LM2903YPT comparator functionality

[Mario84]

Hello,

I am redesigning a circuit that uses the comparator LM2903YPT, because the previous circuit had an undefined output because of the inputs that were outside the common mode range.

According to the datasheet the input common mode range for my application is between -5V53 to -2V, considering also the entire operating temperature range.

I expected that the output should change between Vol and Voh, where Voh is similar to Vcc+ and Vol is -5V5+0.25V=-5V25.

As I can see in the simulation the output changes between  ~0V and -304mV. The inputs are now inside of the common mode range.

Could you hep me please to understand the behavior of the comparator? Or if there is any issue in my circuit?

 

Thank you in advance!

[Peter BENSCH]

Welcome ​@Mario84, to the community!

At least I can make nothing of your simulation circuit. If by pin numbers 30, 44 and 55 you mean pins 3, 4 and 5, then the LM2903 is connected completely wrongly. Please check your schematics again against the datasheet.

Regards
/Peter

[Mario84]

Hi Peter,

Thank you.

Please see below the meaning of pins 44,55 and 30 from lm290x_spice.txt model that I downloaded from your website:

LM2901/LM2903/LM2901H/LM2903H spice macromodel
* CONNECTIONS : (corrected feb/17)
* 2 NON-INVERTING INPUT
* 1 INVERTING INPUT
* 44 POSITIVE POWER SUPPLY
* 55 NEGATIVE POWER SUPPLY
* 30 OUTPUT
 

Best regards,

Marius 

[Peter BENSCH]

Thank you for the clarification.

Yes, your circuit has a problem and cannot work in this form. First of all, because of its bipolar design, the comparator only operates from a voltage that is not specified in the datasheet, probably from around 1.5...2 V.

But even if one assumes operation of the LM2903 theoretically from 0V, node Vref always sees a more positive potential than node Vn, because the voltage divider R5/R4 divides less than R3/R1, which leads to a high output at the comparator output, i.e. your simulation result. If Vout would be high, Vref becomes even more positive, which reinforces the effect.

If you describe the overall function you would like to achieve with it, someone might be able to make a suggestion.

Regards
/Peter

[Mario84]

Hi Peter,

Thank you very much for your feedback.

There are two isolated DCDCs on my board that make the +5V5 and -5V5 voltage rails. These voltage rails are used in the bias circuit for the RF components.

The negative voltage rail shall be produced first and after that the positive one.

The comparator shall detect the presence of the -5V5, and its output that is routed through a digital isolator shall enable the +5V5 DCDC.

Best regards,

Marius

[Mario84]

Hi,

I did the following changes to the circuit:

- I changed the zener diode to one that has a lower Vbr, like 2.4V

- I updated the values of resistors R2 (330 -> 620 Ohms) and R1( 470k- > 330k)

The inputs of the comparator are now inside of the CMVR: IN- : -2.2V and IN+: -2.938V

I think that the circuit works fine now. What do you think?