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LM2577 - SNOS658D Rev Apr 2013: Error in Formula

Other Parts Discussed in Thread: LM2577, LM2596, LM2587

Hi

I'm re-purposing a transformer from a Teac product which has secondaries of 20V-0-20V, 26V-0-26V, 5V and 12V5.  The 20V-0-20V will be used for an audio power amplifier and the 26V-0-26V for a DAB digital radio front-end requiring 3V3 and 5V.

I'm planning to use LM2596 for step-down/buck (5V from the 12V5 winding) and LM2577 for step-up/boost (12V from the 5V winding).  This may seem odd but it's for reasons of spreading the load on the transformer, keeping digital and analogue grounds separate, etc.

Current requirements are quite modest but I chose these TO220 devices for power dissipation reasons - although there are no figures for these in either SNOS658D, rev Apr 2013 (LM2577) or SNVS124C, rev Apr 2013 (LM2596) - just says "Internally Limited".

I appreciate the LM2577 is an old device and will check-out the LM2587 as well.

Meantime, while wading through the calculations, on page 20 of SNOS658D there are two calculations for checking that the ESR of the output filter capacitor is small enough.  The second has a numerator (the bit on top of the fraction) of "8.7 x (10) - 3 x Vin" which I don't think can be correct.  Assuming it was meant to be "8.7 x (10 - 3 x Vin) I put in my Vin of 3.5 which, of course, yields a negative number.

Can someone advise the correct calculation please?

Regards

Steve