LM339: Designing schmitt trigger

ISHANK VARSHNEY

Part Number: LM339

Hello Sir,

Can i use LM339 to design the Schmitt trigger having different threshold voltages i.e, Vth=10V and Vtl=2V ? If yes, then how could i do ? Or is there any schmitt trigger IC having these threshold voltages ?

over 7 years ago

0 Jonathan Nguyen over 7 years ago

TI__Expert 4555 points

Ishank,

8V is quite a large amount of hysteresis. You could always divide down your input signal to avoid using such a large amount of hysteresis. We have an application note detailing how to design hysteresis using external resistors:

www.ti.com/.../tidu020a.pdf

Regards,

Jonny

0 Paul Grohe over 7 years ago

TI__Guru 69006 points

Hello Ishank,

Yes. This is called "Asymmetrical Hysteresis". Google "Comparator Asymmetrical Hysteresis".

You can use opposing diodes in the feedback path to separate the positive and negative hysteresis feedback paths.

See the attached TINA sim and also the TI Precision labs section 15.2 on comparator hysteresis.

TI Precision Labs - Op Amps: Comparator Applications 2

/cfs-file/__key/communityserver-discussions-components-files/14/Asymm_5F00_Hyst.TSC

So you would run the hysteresis calculations for both desired thresholds, but you need to subtract the diode drop from the output swing for output voltage value in the calculation.

Note that the reference voltage needs to be higher than the upper threshold voltage. When the output goes low, the RFN resistor pulls the positive node low through D2 creating a voltage divider with RS.

The "positive" RFP path is optional, as VREF sets the upper threshold, but the RFP path does add extra hysteresis for noise immunity.

Amplifiers

Amplifiers forum

LM339: Designing schmitt trigger