Datasheet

LT3650-8.2/LT3650-8.4
11
36508284fd
V
IN
Input Supply
The LT3650 is biased directly from the charger input supply
through the V
IN
pin. This supply provides large switched
currents, so a high quality, low ESR decoupling capacitor
is recommended to minimize voltage glitches on V
IN
. The
V
IN
decoupling capacitor (C
VIN
) absorbs all input switching
ripple current in the charger, so it must have an adequate
ripple current rating. RMS ripple current (I
CVIN(RMS)
) is:

I
CVIN(RMS)
 I
CHG(MAX)
•
V
BAT
V
IN






•
V
IN
V
BAT






−1






1
2
which has a maximum at V
IN
= 2 • V
BAT
, where:
I
CVIN(RMS)
= I
CHG(MAX)
/2
The simple worst-case of 1/2 • I
CHG(MAX)
is commonly
used for design.
Bulk capacitance is a function of desired input ripple volt-
age (∆V
IN
), and follows the relation:
C
IN(BULK)
= I
MAX
•
V
BAT
/ V
IN
∆V
IN






µF
( )
10µF is typically adequate for most charger applications.
BOOST Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of switch transistor. Oper-
ating range of the BOOST pin is 0V to 8.5V, as referenced
to the SW pin. Connect a 1µF or greater capacitor from
the BOOST pin to the SW pin.
The voltage on the decoupling capacitor is refreshed
through a diode, with the anode connected to/from either
the battery output voltage or an external source, and the
cathode connected to the BOOST pin. Rate the diode av-
erage current greater than 0.1A, and its reverse voltages
greater than V
IN(MAX)
.
V
IN
/ BOOST Start-Up Requirement
The LT3650 operates with a V
IN
range of 9V to 32V,
however, a start-up voltage requirement exists due to
the nature of the nonsynchronous step-down switcher
topology used for the charger. If there is no BOOST supply
available, the internal switch requires (V
IN
– V
SW
) > 3V
to operate. This requirement does not exist if the BOOST
supply is available and (V
BOOST
– V
SW
) > 2V.
When an LT3650 charger is not switching, the SW pin is at
the same
potential as the battery, which can be as high as
V
BAT(F LT)
. For reliable start-up, the V
IN
supply must be at
least 3V above the SW pin. The minimum start-up speci-
fication of V
IN
at or above 11.5V provides ample margin
to satisfy this requirement. Once switching begins, the
BOOST supply capacitor gets charged such that (V
BOOST
– V
SW
) > 2V, and 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 LT3650 charger output requires bypass capacitance
connected from the BAT pin to ground (C
BAT
). A 10µF ce-
ramic 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 of a no-battery condition (see the Status
Pins section).
If it is desired to operate a system load from the LT3650
charger output when the battery is disconnected, additional
bypass capacitance is required. In this type of application
with the charger being used as a DC/DC converter, exces-
sive ripple and/or
low amplitude oscillations can occur
without
additional output bulk capacitance. For these ap-
plications, place a 100µF low ESR nonceramic 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
through long wires. The voltage rating on C
BAT
must meet
or exceed the battery float voltage.
applicaTions inForMaTion