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Application Brief

Application Brief

Understanding Schmitt Triggers

Most CMOS, BiCMOS and TTL devices require fastedges 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 powerup or when using push-button or manual switcheswith 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 theline. On a normal (non-Schmitt trigger) input, the partwill switch at the same point on the rising edge andfalling edge. With a slow rising edge the part willswitch at the threshold. When the switch occurs, it will
require current from Vcc.

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 amountof 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 thedata 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 ahigher point (Vt+) on the rising edge and at a lower
point (Vt–) on the falling edge. The difference in theseswitching points is called Hysteresis (^Vt). Here is an
example of Schmitt trigger specs:

When current is forced from VCC, the VCC levelcan drop and cause the threshold to shift. When thethreshold shifts it will cross the input again causing
the part to switch again. This pattern can continuecausing 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.
Table 1. Example of Schmitt Trigger Specs
PARAMETER

VT+ (Positive-going input threshold voltage)

VT– (Negative-going input threshold voltage)

Vcc

MINMAX

1.65 V

0.76

1.13

2.3 V

1.08

1.56

3V

1.48

1.92

4.5 V

2.19

2.74

5.5 V

2.65

3.33

1.65 V

0.35

0.59

2.3 V

0.56

0.88

3V

0.89

1.2

4.5 V

1.51

1.97

4.5 V

1.88

2.4

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UNIT

V

V

Understanding Schmitt Triggers

1Application Brief

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Table 1. Example of Schmitt Trigger Specs (continued)
PARAMETER

ΔVT Hysteresis (VT+ – VT–)

Vcc

MINMAX

1.65 V

0.36

0.64

2.3 V

0.45

0.78

3V

0.51

0.83

4.5 V

0.58

0.93

5.5 V

0.69

1.04

It is important to remember (Vt+ max) = Vih and (VT–min) = Vil. In the specs, multiple limits are related
to the Schmitt trigger inputs. All of the limits areimportant for different reasons. On the input risingedge, the part will switch between (Vt+ min) and (Vt+max). On the falling edge, the part will switch between(Vt– max) and (Vt– min). The part will not switchbetween (Vt– max) and (Vt+ min). This is importantfor noise rejection.

UNITV

transistors are partially on. The plot below shows Icc
across the input voltage sweep.

The hysteresis is the delta between where the partswitches on the rising edge and where it switches on
the falling edge. The hysteresis will be at least the min
and no more than the max (^Vt) spec.

Figure 2. Supply Current as a Function of Input
Voltage

Figure 1.
In the figure above, the input levels Vih and Vil must
be greater than (VT+ max) and less than (VT– min)to ensure the part will switch. The switching points on
the above plot are separated to give a clearer visual
picture. In reality, the (VT+ min) and (VT– max) mayoverlap.
Input Voltage

Sine Waves
Use Schmitt triggers to translate a sine wave into asquare wave as shown in this oscillator application.
Also, use Schmitt triggers to speed up a slow ornoisy input, or clean up an input, as in the switch
de-bouncer circuit.

One common misconception is that the current
consumption will be less when switching a slow signal
into a Schmitt trigger. This misconception is partly true
because the Schmitt trigger prevents oscillation which
can draw a lot of current; however, the Icc current
may still be higher due to the amount of time the input
is not at the rail. This is Delta Icc. Delta Icc is wherethe inputs are not at the rails and upper or lower drive
2Understanding Schmitt Triggers

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Application Brief

Figure 6. Clean Noisy Signals

Figure 3. Oscillator Application Using Schmitt
Trigger Inverter

Figure 7. Convert Slow Edges
We specify the part will switch on the rising edge
between (VT+ min) and (VT+ max). We specify thepart will switch on the falling edge between (VT– max)and (VT– min).
Between (VT+ min) and (VT– max), we specify thepart will not switch. This specification can be used fornoise rejection. These 2 limits can overlap.
We specify a minimum amount of hysteresis as delta
VT min.
••Figure 4. Switch De-bouncer Using Schmitt
Trigger Inverter

Conclusion
Schmitt triggers can be used to change a sine wave
into a square wave, clean up noisy signals, andconvert slow edges to fast edges.

Vih = (VT+ max)
Vil = (VT– min)Texas Instrument Schmitt trigger functions areavailable in most all technology families from the 30year old 74XX family to the latest AUP1T family.
These two Schmitt-trigger functions are available inmost families:••

14 for inverting Schmitt trigger
17 for non-inverting Schmitt trigger

Texas Instrument also has a complete line of little
logic products with Schmitt trigger inputs.
Configurations
SN74LVC1G57, SN74LVC1G58, SN74LVC1G97,
SN74LVC1G98, SN74LVC1G99 SN74AUP1G57,
SN74AUP1G58, SN74AUP1G97, SN74AUP1G98,
SN74AUP1G99
Low to High Translators

Figure 5. Sine Wave to Square Wave

SN74AUP1T02, SN74AUP1T04, SN74AUP1T08,
SN74AUP1T14, SN74AUP1T157, SN74AUP1T158,
SN74AUP1T17, SN74AUP1T32, SN74AUP1T86

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Understanding Schmitt Triggers

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