LM139 Unused Pin Termination

Josh,

There are few correct answers to the question.

In order from most caring to least.

Connect the inputs to two different (not low impedance) voltages. If connected to low impedance source use a series resistor. This gives a known and stable output that is not used.

Connect one input to GND/VCC- pin through series resistance and the other to VCC thorugh a series resistance. This gives a known and stable output that is not used.

Same as above but lose the resistors (be sure there no ground shift between low input and LM139 GND/VCC- pin). This gives a known and stable output that is not used.

Connect both input pins to GND/VCC- pin. This gives an unknown and possibly unstable output that is not used.

Leave both pins open. The ouput that is not used will be stable.

Never ever connect an input directly to ground when the LM139 is powered by dual supplys, +/-15V for example. If the -15V supply comes up last, the -15V could go to positvie voltage and damage the input.

Josh,

The output can be left open.

Excuse me for squeezing into this discussion, but I just want to clarify what Ron is suggesting here versus what the LM139 datasheet is recommending.

In the datasheet SNOSBJ3C (5/24), under the Application Hints Section, it states there that "All pins of any unused comparators should be tied to the negative supply". Does it mean that the output pin of the unused comparator can be tied to the negative supply too? Is this suggestion true or is it a typo?

Regards,

Jojo

Joselito,
Welcome to the thread.

There are many ways to terminate unused comparators. Almost all ways are safe in normal circumstances.
It is only when abnormal circumstances are considered that the differences will matter.

For example if DUT VCC- and all three comparator pins are connected to ground. But each connection to ground was at a (slightly) different location. 
If a high current event happened in the location between the different ground locations then the voltages on the 4 "grounded" DUT pin would be at a different voltage. If this difference was large enough (>1V) then device damage could occur. 

Ron,

I'm interested in terminating the LM139 safely as well as efficiently. I don't want to damage the comparator and I don't want it to consume unnecessary power as well as generate EMI concerns. I have dual power supply (±12V) powering up the LM139 Quad pack, but I am only using one comparator in the package. So I need to properly terminate the 3 unused comparators. The way I terminated them is I tied all the unused inverting inputs together and placed a series resistor and tied this to 5V. I also terminated all the unused non-inverting inputs and tied them together and placed a series resistor and also tied this to 5V. This should give me a constant and stable 0V on the output of each of the unused comparators which is mid-point of the supply voltage of the comparators. The output pin of all the unused comparators are left floating/unconnected. I'm thinking this would be safe and will keep the unused comparators from consuming unnecessary power. Can you please confirm if my termination configuration is correct and achieves the desired performance for the unused comparators?

Thanks,

Jojo

Joselito,

If the inverting and non-inverting inputs are both tied to 5V then the input differential voltage is 0V. The output will not be known.
To ensure the outputs are a static low, the inverting input must be greater than non-inverting input by more than the maximum VIO (DS offset voltage).

Ron,

Thanks for your feedback. So if I follow your suggested configuration to ensure the outputs are at static low, will this guarantee the unused comparators not to consume unnecessary power and stay quiet at all times?

Thanks,

Jojo

Yes.

 Thanks!

I may have the problem described earlier for a unused LM139 unused compartor  "Never ever connect an imput to ground when the LM139 is powered by dual supplies. If the -15V supply comes up last, the -15V could go positive voltage and damage the input.

I have two unused comparators in different IC's with damaged inputs. The power supplies are +15V and -5V the -ve imput is connected to +15 and the +ve input to GND.

Can you provide more detail as to what happens if the -5V supply does not come up as fast as the +15V supply.

Both of the comparators unused inputs are connected the same way.

John,

The problem (when it occurs) happen like this. The 15V supply comes on and there are loads connected from +15V to -5V supply. Therefore current flows to -5V supply. If the -5V turn on is delayed then the -5V node may become a positive voltage.

The input pins on LM139 has a parasitic diode from input (cathode) to LM339 VCC- (anode). The positive voltage on -5V supply (VCC-) will flow through this diode to (+ve) ground and if the current is large can cause device damage.

The damage will occur internally near the input pin that was connected to ground.

It is a common wisdom to add a clamping diode across negative rail to prevent it being pulled to positive by loads like OP Amps and comparators (powered between pos and neg rail i.e. capable of doing this)

Some datasheets recommend:

"Termination of Unused Inputs
Proper termination of unused inputs is a good practice to
keep the output from ‘chattering.’ For example, if one
channel of a dual or quad package is not being used, then the
inputs must be connected to a defined state. The
recommended connections would be to tie one input to VCC
and the other input to ground."

So, I never leave both inputs pins open.

Hi Ron,

    Form you email: Never ever connect an input directly to ground when the LM139 is powered by dual supplys, +/-15V for example. If the -15V supply comes up last, the -15V could go to positvie voltage and damage the input.

   Does it mean that ground cannot be used a 0V reference voltage connected to LM139's input when it is powered by dual power supply +/-15V?

 Regards

 Jim Wei

Jim,

The key word is 'directly'. Add a resistor between ground and the input and everything is good again.

1k ohm is plenty to keep any current flow to safe level.