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Original question:

LM2903-Q1: Input Leakage current of LM2903-Q1

LM2903B-Q1: Input stage behaviour of LM2903B-Q1 during power OFF condition

Manoj Turpati
Prodigy 230 points
Part Number: LM2903B-Q1
Other Parts Discussed in Thread: LM2903B, LM193

Hello TI,

Apologies for opening a new request, since i am not able to reopen the previous thread LM2903-Q1: Input Leakage current of LM2903-Q1 - Amplifiers forum - Amplifiers - TI E2E support forums 

I have a question regarding internal circuit of LM2903B-Q1. I have connected 15V to the non-inverting input of LM2903B-Q1 which has 400nF capacitor and 10Meg resistance in parallel, when there is a power down scenario occurs, 15V will discharge through 10Meg over period of 5 seconds. I would like to understand how the input stage behaves during this condition. since as per below diagram the internal circuit of LM2903B-Q1 has diode D1 which will be forward biased during power off condition and due to this, the transistor Q2 base will see a voltage and its collector&emitter are at ground potential. So i would like to know base to emitter breakdown voltage of Q2. 

Best Regards,

Manoj.

  • 0
    TI__Guru 53776 points

    Hello Manoj,

    We cannot comment on specific internal device characteristics...but suffice to say that these PNP transistors have a large reverse VBE tolerance to withstand the 40V Abs Max.

    The inputs were designed to handle 40V with the supply off without damage ("failsafe" input) .

    Leakages are in the pA's at room temperature, and rise up to 15-20nA at 125°C. At 15V, it looks to be about 12nA.

    Note that there will be bias current (~3.5nA) flowing OUT of the input towards ground while the power is on (helping charging the cap). When the supply collapses, the current will stop when the input is about 1V below Vcc so the supply collapse rate may also affect your timing.

  • 0 in reply to Paul Grohe
    Prodigy 230 points

    Hello Paul,

    Thanks for the information. This will help my tolerance calculations.

    Best Regards,

    Manoj.

  • 0 in reply to Manoj Turpati
    Prodigy 230 points

    Hello Paul,

    I would like to do a quick test for the leakage current measurement, but unfortunately i do not have LM2903B Ics with me. I only have LM193 with me so I would like to do the test with LM193, assuming both Ics internal input circuitary are identical. Is my understanding correct ? 

    Best Regards

    Manoj.

  • 0 in reply to Manoj Turpati
    TI__Guru 53776 points

    Hello Manoj,

    The original LM193 and the LM2903B are on different fab processes. The LM193 is on a 50 year old bipolar process, and the "B" on a modern bipolar process. The internal leakages will not be the same between the two - maybe up to 10x different.

    The pA's to 15nA above was from a measurement I made a few years ago on the LM2903B - so that is what you should see. But both should be in the low pA's at room temperature - hot is where you would see a difference.