LM293: output pin short to VCC

George Zhao

Part Number: LM293
Other Parts Discussed in Thread: LM339

Hi team,

My customer is using LM293, in their PCB they are leaving VOUT2 floating. But in the factory fabricate, sometimes the VOUT2 may be shorted to VCC due to improper soldering. Is there potential risk?

Thanks!

George

over 2 years ago

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi George,

I have never seen shorted pins due to improper soldering in our products the last 25 years.

A short-circuit during normal operation should be avoided, of course! But even if you wire the inputs in that way, that the output is always high state, during power-up and power-down there's a risk of short-circuiting the output. This can also result in issues elsewhere on the board (ground bounce, latch-up, etc.)

But hey, we have 2021, there shouldn't be any problem with the soldering.

Kai

+1 Clemens Ladisch over 2 years ago

Guru 243910 points

Shorting VOUT2 to VCC is harmless only if this output never drives low, i.e., if that comparator always has VIN+ > VIN−. How are the inputs connected?

+1 Paul Grohe over 2 years ago

TI__Mastermind 49946 points

Hello George,

Yes. There is a BIG, potentially damaging risk.

Shorting to VCC can blow the output as the output may attempt to short the supply if the output tries to go low.

As mentioned above, they would have to *GUARANTEE* the output is NEVER set low...and that may not be completely possible during power-up or power-down.

They need to clean-up their soldering and inspection process...

0 George Zhao over 2 years ago in reply to Paul Grohe

TI__Intellectual 1635 points

Hi Paul,

Thanks for the reply. Yes I understand the risk now. since they don't use channel 2, they are shorting both the + and - inputs to GND. I think even this cannot guarantee the output never go to low because of any kind of interference.

I will let them aware of the risk.

0 George Zhao over 2 years ago in reply to Clemens Ladisch

TI__Intellectual 1635 points

Hi Clemens, thanks for the reply. They are shorting both the VIN+ and VIN- to GND.

+1 Paul Grohe over 2 years ago in reply to George Zhao

TI__Mastermind 49946 points

Hi George,

Shorting the pins together gives you a 50/50 chance of the output high or low, due to the individual internal offset voltage of that device, which can be -9mV to +9mV.

This is covered in the LM339 family application note, section 5.1:

Application Design Guidelines for LM339, LM393, TL331 Family Comparators

0 George Zhao over 2 years ago in reply to Paul Grohe

TI__Intellectual 1635 points

Thanks Paul, got it!

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LM293: output pin short to VCC