Datasheet

LT3652HV
14
3652hvfa
APPLICATIONS INFORMATION
The minimum average diode current rating (I
DIODE(MAX)
)
is calculated with maximum output current (I
CHG(MAX)
),
maximum operational V
IN
, and output at the precondition
threshold (V
BAT(PRE)
, or 0.7 • V
BAT(FLT)
):
I
DIODE(MAX)
> I
CHG(MAX)
• (V
IN(MAX)
– V
BAT(PRE)
) / V
IN(MAX)
) (A)
For example, a rectifier diode for a 7.2V, 2A charger with
a 25V maximum input voltage would require:
I
DIODE(MAX)
> 2 • (25 – 0.7[7.2]) / 25), or
I
DIODE(MAX)
> 1.6A
Battery Float Voltage Programming
The output battery float voltage (V
BAT(FLT)
) is programmed
by connecting a resistor divider from the BAT pin to V
FB
.
V
BAT(FLT)
can be programmed up to 18V.
Figure 4. Feedback Resistors from BAT to V
FB
Program Float Voltage
BAT
+
V
FB
R
FB2
R
FB1
LT3652HV
3652 F04
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.
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
BAT
+
V
FB
R
FB2
R
FB3
R
FB1
LT3652HV
3652 F05
Figure 5. A Three-Resistor Feedback Network Can
Ease Component Selection
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
= (R1 • (2.5 • 10
5
)) / (R1- (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.