Can we use the LT3045-1 in place of the LT3045 without layout change?

Hi,

In the MSOP12 package, the only difference between the LT3045 and the LT3045-1 is the VIOC pin.

If we have a PCB made for the LT3045 with IN layouted to pins 1, 2 and 3, but we now use mostly the LT3045-1 (on other PCBs), can we just use the LT3045-1 on the older PCBs?

The VIOC pin can sink 15µA, so 15µA current would be wasted, but that would be OK.

In some application circuits in the datasheet, a resistor (R3) is used to pull VIOC at least to some extent towards IN in order to limit the pre-regulator's output voltage.

On the other side, the absolute maximum ratings section states -0.3 to 4V for the VIOC pin, but the 4V are also stated in the typical operation conditions and might just be the voltage that can be supplied by the VIOC pin.

So, what happens if VIOC is connected directly to IN?

Best Regards,

Gerd

  • 0
    •  Analog Employees 
    on Oct 3, 2020 12:37 AM 2 months ago

    Please respect the abs. max. rating of VIOC that I show in the image below that is an excerpt from the LT3045-1 datasheet.  Note 10 mentioned in the excerpt just details the parasitic diode that clamps VIOC and other pins to a diode drop below GND.   I will forward this post to the designer and I will post more if the designer provides input to address your comments.  From my perspective, VIOC is the output of an amplifier designed for 4VOUT and consequently it seems like a bad idea to connect VIOC to VIN without limiting the current into VIOC.  My guess is that the devices of the VIOC amplifier output stage will become reverse biased and breakdown eventually if the VIN voltage is high enough.   

    LT3045-1 abs max_1.png

  • Thanks for forwarding it to the designer.

    As you are already at it, could you also ask the designer something about the IPGFB?
    The datasheet only notes 25nA typical. From the equation on the lower left of page 21, I assume that the 25nA flow into the PGFB pin.
    Does the IPGFB change significantly with temperature or with the voltage applied to the pin? Like may it double or even tripple at low or high temperatures? Or if it is tied to OUT with an output voltage of 12V or 15V?

    Regards,
    Gerd

  • 0
    •  Analog Employees 
    on Oct 12, 2020 3:23 PM 1 month ago in reply to GerdF

    I am still waiting for the designer to respond to your original question.

  • +1
    •  Analog Employees 
    on Oct 14, 2020 3:26 PM 1 month ago in reply to GerdF

    The designer says VIOC cannot be taken beyond its abs. max. rating of 4V - doing so will damage the internal circuitry.  

    PGFB current actually flows out of the pin so there is a typo in the equation - it should be -IPGFB*RPG2.  Across process and temp, that current can change 2x to 3x as the beta of the internal transistor changes.

  • That's good to know, thanks.

    I've just seen that with the LT3094 the maximum PGFB current is specified as 100nA (30nA typical), which would match with the 2x to 3x the designer told for the LT3045.

    I assume that with the LT3094 the current flows into the IC, right? Because if it is a differential bipolar transistor pair input, it would have to be NPN transistors with the LT3094 as it would have to be PNP transistors with the LT3045 because the threshold voltage is so close to GND.

    So, if the bias current is actually flowing out of the pin (with the LT3045), this is actually kind of a pull-up current and if I wouldn't need the PGFB functionality, I could float the PGFB pin, because due to the bias current it would rise above 0,3V anyway and the PGFB pin is actually allowed to go up to the supply voltage, right?