# How does this circuit turn on an LED?

#### matthej

Joined Oct 10, 2020
59
Hi,

I am looking at this eval board which supposedly works and I am wondering how the LED circuit operates. It seems to me that the U537 inverter will not go to a low state which turns on the LED. The output of U526 is an open drain output which gets pulled up to 1.8V thru a 10K resistor. Won't that mean that the input to U537 will be 1.8V max? The U537 is operating at 3.3V which has a Vih = 2.0V (min) and Vil = .8V max.

Thanks!

#### dl324

Joined Mar 30, 2015
13,836
The output of U526 is an open drain output which gets pulled up to 1.8V thru a 10K resistor. Won't that mean that the input to U537 will be 1.8V max? The U537 is operating at 3.3V which has a Vih = 2.0V (min)
It looks like another example of a poor design that depends on a voltage below the minimum guaranteed high level input voltage being recognized as a HIGH.

#### matthej

Joined Oct 10, 2020
59
It looks like another example of a poor design that depends on a voltage below the minimum guaranteed high level input voltage being recognized as a HIGH.
I was thinking of changing U537 to an open drain inverter running at 1.8V. Would that work?

#### Papabravo

Joined Feb 24, 2006
17,580
Hi,

I am looking at this eval board which supposedly works and I am wondering how the LED circuit operates. It seems to me that the U537 inverter will not go to a low state which turns on the LED. The output of U526 is an open drain output which gets pulled up to 1.8V thru a 10K resistor. Won't that mean that the input to U537 will be 1.8V max? The U537 is operating at 3.3V which has a Vih = 2.0V (min) and Vil = .8V max.

Thanks!

View attachment 243724
Those are TTL thresholds. The 74LVC1G04 has a threshold at Vcc/2 = 1.65V. If you want some hysteresis you can use the 74LVC1G14 Schmitt Trigger. The 1.8V pullup will get you above the 1.65V threshold, but can't your pullup go to 3.3V, or what is the point of having an open drain output?
The recommended max Vo is 3.6Volts and the absolute max (not to exceed) is 4.6V and since the resistor limits the current you're probably safe.

Edit: I see the problem is U528. So what is the maximum input voltage on the 74AUP1G11?
Looks like absolute max is 4.6 and recommended max is 3.6 It might be OK to pull up to a higher voltage than the 1.8V. I would check with an FAE before you decide to do this however. Many devices nowadays will tolerate input voltages higher than Vcc. 50 years ago that was seldom the case.

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#### dl324

Joined Mar 30, 2015
13,836
I was thinking of changing U537 to an open drain inverter running at 1.8V. Would that work?
That should work.

#### matthej

Joined Oct 10, 2020
59
Those are TTL thresholds. The 74LVC1G04 has a threshold at Vcc/2 = 1.65V. If you want some hysteresis you can use the 74LVC1G14 Schmitt Trigger. The 1.8V pullup will get you above the 1.65V threshold, but can't your pullup go to 3.3V, or what is the point of having an open drain output?
Where are you seeing the Vcc/2 thresholds?
My pullup cannot go to 3.3V since it is feeding the circuit above and its at 1.8V

#### Papabravo

Joined Feb 24, 2006
17,580
Where are you seeing the Vcc/2 thresholds?
My pullup cannot go to 3.3V since it is feeding the circuit above and its at 1.8V
You are ASSUMING that the limit on input voltage for the 74AUP1G11 is Vcc. That is not necessarily the case, so long as the current into the gate is limited. With a 10K pullup to 3.3 volts the max current between 3.3V and 1.8V will be 150 μa which sounds pretty limited to me. Easily handled by the anti-static diodes.
For 3.3 volts the thresholds are not at Vcc/2, they are asymmetric as you stated originally. I was thinking of HC parts.
I also thought that the TTL threshold parts had an extra "T" in the part number as in "74LVC1GT04", maybe that was a once upon a time fever dream.

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