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Adjusting charge current by adjusting control voltage into LT1248 PFC controller

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ocset:
Hi,

We have batteries which are 400v +/-5v ( no matter what the state of charge)
We wish to charge them with our “controlled open loop” method…depicted as attached. (LTspice sim and pdf schem)
Its non isolated but we’ll be ok.

We simply vary the voltage on the SS pin of the LT1248 Boost PFC controller  in order to change the charge current.
Do you see any problems with this application of a control voltage into the SS pin of the LT1248?
(LTspice sim works fine, but you never know  with  the real thing)

LT1248 datasheet
https://www.analog.com/media/en/technical-documentation/data-sheets/1248fd.pdf

moffy:
I don't understand why you don't just use the Vsense pin for your control. The SS pin is used for under/overvoltage lockout, you lose that functionality, plus you run the high risk of shorting the output of your opamp via the transistor pull down and doing some damage.

ocset:
Thanks, we need to "inflict" a voltage at the VAOUT pin in order to control the charge current into the battery.
(we can add in our own under/over voltage lockout)
I cannot see how we risk shorting the output of the EA opamp.
There are 5 ways to "Inflict" a voltage onto the VAOUT pin...these are as above in #1, and as attached.
Which do you think is the best method?

moffy:
Nicely drawn, your options. Number 5 seems the most obvious. Internal to the chip LT1248 and connected to the SS pin is an NPN transistor which shorts it to ground in the event of under/over voltage. That is where the opamp output could be shorted to ground. I would leave that pin alone as an input and use it only as designed, as an output. 8)

ocset:

--- Quote ---Internal to the chip LT1248 and connected to the SS pin is an NPN transistor which shorts it to ground in the event of under/over voltage. That is where the opamp output could be shorted to ground. I would leave that pin alone as an input and use it only as designed, as an output
--- End quote ---
Thankyou very much moffy!, yes, very nicely spotted!!...my  circuit number 1 (of post #2) is thus kaput....
...However, circuit 2  uses the SS pin and solves it with a diode....circuits 3 & 4 can also use the SS pin and not be affected by that internal NPN transistor (due again to the diode use).

There are diodes (near the Error amplifier and SS pins) shown in the datasheet block diagram of LT1248 that dont actually seem to be "real" diodes. The datasheet doesnt really explain them...but they cant possibly be there in reality. I've never seen this sort of thing on other PFC controller datasheets.

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