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LT3652EMSE

Power Tracking 2A Battery Charger for Solar Power

Linear Technology 

LT3652

APPLICATIONS INFORMATION

Using a resistor divider with an equivalent input resistance

at the VFB pin of 250k compensates for input bias current

error. Required resistor values to program desired VBAT(FLT)

follow the equations:

RFB1 = (VBAT(FLT) • 2.5 • 105) / 3.3

(Ω)

RFB2 = (R1 • (2.5 • 105)) / (R1- (2.5 • 105)) (Ω)

The charge function operates to achieve the final float

voltage of 3.3V on the VFB pin. The auto-restart feature

initiates a new charging cycle when the voltage at the VFB

pin falls 2.5% below that float voltage.

Because the battery voltage is across the VBAT(FLT) pro-

gramming resistor divider, this divider will draw a small

amount of current from the battery (IRFB) at a rate of:

IRFB = 3.3 / RFB2

Precision resistors in high values may be hard to ob-

tain, so for some lower VBAT(FLT) applications, it may be

desirable to use smaller-value feedback resistors with an

additional resistor (RFB3) to achieve the required 250k

equivalent resistance. The resulting 3-resistor network,

as shown in Figure 5, can ease component selection

and/or increase output voltage precision, at the expense of

additional current through the feedback divider.

BAT

LT3652

VFB

3652 F05

RFB3

+

RFB1

RFB2

Figure 5. A Three-Resistor Feedback Network Can

Ease Component Selection

For a three-resistor network, RFB1 and RFB2 follow the

relation:

RFB2/RFB1 = 3.3/(VBAT(FLT) – 3.3)

Example:

For VBAT(FLT) = 3.6V:

RFB2/RFB1 = 3.3/(3.6 - 3.3) = 11.

Setting divider current (IRFB) = 10μA yields:

RFB2 = 3.3/10μA

RFB2 = 330k

Solving for RFB1:

RFB1 = 330k/11

RFB1 = 30k

The divider equivalent resistance is:

RFB1||RFB2 = 27.5k

To satisfy the 250k equivalent resistance to the VFB

pin:

RFB3 = 250k − 27.5k

RFB3 = 223k.

Because the VFB pin is a relatively high impedance node,

stray capacitances at this pin must be minimized. Special

attention should be given to any stray capacitances that

can couple external signals onto the pin, which can pro-

duce undesirable output transients or ripple. Effects of

parasitic capacitance can typically be reduced by adding

a small-value (20pF to 50pF) feedforward capacitor from

the BAT pin to the VFB pin.

Extra care should be taken during board assembly. Small

amounts of board contamination can lead to significant

shifts in output voltage. Appropriate post-assembly board

3652fb

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