Sunny Buddy - LT3652 voltage divider

Components & Accessories Power
1

Hello,

I am looking at the schematic of your Sunny Buddy (PRT-12885) and I am seeing it uses component LT3652. I see in it’s datasheet that one has to set the Battery Floating Voltage with a voltage divider between pins BAT and VFB, and they provide 2 equations to calculate the resistor values (page 14, equations for Rfb1 and Rfb2). Your Sunny Buddy uses R2=Rfb1=47k and R3=Rfb2=220k. I suppose the Vbat(flt) was set around 4v for lithium-ion batteries, but I can’t satisfy those 2 equations with the values selected for the Sunny Buddy. Could you explain how those values were found?

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))

Thanks

2

While their isn’t any annotation/documentation for us to rely on (and the engineer probably won’t stop by)…I’ll take a swing at this.

I assume that equation is using an analog value (correlating to the battery voltage) scaling UP TO 4.2v…but without having access to the actual firmware, it’s a bit of a black box :-/