LT3652HV
13
3652hvfb
For more information www.linear.com/LT3652HV
APPLICATIONS INFORMATION
In low V
IN
applications, the BOOST supply can be powered
by an external source for start-up, eliminating the V
IN
start-up requirement.
V
BAT
Output Decoupling
An LT3652HV charger output requires bypass capacitance
connected from the BAT pin to ground (C
BAT
). A 10µF
ceramic capacitor is required for all applications. In systems
where the battery can be disconnected from the charger
output, additional bypass capacitance may be desired for
visual indication for a no-battery condition (see the Status
Pins section).
If it is desired to operate a system load from the LT3652HV
charger output when the battery is disconnected, additional
bypass capacitance is required. In this type of application,
excessive ripple and/or low amplitude oscillations can oc
-
cur without additional output bulk capacitance. For these
applications, place a 100µF low ESR non-ceramic capacitor
(chip tantalum or organic semiconductor capacitors such
as Sanyo OS-CONs or POSCAPs) from BAT to ground,
in parallel with the 10µF ceramic bypass capacitor. This
additional bypass capacitance may also be required in
systems where the battery is connected to the charger
with long wires. The voltage rating of C
BAT
must meet or
exceed the battery float voltage.
Inductor Selection
The primary criterion for inductor value selection in an
LT3652HV charger is the ripple current created in that
inductor. Once the inductance value is determined, an
inductor must also have a saturation current equal to or
exceeding the maximum peak current in the inductor. An
inductor value (L), given the desired amount of peak-to-
peak inductor ripple current (ΔI
L
) can be approximated
using the relation:
L =
10 •R
SENSE
ΔI
L
I
CHG(MAX)
• V
BAT(FLT)
• 1–
V
BAT(FLT)
V
IN(MAX)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
µH
( )
In the above relation, V
IN(MAX)
is the maximum operational
voltage. Ripple current is typically set within a range of
25% to 35% of I
CHG(MAX)
, so an inductor value can be
determined by setting 0.25 < ΔI
L
/I
CHG(MAX)
< 0.35.
Figure 3. 14.4V at 1.5A Switched Inductor Values
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
18 20 22
4
SWITCHED INDUCTOR VALUE (µH)
12
10
14
24
16
18
20
22
34
3652 F03
8
6
26 2824 30 32
Magnetics vendors typically specify inductors with maxi-
mum RMS and saturation current ratings. Select an inductor
that has a saturation current rating at or above I
CHG(MAX)
+ ∆I
L
/I
CHG(MAX)
, and an RMS rating above I
CHG(MAX)
. In-
ductors 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 LT3652HV 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
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.
