Why will LT8312 based BCM Boost PFC sim in LTspice not work?

[treez]

Hello,
Do you know why this (attached) LT8312 based BCM Boost PFC LTspice simulation won’t work?
It gives “Singular matrix” errors.
I am trying to adjust the output power by setting the Vc pin voltage by setting up the internal error amplifier as an inverting opamp, and inputting voltages into that.
I appreciate that the internal error amp is of the transconductance variety, however, LTC1871 also has this, and can be set up as shown here.

LT8312 datasheet
https://www.analog.com/media/en/technical-documentation/data-sheets/8312fa.pdf

 

[FvM]

Don't get singular matrix error.

 

[treez]

Thanks...just opened LTspice back up again, and now i'm not getting the error any more either.....hope its gone for good.

 

[treez]

Actually , for it to work like intended, the resistors R2 & R3 need to be 330k in value. This gives a ridiculously small current at the output of the transconductance error amp, and it would be noise sensitive. As such, I’m going to leave it, it appears its not a way to control the LT8312.

 

[treez]

..Having said that, the current out of the transconductance error amp in its "normal" setup is going to be sub 1uA in steady state...so in fact, this could actually be a good way to control the LT8312 in an "open loop" fashion, would you agree?

Updated LTspice sim and schem as attached

 

[FvM]

Actually , for it to work like intended, the resistors R2 & R3 need to be 330k in value.

According to which criteria?

However, I see that you are operating the error amplifier far from intended usage. Complaining that "it doesn't work" is somehow pointless. It's a low gm OTA with limited output current.

To give fixed setpoint to the PFC multiplier, you can drive a voltage to Vc, I presume.

 

[treez]

To give fixed setpoint to the PFC multiplier, you can drive a voltage to Vc, I presume.

Thanks, i tried that, with an opamp buffer output connected to Vc, but it wouldnt work......The Vc pin spews out 32mA and it just doesnt work.

According to which criteria?

If you change R2 and R3 to 10k, it doesnt work well, ...but if you change them to 330k, then the OTA amp acts pretty much just as if it were a voltage outputting error amp, and it works well.

 

[treez]

The OTA inside the LT8312 has a maximum possible output current of 63uA.....at least the LTspice sim model of LT8312 does.
How, from the datasheet, could this figure of 63uA possibly be calculated?
9LTspice sim of LT8312 attached)