Testing to prove EN/UVLO pin can be pulled low and measuring the corresponding output voltage point that will occurs

Thread Starter

jim0000

Joined Oct 28, 2020
130
Goal:
Measure EN/UVLO node to show how it can be pulled low
Measure Vbattery(would be power supply in lab) to show the corresponding battery voltage this effect will occur at (ideally 13.5V)
Measure Vgate to make sure voltage divider with the diodes provides the correct base voltage to transistor

Note: V1 and V2 will be two different power supplies since I will need to sweep v1 from 0 to at least 15. V2 can be a constant 17V

The circuit was simulated on LTspice and it worked fine. I am not sure if that will work like it did in simulation though? If I were to add LED indicators, could I just put one on the EN/UVLO node without any additional circuitry?



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WBahn

Joined Mar 31, 2012
27,903
What is this "LED indicator" you are talking about and what does it mean to put it on a node?

Whatever this "LED indicator" is, it can't present much of a load on the node at all, not with hundreds of kilohms of impedance running around.

If you want an indicator that isn't going to impact the behavior, you want it to present a very high impedance load to the node. Why not use a comparator set to trigger at about 2 V?

What is it that concerns you that it's not going to work very similarly to the simulation?

To pull the node down to common, the transistor has to sink about 45 µA of current. If we use a saturation beta of 10, which is the norm,. that means a base current of about 4.5 µA.

But let's estimate the battery voltage at which the node is pulled half way, or about 2 V. That would have about 16.5 µA flowing in R4 and 40 µA flowing in R3, thus requiring about 23.6 µA of collector current.

Looking at the curves in the data sheet (from ON Semi), the DC current gain is probably only going to be around 50 at that small of a current (making the base current about 0.5 µA) and the base-emitter voltage is going to be 600 mV or perhaps a bit less. So let's call it 600 mV. That means that there will be about 2.22 mA of current flowing in R2, making the base current negligible. That would make the voltage at the top of R1 equal to about 1.2 V. At a current of 2 mA, the voltage across the Schottky (at room temp) would typically be about 0.2 V. The voltage across the zener looks to be just a slight bit less than the nominal 12 V, so let's call it 11.9 V. The all adds up to a battery voltage of about 13.3 V, which is in pretty good agreement with your simulation, which shows about 13.4 V.

But it doesn't take into account variation from device to device or variation over temperature.

What you are needs? If had a million of these devices manufactured and characterized, what is the minimum voltage and maximum voltage that acceptable devices would have to fall between?

THAT is what will dictate whether your design is adequate.
 

Thread Starter

jim0000

Joined Oct 28, 2020
130
What is this "LED indicator" you are talking about and what does it mean to put it on a node?

Whatever this "LED indicator" is, it can't present much of a load on the node at all, not with hundreds of kilohms of impedance running around.

If you want an indicator that isn't going to impact the behavior, you want it to present a very high impedance load to the node. Why not use a comparator set to trigger at about 2 V?

What is it that concerns you that it's not going to work very similarly to the simulation?

To pull the node down to common, the transistor has to sink about 45 µA of current. If we use a saturation beta of 10, which is the norm,. that means a base current of about 4.5 µA.

But let's estimate the battery voltage at which the node is pulled half way, or about 2 V. That would have about 16.5 µA flowing in R4 and 40 µA flowing in R3, thus requiring about 23.6 µA of collector current.

Looking at the curves in the data sheet (from ON Semi), the DC current gain is probably only going to be around 50 at that small of a current (making the base current about 0.5 µA) and the base-emitter voltage is going to be 600 mV or perhaps a bit less. So let's call it 600 mV. That means that there will be about 2.22 mA of current flowing in R2, making the base current negligible. That would make the voltage at the top of R1 equal to about 1.2 V. At a current of 2 mA, the voltage across the Schottky (at room temp) would typically be about 0.2 V. The voltage across the zener looks to be just a slight bit less than the nominal 12 V, so let's call it 11.9 V. The all adds up to a battery voltage of about 13.3 V, which is in pretty good agreement with your simulation, which shows about 13.4 V.

But it doesn't take into account variation from device to device or variation over temperature.

What you are needs? If had a million of these devices manufactured and characterized, what is the minimum voltage and maximum voltage that acceptable devices would have to fall between?

THAT is what will dictate whether your design is adequate.
Transistor Calculation:
I was a little confused by the transistor calculation you did, I am new to BJTs but learning. Would the transistor configuration work well? I was hoping to have the transistor fully on so I shot for a Vbase of 0.9V which would give a Vbe of about 0.9V since the emitter is grounded.

Needs:
The maximum I want this effect to take place is 13.5V, although 13.8V is the absolute maximum since battery degradation will happen. The minimum is 13V since I want to make sure the battery is fully charged (12V lead acid battery, for my uses its best not to do stage charging but to instead isolate it from the power when its fully charged). Thats the reasoning for designing the effect of pulling the EN/UVLO pin low at approximately 13.5V.

Potential LED indicator addition:
For the LED indicator I was thinking about putting it on the collector of the transistor (what I was calling the EN/UVLO node, I am referring to nodal voltage terminology from KCL). I am not sure what specific impedance you are talking about that could mess up an LED functioning? What impedance would cause one to not work on the collector? What would be a fix? I saw you mentioned a comparator, perhaps that is what I need to do. What would a common configuration be to include an LED with a comparator if a comparator is indeed needed?

Temperature Compensation:
I am hoping to compensate changes with temperature with the 2nd diode. Typically, zeners can of a 2mV/deg C change in voltage drop, and then regular diodes would have about -2mV/deg C from what I was reading at least. I was trying to counteract the changes that can happen with temperature in my zener. I didnt want a change in temperature to effect my transistor turning off or on.
 

MrChips

Joined Oct 2, 2009
27,695
R3 and R4 add up to about 500kΩ. The total current is about 34μA.
To pull the node to 0V the transistor has to sink 45μA. Is this what you expect to happen?
 

Thread Starter

jim0000

Joined Oct 28, 2020
130
R3 and R4 add up to about 500kΩ. The total current is about 34μA.
To pull the node to 0V the transistor has to sink 45μA. Is this what you expect to happen?
Oh I see, so I my transistor needs to sink more current than there is available here (at least in that part of the circuit)? How did you find that there is 34μA due to the r3 and r4 voltage division?
 

WBahn

Joined Mar 31, 2012
27,903
Transistor Calculation:
I was a little confused by the transistor calculation you did, I am new to BJTs but learning. Would the transistor configuration work well? I was hoping to have the transistor fully on so I shot for a Vbase of 0.9V which would give a Vbe of about 0.9V since the emitter is grounded.
I tried to explain what I was doing at each step. Please indicate where you stopped following and I'll try to be more explicit.

Don't try to directly control the DC base-emitter voltage on the transistor -- that is a fool's errand. There is too much variability from device to device and over temperature for that to be effective.

Needs:
The maximum I want this effect to take place is 13.5V, although 13.8V is the absolute maximum since battery degradation will happen. The minimum is 13V since I want to make sure the battery is fully charged (12V lead acid battery, for my uses its best not to do stage charging but to instead isolate it from the power when its fully charged). Thats the reasoning for designing the effect of pulling the EN/UVLO pin low at approximately 13.5V.
So perhaps shoot for a design that, under worst-case for lowest transition point occurs at 13.2 V and worst case for highest transition point is 13.6 V. That gives you a bit of margin on either side.

IF you can design a circuit that meets those goals, taking min/max values for everything into account, including over temperature, then you are good to go. But you might well find that you can't guarantee that and so you need a more precise method. One such method is to use a precision voltage reference and a comparator.

Potential LED indicator addition:
For the LED indicator I was thinking about putting it on the collector of the transistor (what I was calling the EN/UVLO node, I am referring to nodal voltage terminology from KCL). I am not sure what specific impedance you are talking about that could mess up an LED functioning? What impedance would cause one to not work on the collector? What would be a fix?
I still don't know what you are thinking of as this "LED indicator". Is it just an LED tied to some node? With a resistor? With a transistor switch? Is it intended to turn on when the battery is above the switch-over voltage, or when it is below that voltage? There are lots of ways to add an LED indication and some of them will work great and others will completely disrupt your circuit's behavior.

I saw you mentioned a comparator, perhaps that is what I need to do. What would a common configuration be to include an LED with a comparator if a comparator is indeed needed?
Many comparators use open collector outputs, which would work well for you since you are trying to pull a node LO. You can use that same output to sink current from an LED (with appropriate transistor) if you want the LED to turn on when the node is being pulled LO. But where will the LED get its power from? From the 17 V source? Or from the battery?

Temperature Compensation:
I am hoping to compensate changes with temperature with the 2nd diode. Typically, zeners can of a 2mV/deg C change in voltage drop, and then regular diodes would have about -2mV/deg C from what I was reading at least. I was trying to counteract the changes that can happen with temperature in my zener. I didnt want a change in temperature to effect my transistor turning off or on.
I suspected that you might be trying to do temperature compensation, but you have bigger fish to fry.

First off, what is the temperature range over which your circuit needs to operate?

Next, how much good is it to compensate changes of, say, 50 mV in zener voltage due to a 25°C temperature change when the out-of-the-box variation in the diode probably spans a range of about 1.5 V? Or the effect of the tolerances of your resistors?

If you are at a point where the temperature coefficient of your zener needs to be compensated for, then you almost certainly need a much better approach (again, consider using a precision voltage reference and a comparator).
 

MrChips

Joined Oct 2, 2009
27,695
Oh I see, so I my transistor needs to sink more current than there is available here (at least in that part of the circuit)? How did you find that there is 34μA due to the r3 and r4 voltage division?
Knowing how to apply Ohm’s Law can do wonders.

And in response to the other thread, when you connect to an ideal voltage source make sure that you have enough series resistance to limit the source current otherwise you are bound to let out the magic smoke.
 

Thread Starter

jim0000

Joined Oct 28, 2020
130
Knowing how to apply Ohm’s Law can do wonders.

And in response to the other thread, when you connect to an ideal voltage source make sure that you have enough series resistance to limit the source current otherwise you are bound to let out the magic smoke.
Would the series resistance ever be needed in the lab? I was thinking the power supply would have enough but maybe I am wrong.
 

MrChips

Joined Oct 2, 2009
27,695
Would the series resistance ever be needed in the lab? I was thinking the power supply would have enough but maybe I am wrong.
Yes. The power supply needs a load otherwise the supply current is infinite when the supply is shorted.

We are talking about real circuit design.

1670256891168.png

You need a load for the circuit to operate properly.

For VBB, the load is RB plus the base-emitter junction of the BJT.
For VCC, the load is RC plus the collector-emitter junction of the BJT.
 

Thread Starter

jim0000

Joined Oct 28, 2020
130
I tried to explain what I was doing at each step. Please indicate where you stopped following and I'll try to be more explicit.

Don't try to directly control the DC base-emitter voltage on the transistor -- that is a fool's errand. There is too much variability from device to device and over temperature for that to be effective.



So perhaps shoot for a design that, under worst-case for lowest transition point occurs at 13.2 V and worst case for highest transition point is 13.6 V. That gives you a bit of margin on either side.

IF you can design a circuit that meets those goals, taking min/max values for everything into account, including over temperature, then you are good to go. But you might well find that you can't guarantee that and so you need a more precise method. One such method is to use a precision voltage reference and a comparator.



I still don't know what you are thinking of as this "LED indicator". Is it just an LED tied to some node? With a resistor? With a transistor switch? Is it intended to turn on when the battery is above the switch-over voltage, or when it is below that voltage? There are lots of ways to add an LED indication and some of them will work great and others will completely disrupt your circuit's behavior.



Many comparators use open collector outputs, which would work well for you since you are trying to pull a node LO. You can use that same output to sink current from an LED (with appropriate transistor) if you want the LED to turn on when the node is being pulled LO. But where will the LED get its power from? From the 17 V source? Or from the battery?



I suspected that you might be trying to do temperature compensation, but you have bigger fish to fry.

First off, what is the temperature range over which your circuit needs to operate?

Next, how much good is it to compensate changes of, say, 50 mV in zener voltage due to a 25°C temperature change when the out-of-the-box variation in the diode probably spans a range of about 1.5 V? Or the effect of the tolerances of your resistors?

If you are at a point where the temperature coefficient of your zener needs to be compensated for, then you almost certainly need a much better approach (again, consider using a precision voltage reference and a comparator).
Switching to comparator design:
Okay thank you, I think I would like to just switch to a design that uses a comparator. It seems like that would be best practice anyway and then I wouldn't really have to worry about fluctuations with temperature. This might require a little bit of reading since I am unsure how to use the comparator to pull the pin low. I am limited on voltage regulators and op amps but I have a couple to choose from:
Comparator:
lm324 or lm339
Regulator:
lm431 or lm117

Temp range:
The temperature range would be anywhere from 25C to 50C worst case scenario

LED indicator:
As long as it didnt change my circuit I was hoping to just have an LED on the pin that will be pulled low to show this effect. So on the EN/UVLO node (at the collector right now) which would be on due to the 17V source at first, and then off once the pin is pulled low.
 

Thread Starter

jim0000

Joined Oct 28, 2020
130
Yes. The power supply needs a load otherwise the supply current is infinite when the supply is shorted.

We are talking about real circuit design.

View attachment 282244

You need a load for the circuit to operate properly.

For VBB, the load is RB plus the base-emitter junction of the BJT.
For VCC, the load is RC plus the collector-emitter junction of the BJT.
Oh okay yeah that does make sense. It seems like as long as my circuit isn't shorted to ground somewhere I should be protected from the infinite current.
 

WBahn

Joined Mar 31, 2012
27,903
Switching to comparator design:
Okay thank you, I think I would like to just switch to a design that uses a comparator. It seems like that would be best practice anyway and then I wouldn't really have to worry about fluctuations with temperature. This might require a little bit of reading since I am unsure how to use the comparator to pull the pin low. I am limited on voltage regulators and op amps but I have a couple to choose from:
Comparator:
lm324 or lm339
Regulator:
lm431 or lm117
The LM324 is not a comparator and really shouldn't be used as one. If you need a comparator, use a chip that is designed to be a comparator, such as the LM339.

Don't use an adjustable regulator to get your voltage reference. The fact that it is adjustable means that there is going to be greater uncertainty in what the voltage actually is. If you want a reference voltage, then use an IC that is designed to be used as a reference voltage, such as the LT1236-5.

https://www.analog.com/en/products/lt1236.html#product-overview

There are many other similar devices, notably from Linear Technologies, Analog Devices/Maxim.

The cost depends on how tight you want the tolerance to be. If you are willing to spend upwards of $20, you can get 0.02% with a tempco of 2 ppm/°C. If 2% and 250 ppm/°C is good enough, then you are under $2.

Temp range:
The temperature range would be anywhere from 25C to 50C worst case scenario
So to keep the temperature variation over that range in the 1% range, you would need something with less than 200 ppm/°C.

LED indicator:
As long as it didnt change my circuit I was hoping to just have an LED on the pin that will be pulled low to show this effect. So on the EN/UVLO node (at the collector right now) which would be on due to the 17V source at first, and then off once the pin is pulled low.
When your EN/UVLO pin is at 4 V, it can't drive an LED because there is a very high resistance from that node to your 17 V source. That node is not suitable for driving anything that needs anything more than a few microamps of current.
 
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