Author Topic: Fun with Linear datasheet decoding....  (Read 1480 times)

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Offline rx8pilot

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Fun with Linear datasheet decoding....
« on: December 15, 2017, 05:15:50 am »
I am hoping someone can help me understand what I am missing on this datasheet for a Linear LT3652 charger.

Trying to layout a test circuit with this chip and the method to calculate the inductor does not appear to match the examples given.

The formula to determine the inductor given in IMG-A.PNG
Values used in the sample schematic
Rsense = .05 (50mOhm) Current sense
ICHG(max) = 2amps charge current
Vbat(flt) = 8.3v Float voltage
VIN(MAX) = 12v Max input voltage

IMG-B.PNG is the sample schematic where the inductor is 8.2uH
It also shows the rather miraculous setup where 12 watts goes in and 16.6 watts comes out - it makes power I guess. Convenient for sure  :-DD

I did not get a number even in the ballpark of 8.2uH - closer to 1. This looks simple, but clearly, I am messing it up.

http://cds.linear.com/docs/en/datasheet/3652fe.pdf


« Last Edit: December 15, 2017, 05:23:13 am by rx8pilot »
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Offline rx8pilot

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Re: Fun with Linear datasheet decoding....
« Reply #1 on: December 15, 2017, 06:24:01 am »
If you set the ripple current at 25% and VIN(MAX) at 16V, you get the 8uH value.

(12V is the nominal input, not the maximum)

As I sit here staring at the data sheet....I see this. IMG-C
20v max - ripple at 30% = 8.1uH

Damn sneaky details. Thanks for taking a look. Still trying to figure out the 120% efficiency, lol.
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Offline ConKbot

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Re: Fun with Linear datasheet decoding....
« Reply #2 on: December 15, 2017, 02:33:03 pm »
"Vin_reg loop servos maximum charge current to prevent AC adapter outlet from drooping lower than 12v"

The charger will just thrash on the adapter for all it's worth while keeping it pulled down to 12v.   Given that it will be power limited at 3v/cell (less after efficiency takes it's toll)  calling that 2A output with that adapter is a tad dubious.  But its peak output current will indeed be 2A for a small bit of the charge, and they aren't making the final product, just showing off one of its capabilities of that IC. The circuit will still run fine when the beancounters "value engineer" the wallwart from a nice 1.5A unit down to a 1A unit.
« Last Edit: December 15, 2017, 02:35:07 pm by ConKbot »
 
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