continue
causing oscillation, which can cause excessive
current. This pattern can also happen if noise is on
the input. The noise can cross the threshold multiple
times and cause oscillation or multiple clocking.
Hysteresis
The solution to these problems is to use a Schmitt
trigger device to translate the slow or noisy edges into
something faster that will meet the input rise and fall
specifications of the following device. A true Schmitt
trigger does not have rise and fall time limitations.
Parts with Schmitt trigger action have a small amount
of hysteresis that helps with noise rejection but
still have an input rise and fall time-limit. These
parts usually do not have VT specifications in the
data sheet and have rise and fall time limitations
specified for the inputs in the recommended operating
conditions.
The true Schmitt trigger input has the switching
threshold adjusted where the part will switch at a
higher point (Vt+) on the rising edge and at a lower
point (Vt–) on the falling edge. The difference in these
switching points is called Hysteresis (^Vt). Here is an
example of Schmitt trigger specs:
Table 1. Example of Schmitt Trigger Specs
PARAMETER V
cc
MIN MAX UNIT
V
T+
(Positive-going input threshold voltage)
1.65 V 0.76 1.13
V
2.3 V 1.08 1.56
3 V 1.48 1.92
4.5 V 2.19 2.74
5.5 V 2.65 3.33
V
T–
(Negative-going input threshold voltage)
1.65 V 0.35 0.59
V
2.3 V 0.56 0.88
3 V 0.89 1.2
4.5 V 1.51 1.97
4.5 V 1.88 2.4
www.ti.com Application Brief
SCEA046A – SEPTEMBER 2011 – REVISED NOVEMBER 2022
Submit Document Feedback
Understanding Schmitt Triggers 1
Copyright © 2022 Texas Instruments Incorporated
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