LM393: Problem of abnormal output of LM393

user5840537

Part Number: LM393
Other Parts Discussed in Thread: TLV1812, TLV1822, LM339

Tool/software:

AB/BA interchange has been done. This time, 100% of the newly purchased batches are abnormal. The outputs of the old batches and the ON brand's LM393 are normal.

Application Circuit: Build a comparator circuit as shown in the following figure using LM393 to achieve overcurrent protection. Set a 6V reference voltage at the inverting input terminal, and connect the current detection at the non-inverting input terminal. The output terminal is pulled up to a 12V power supply through a 1k resistor, and a diode 1N4148 is used to feed the output voltage back to the non-inverting terminal to realize the overcurrent protection lockout function.

Problem Description:

Using the original materials, the circuit functions normally: After the auxiliary power supply of 12V is powered on, the output of the comparator is at a low level. When an overcurrent occurs in the circuit, the output of the comparator changes to a high level.
Using the current materials, the circuit functions abnormally: After the auxiliary power supply of 12V is powered on, the comparator outputs a high level immediately. At this time, there is no current in the circuit, and the current detection signal is 0.

Raw material model and test waveform:

Note: Channel 4 is the output; Channel 3 is the inverting terminal; Channel 2 is the non-inverting terminal

Current material model and test waveform:

Note: Channel 4 is the output; Channel 3 is the inverting terminal; Channel 2 is the non-inverting terminal

7 months ago

+1 Clemens Ladisch 7 months ago

Guru 289160 points

Below the minimum recommended supply voltage of 2 V, correct operation is not guaranteed. And it is reasonable for the output to be inactive (i.e., high) during power up. Your latching circuit relies on implementation details that are not actually guaranteed by the datasheet. (Also see section 5.3 of the Application Design Guidelines for LM339, LM393, TL331 Family Comparators.)

There are comparators with a power-on reset circuit, but comparators wioh open-drain outputs (e.g. TLV1822) make them inactive during reset, which is not what you want. But the TLV1812 (with push/pull outputs) forces its outputs low during reset and should work.

+1 Michael Hawkins 7 months ago

TI__Intellectual 1320 points

Hello, as Clemens suggested, correct operation may not apply below a supply voltage of 2V, and using a power-on reset comparator such as TLV1812 should work.

Best,

Michael

+1 Paul Grohe 7 months ago

TI__Guru 54316 points

Please also see sections 3 and 5.10 of the LM339 family appnote:

Application Design Guidelines for LM339, LM393, TL331 Family Comparators

The start-up behavior between the "classic" and "new" die are different.

The "classic" die output would flop low when the supply voltage was under 2V and the new devices will stay high. Unfortunately, the new devices behavior will continue into the future.

The material datecodes shown have the "PCN#1" "new", or "B" die.

As Clemens mentioned, you could replace it with the TLV1812, which will hold the output low and ignore the inputs until after the POR period. The 1k pull-up resistor would no longer be needed.

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LM393: Problem of abnormal output of LM393