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# LT3652 voltage divider

Hello,

I am looking at LT3652 datasheet, and I see the Battery Floating Voltage is set with a voltage divider between pins BAT and VFB. 2 equations are provided to calculate the resistor values (page 14, equations for Rfb1 and Rfb2). The 2 equations on page 14 are RFB1 = (VBAT(FLT) • 2.5 • 10^5)/3.3 and RFB2 = (RFB1 • (2.5 • 105))/(RFB1 - (2.5 • 10^5))

I am trying to use these equations to match the typical application example on page 26, where rfb1 is 309k and rfb2 is 100k, but I can't satisfy the 2 equations. The only thing I notice is that with a voltage of 13.5v applied to that voltage divider, output is 3.3v.

Thanks

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• Hello,

the example on page 26 is not typical as the desired float voltage changes on battery status resp. temperature. CHG pin influences the feedback divider. Details on this configuration can be found on page 17 in the datasheet.

If you use the formula for the upper example on page 22 the values fit nicely.

• Thank you for your answer Markus. I'm actually looking to implement LT3652 for a li-ion battery solar charge controller (7.4v 2S battery). I though the equations under the "Battery Float Voltage Programming" would apply for any situation. I see on page 22 there is an example for "2-Cell Li-Ion Charger (8.3V at 2A)", but this one uses an AC adapter input.

Do you have any suggestions about which approach to take to implement this charge controller? I was able to find an example in here: https://workspace.circuitmaker.com/Projects/Details/Craig-Peacock-4/Solar-Power-Charger-2A-Lithium-SLA

that suggests that for a 2S LiPo battery, with a charge voltage of 8.4v, rfb1 should be 154k and rfb2 100k. I want to corroborate this values, but I haven't been able to match them to any of the equations of the datasheet.