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Interpretation of equation(s) for Multiplier current (IM) in LT1248 PFC controller

treez

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Hi,
We are trying to set up the LT1248 PFC controller, however, the datasheet appears incorrect…

The equation for “IM” (multiplier output current) on page 7 of the LT1248 datasheet appears to be wrong.

The equation given on page 7 is as follows….

“IM = (IAC • IEA^2)/((200mA)^2), with IEA = (VAOUT– 2V)/25k.”
(near bottom right of page 7)

However, if you observe the graph in Fig 1 of page 7, and pick off values from that graph, then clearly, the given equation for “IM” is wrong.

…For example, If I pick off the graph, VAOUT = 7V and IAC = 500uA and put these into the equation above, then I get IM = 500uA……however, the graph gives ~268uA.

However, the equation is a lot more accurate if I pick a part of any graph that’s not levelled off…
…For example, at VAOUT=7V and IAC =250uA than IM = 250uA and the graph almost confirms that and is ~230uA, …quite accurate.

So anyway, why does the IM value on the graph level off at 268uA?
Nothing in the datasheet suggests that IM is clamped at 268uA.

In fact, at the top of page 7, there is another equation for IM as follows….

IM(MAX) = 3.75V/RSET
(top left of page 7)


….now, RSET can have a “Legal” value of 10k…(RSET is shown as being 10k in the graph at top right of page 5)…….that would; give IM(MAX) as 375uA ….so why does the graph on page 7 show IM being clamped to 268uA ?

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