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Problem of surge currents flowing in the sense resistor of LT1248 and LT1249 PFC's

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treez

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The LT1249 and LT1248 are both CCM PFC controllers. They are very similar chips.
Why is it that the LT1249 datasheet, on page 10, recommends a surge current bypass circuit (three diodes) for the sense resistor , but the LT1248 makes no mention of any need to protect from such surge currents? (eg the rush of current through the sense resistor if the PFC is switched on at Mains peak)
Also, The LT1248 leaves one with a choice of what maximum sense resistor voltage to employ…but what is the maximum? :-?

LT1249 datasheet:
http://cds.linear.com/docs/en/datasheet/1249fbs.pdf

LT1248 datasheet
http://cds.linear.com/docs/en/datasheet/1248fd.pdf
 

LT1248 and LT1249 are similar architecture except the number of pins.
Generally, R01 should be improvisation of R00, in this case the converter part count will reduce in LT1249 than LT1248(Number of pins will decide the overall part count).

According to LT1249 data sheet (page 10) External clamp circuit (fig -6) only applicable when VCC pin is powered from some external housekeeping supply. This scenario applicable to LT1248 also

If VCC powered from the same line, the thermistor will take care of these surge. Thermistor can be placed before the bridge or after the bridge
 
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Well, look at that. Right there on page 8 is a formula for calculating Rs.
Click to expand...
Thanks, yes i agree, the calculation of Rs is extremely simple.....but the protection of LT1248 MOUT pin from (negative voltages instigated by) surge currents flowing through Rs is less simple....i think the LT1248 will have to have a current clamp upstream of it. Do you agree?
 

Look at the application circuit in the LT1248 datasheet. It shows that the shunt is connected to the IC via two 4K resistors. During surges these likely limit the current into those pins to less than the specified +/-5mA maximum.
 
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