Datasheet

LT3652
14
3652fd
APPLICATIONS INFORMATION
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above (1+
∆I
MAX
/2) • I
CHG(MAX)
, and an RMS rating above I
CHG(MAX)
.
Inductors must also meet a maximum volt-second product
requirement. If this specification is not in the data sheet of
an inductor, consult the vendor to make sure the maximum
volt-second product is not being exceeded by your design.
The minimum required volt-second product is:
V
BAT(FLT)
• (1 – V
BAT(FLT)
/V
IN(MAX)
) (V • μS)
Rectifier Selection
The rectifier diode from SW to GND, in a LT3652 battery
charger provides a current path for the inductor current
when the main power switch is disabled. The rectifier is
selected based upon forward voltage, reverse voltage, and
maximum current. A Schottky diode is required, as low
Figure 3. 7.2V at 1.5A Switched Inductor Values
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
12 16
4
SWITCHED INDUCTOR VALUE (μH)
12
10
14
16
32
3652 F03
8
6
2420 28
Figure 4. Feedback Resistors from BAT to V
FB
Program Float Voltage
BAT
+
V
FB
R
FB2
R
FB1
LT3652
3652 F04
forward voltage yields the lowest power loss and highest
efficiency. The rectifier diode must be rated to withstand
reverse voltages greater than the maximum V
IN
voltage.
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 14.4V.