Datasheet

8
LT1513/LT1513-2
sn1513 1513fas
APPLICATIONS INFORMATION
WUU
U
shown in the maximum charging current graph. Higher
inductance values give slightly higher maximum charging
current, but are larger and more expensive. A low loss toroid
core such as Kool Mµ
®
, Molypermalloy or Metglas
®
is
recommended. Series resistance should be less than 0.04Ω
for each winding. “Open core” inductors, such as rods or
barrels are not recommended because they generate large
magnetic fields which may interfere with other electronics
close to the charger.
Input Capacitor
The SEPIC topology has relatively low input ripple current
compared to other topologies and higher harmonics are
especially low. RMS ripple current in the input capacitor is
less than 0.25A with L = 10µH and less than 0.5A with
L = 5µH. A low ESR 22µF, 25V solid tantalum capacitor (AVX
type TPS or Sprague type 593D) is adequate for most
applications with the following caveat. Solid tantalum
capacitors can be destroyed with a very high turn-on surge
current such as would be generated if a low impedance input
source were “hot switched” to the charger input. If this
condition can occur, the input capacitor should have the
highest possible voltage rating, at least twice the surge input
voltage if possible. Consult with the capacitor manufacturer
before a final choice is made. A 4.7µF ceramic capacitor such
as the one used for the coupling capacitor can also be used.
These capacitors do not have a turn-on surge limitation. The
input capacitor must be connected directly to the V
IN
pin and
the ground plane close to the LT1513.
Output Capacitor
It is assumed as a worst case that all the switching output
ripple current from the battery charger could flow in the
output capacitor. This is a desirable situation if it is neces-
sary to have very low switching ripple current in the battery
itself. Ferrite beads or line chokes are often inserted in series
with the battery leads to eliminate high frequency currents
that could create EMI problems. This forces all the ripple
current into the output capacitor. Total RMS current into the
capacitor has a maximum value of about 1A, and this is
handled with the two paralleled 22µF, 25V capacitors shown
in Figure 1. These are AVX type TPS or Sprague type 593D
surface mount solid tantalum units intended for switching
applications. Do not substitute other types without ensuring
that they have adequate ripple current ratings. See Input
Capacitor section for details of surge limitation on solid
tantalum capacitors if the battery may be “hot switched” to
the output of the charger.
Coupling Capacitor
C2 in Figure 1 is the coupling capacitor that allows a SEPIC
converter topology to work with input voltages either higher
or lower than the battery voltage. DC bias on the capacitor is
equal to input voltage. RMS ripple current in the coupling
capacitor has a maximum value of about 1A at full charging
current. A conservative formula to calculate this is:
I
IVV
V
COUP RMS
CHRG IN BAT
IN
()
()(.)
()
=
+ 11
2
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
With I
CHRG
= 1.2A, V
IN
= 15V and V
BAT
= 8.2V, I
COUP
= 1.02A.
The recommended capacitor is a 4.7µF ceramic type from
Marcon or Tokin. These capacitors have extremely low ESR
and high ripple current ratings in a small package. Solid
tantalum units can be substituted if their ripple current rating
is adequate, but typical values will increase to 22µF or more
to meet the ripple current requirements.
Diode Selection
The switching diode should be a Schottky type to minimize
both forward and reverse recovery losses. Average diode
current is the same as output charging current, so this will be
under 2A. A 3A diode is recommended for most applications,
although smaller devices could be used at reduced charging
current. Maximum diode reverse voltage will be equal to
input voltage plus battery voltage.
Diode reverse leakage current will be of some concern
during charger shutdown. This leakage current is a direct
drain on the battery when the charger is not powered. High
Kool Mµ is a registered trademark of Magnetics, Inc.
Metglas is a registered trademark of AlliedSignal Inc.