I think I actually did this once on the UC3854 - which is what this chip is a copy of -
Thanks very much.
Its very interesting that UC3854 and LT1248 do indeed appear to be “twins”
The UC3854 looks to be less than half the price of the LT1248.
The differences look very minor...
A…though the UC3854 has no overvoltage pin…so that would need an external comparator…though in UC3854’s favour, it does have line voltage feed-forward via the VRMS pin, which the LT1248 doesn’t have.
B….The LT1248 is sync-able, but UC3854 doesn’t seem to be.
C….Bias current is very slightly higher in UC3854.
D……The UC3854 offers to reduce line current dead zone by connecting a resistor from VREF to IAC…..presumably this can also be done with the LT1248, though its datasheet makes no mention.
E……The LT1248 also appears to have biasing which prevents its Vout rising above the regulation level during light-load or no-load….UC3854 doesn’t appear to have this.
F……The LT1248 MOUT pin is clamped internally at -0.6V, whereas the UC3854 needs an external Schottky to be placed on its equivalent pin………I am not sure I like this, Schottkys have high leakage current at high temperature and this could mess up the MOUT pin (called “MULTOUT” in UC3854). The multiplier output current is only 200uA maximum so I don’t like the idea of having a Schottky on the MULTOUT pin.
G…..The LT1248 datasheet says it needs a Schottky from Gate drive pin to ground, whereas UC3854 datasheet makes no mention of this.
H…..strangely the UC3854 datasheet says it mustn’t be placed within 1 inch of the boost inductor, and musnt be placed under the boost inductor….but LT1248 makes no mention of this. It seems odd because eg the Innoswitch range of flyback controllers from power integrations actually state (on page 32 of its datasheet) that the innoswitch controller should be placed directly under the flyback transformer.
Due to A and F above, would you say the LT1248 would be the preferred choice?
LT1248 datasheet
https://www.analog.com/media/en/technical-documentation/data-sheets/1248fd.pdf
UC3854 datasheet
https://www.ti.com/lit/ds/symlink/uc3854.pdf
Inno switch datasheet
**broken link removed**