External error amplifier in LT8705 Boost converter

[Flyback]

Hello,
We need to use an external error amplifier with LT8705 Boost converter as in the below schematic.
(vin=28v, vout=30.5v, 610W output, Fsw = 160KHz, continuous current mode)
What is the maximum value for the series resistor R21 (ringed in schematic).

LT8705 datasheet:
https://www.linear.com/product/LT8705

I also attach the LTspice simulation of this circuit.

 

[crutschow]

You make your schematics so large with so much room between parts that its difficult to follow the circuit. You could probably compress it to about 1/3 of what you show.

Why are you connecting the error amplifier out to Vc?

 

[Flyback]

..because this allows a type 3 error amp compensation network to be used.....and such is needed because even though this is a current mode converter, the ramp on the sense resistor is just a few mV, and so this ramp is likely to be drowned by noise, and thus the converter will end up getting controlled by the LT8705's internal slope compensatory ramp.....as such the converter will be in voltage mode, and so a type 3 erroramp compensator is needed.

Also, the actual load for this circuit will be a BLDC and its inverter, and the Vc pin will be used with the external error amplifier like as shown, to regulate the current to the BLDC.

Sorry about the space, originally we had external FET drivers driving the FETs, but the dead time in the LT8705 is far too small (<70ns) to allow us to use external FET drivers, so therefore we had to remove the external FET drivers.