# application specific questions about schmitt trigger threshold and hysterisis

Discussion in 'General Electronics Chat' started by donut, Feb 7, 2013.

1. ### donut Thread Starter Member

May 23, 2012
51
0
Im using the NL27WZ14 in a test application. I have a question about the DC electrical characteristics of the of the part as described on page 3 of the data sheet.

http://www.datasheetcatalog.org/datasheet2/7/0t78zz74e8fqk7ig5dtpetxq3ipy.pdf

First off I see that the Min and Max ranges that causes the trigger to switch is dependent upon VCC. The board I am testing has 5V VCC but there is no 5 V VCC in the data sheet only 4.5V VCC and 5.5V VCC so what min and max voltage threshold should I use? The one for 4.5V VCC or 5.5V VCC?

Second. I went ahead and decided to follow the 4.5V VCC voltage thresholds. -40C to 85C.
Vt-Min = 1V
Vt-Max = 2V
So this means that I will switch off(negative) between 1V and 2V?

Vt+Min = 1.9V
Vt+Max = 3.1V
So this means that I will switch on(positive) between 1.9V and 3.1V?

I see that my Vt-Max (2V) is larger than my Vt+Min (1.9V). Can you do that? Can you say that I want to turn on (go high) at 1.9V but then turn off (go low) at 2.0V? I thought there has to be some voltage delta between the Vt+min and Vt-max that keeps the NL27WZ14 in a none switching state?

So what is VH = 0.6 Min and 1.5 Max?
Does VH mean the voltage range will not switch on or off between 0.6V and 1.5V?

2. ### w2aew Active Member

Jan 3, 2012
219
64
The values for the switching thresholds at 5V will be midway between the values shown for 4.5V and 5.5V. Figure 5 in the datasheet tends to confirm the linear relationship between VCC and the threshold voltages, so figuring them as I described will be fine.

Don't worry that the min and max of these thresholds seem to "cross" each other. The thresholds will never be at the opposite extremes. They will tend to track each other over temperature.

Further proof of this is that the VH, or hysteresis voltage, shown in the table (which is just the difference between the two threshold voltages) is guaranteed to be a positive voltage, indicating that you will always have the non-switching zone between the thresholds. For the case you mentioned (4.5V VCC, -40 to +85C), the hysteresis will be at least 600mV, and no more than 1.5V.

3. ### donut Thread Starter Member

May 23, 2012
51
0
Understood. But what about the Output High voltage (Voh) and the Out Low Voltage (Vol)?

The spec says two different things that seems to be conflicting for me:

For Voh min -40 C to +85 C it says:

1) Voh min will be Vcc-0.1 for a VCC of 1.65 to 5.5
2) Voh min will be 3.8 for a VCC of 4.5V

what does 1 mean and which one is more applicable?

For Vol min -40 C to +85 C it says:

1) Vol max will be 0.1 for a VCC of 1.65 to 5.5
2) Vol max will be .55 for a VCC of 4.5V

what does 1 mean and which one is more applicable?

4. ### w2aew Active Member

Jan 3, 2012
219
64
You have to look at the conditions column. The first line which gives a spec over VCC from 1.65 o 5.5 applies when the load current (source or sink) is less than 100uA. The other values apply for the load currents shown.

5. ### donut Thread Starter Member

May 23, 2012
51
0
So for I(oL) of 32mA does that mean the NL27WZ14 is sinking or source that much current?

6. ### w2aew Active Member

Jan 3, 2012
219
64
The convention used in the datasheet is that positive current is sinking current, and negative current is sourcing current. The Maximum ratings table show that the max output current is +/-50mA.