EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: smoothVTer on November 09, 2022, 09:25:16 pm
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I need to interface the LMT86 temperature sensor with a LTC6992-1 voltage-to-pwm converter for an application which will not be using any embedded processor. Everything is on a 3.3V single-supply. I can use op amps, references and comparators to achieve the final solution. Bandwidth is not an issue for this purpose.
The LMT86 outputs 2.6V @ -50C and 0.4V @ 150C, so it has a negative tempco. The LTC6992-1 takes 0V-1V on its MOD pin to control duty cycle proportionally. I am aware that voltage on MOD can go as high as the supply - 0.3V, while the duty cycle is maxed at 100%, but for the sake of self edification I want to clamp it to 1V. As for the PWM signal itself, I need at minimum 30% duty cycle and at maximum 100% duty cycle. So, anywhere from 300mV to 1V input to the LTC6992-1. Thus the output from any signal conditioning circuitry needs to clamp to 300mV at the low end and to 1.0V at the high end. Stated another way: if the temperature >= 75C (LMT86 outputs 1.25V), I want the duty cycle to top out to 100% If the temperature is <= 25C ( LMT86 outputs 1.75V ), I want the duty cycle to bottom out to 30%. In between 25C - 75C, I want the duty cycle to scale proportionally.
Thus, transfer characteristic I am trying to get is:
(https://www.eevblog.com/forum/projects/signal-conditioning-temp-sensor-output-with-high-low-clamp/?action=dlattach;attach=1635761;image)
The sloped portion of the line has the equation of y = -1.4x + 2.75, so I gather than the op amp needs an Av = 1.4 with an offset of 2.75V. I know I probably need to use an op amp configured as a summing amplifier, with some circuitry sprinkled in to sense and apply the clamps. However, there are some considerations I cannot figure out. Namely,
-- Since this slope is negative, I don't know how to 'subtract from' using an op amp with this particular signal. If all I needed was a gain of 1.4 and an offset of 2.75, I think I could do this part of the design, but I don't understand how the negative way would be designed.
-- How do I achieve the clamps at 300mV and 1V? There will be a fairly tight LDO used, so I can derive 300mV and 1V reference levels, buffer them, and use them ... but how would I clamp at the low and high ends?
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So, I actually figured this problem out and I am posting the solution here for posterity. Funny how, once you ask the question out loud, the answers sort of come to you :box:
Overall here is the circuit solution and sim results, at least for DC/low freq purposes:
(https://www.eevblog.com/forum/projects/signal-conditioning-temp-sensor-output-with-high-low-clamp/?action=dlattach;attach=1636112;image)
(https://www.eevblog.com/forum/projects/signal-conditioning-temp-sensor-output-with-high-low-clamp/?action=dlattach;attach=1636118;image)