LT3652 LINER [Linear Technology], LT3652 Datasheet - Page 15

• Manufacturer Part Number: LT3652
• Description: Power Tracking 2A Battery Charger for Solar Power
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Using a resistor divider with an equivalent input resistance
at the V
error. Required resistor values to program desired V
follow the equations:
The charge function operates to achieve the final float
voltage of 3.3V on the V
initiates a new charging cycle when the voltage at the V
pin falls 2.5% below that float voltage.
Because the battery voltage is across the V
gramming resistor divider, this divider will draw a small
amount of current from the battery (I
Precision resistors in high values may be hard to ob-
tain, so for some lower V
desirable to use smaller-value feedback resistors with an
additional resistor (R
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.
R
R
I
RFB
FB1
FB2
Figure 5. A Three-Resistor Feedback Network Can
Ease Component Selection
= 3.3 / R
FB
= (V
= (R1 • (2.5 • 10
pin of 250k compensates for input bias current
BAT(FLT)
LT3652
FB2
• 2.5 • 10
3652 F05
BAT
V
FB
FB3
5
) to achieve the required 250k
FB
)) / (R1- (2.5 • 10
R
BAT(FLT)
FB3
pin. The auto-restart feature
5
) / 3.3
R
R
FB1
FB2
applications, it may be
RFB
+
) at a rate of:
5
))
BAT(FLT)
BAT(FLT)
(Ω)
(Ω)
pro-
FB
For a three-resistor network, R
relation:
Example:
Because the V
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 V
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
R
For V
Setting divider current (I
Solving for R
The divider equivalent resistance is:
To satisfy the 250k equivalent resistance to the V
pin:
FB2
R
R
R
R
R
R
R
R
/R
BAT(FLT)
FB2
FB2
FB2
FB1
FB1
FB1
FB3
FB3
FB1
/R
||R
= 3.3/10μA
= 330k/11
= 250k − 27.5k
= 330k
= 30k
= 223k.
= 3.3/(V
FB1
FB
FB2
= 3.6V:
FB1
pin is a relatively high impedance node,
= 3.3/(3.6 - 3.3) = 11.
= 27.5k
:
FB
BAT(FLT)
pin.
RFB
– 3.3)
) = 10μA yields:
FB1
and R
FB2
LT3652
follow the
15
FB
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