LT3652 LINER [Linear Technology], LT3652 Datasheet - Page 13

• Manufacturer Part Number: LT3652
• Description: Power Tracking 2A Battery Charger for Solar Power
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
V
The LT3652 operates with a V
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
to reliably operate. This requirement does not exist if the
BOOST supply is available and (V
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
must be at least 3.3V above V
begins and the BOOST supply capacitor gets charged
such that (V
longer applies.
In low V
by an external source for start-up, eliminating the V
start-up requirement.
V
An LT3652 charger output requires bypass capacitance
connected from the BAT pin to ground (C
ceramic capacitor is required for all applications. In systems
where the battery can be disconnected from the charger
IN
BAT
/ BOOST Start-Up Requirement
BAT(FLT)
Output Decoupling
IN
applications, the BOOST supply can be powered
Figure 2. Zener Diode Reduces Refresh
Voltage for BOOST Pin
. As such, for reliable start-up, the V
BOOST
LT3652
– V
BOOST
SENSE
SW
BAT
3652 F02
SW
) > 2V, the V
IN
BAT(FLT)
range of 4.95V to 32V,
BOOST
IN
. Once switching
– V
IN
requirement no
– V
SW
BAT
SW
) > 2V.
). A 10μF
IN
) ≥ 3.3V
supply
IN
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
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 ripple current (ΔI
can be approximated using the relation:
In the above relation, ΔI
rent, V
V
Ripple current is typically set within a range of 25% to
35% of I
by setting 0.25 < ΔI
F
L = (10 R
is the forward voltage of the rectifying Schottky diode.
[1 – (V
IN(MAX)
CHG(MAX)
SENSE
BAT(FLT)
is the maximum operational voltage, and
, so an inductor value can be determined
/ ΔI
MAX
/ V
MAX
IN(MAX)
MAX
< 0.35.
) • V
is the normalized ripple cur-
)]
BAT(FLT)
(μH)
•
BAT
LT3652
must meet or
13
MAX
3652fb
)