Data Sheet

LT3652
13
3652fe
For more information www.linear.com/LT3652
APPLICATIONS INFORMATION
V
IN
/BOOST Start-Up Requirement
The LT3652 operates with a V
IN
range of 4.95V to 32V,
however, a start-up voltage requirement exists due to
the nature of the non-synchronous step-down switcher
topology used for the charger. If there is no BOOST supply
available, the internal switch requires (V
IN
– V
SW
) ≥ 3.3V
to reliably operate. This requirement does not exist if the
BOOST supply is available and (V
BOOST
– V
SW
) > 2V.
When an LT3652 charger is not switching, the SW pin is
at the same potential as the battery, which can be as high
as V
BAT(FLT)
. As such, for reliable start-up, the V
IN
supply
must be at least 3.3V above V
BAT(FLT)
. Once switching
begins and the BOOST supply capacitor gets charged
such that (V
BOOST
– V
SW
) > 2V, the V
IN
requirement no
longer applies.
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 LT3652 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 LT3652
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
LT3652 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-peek 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 2. Zener Diode Reduces Refresh
Voltage for BOOST Pin
SW
BOOST
SENSE
BAT
LT3652
3652 F02