Data Sheet

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