LT1513/LT1513-2
U
W U U
APPLICATIONS INFORMATION
shown in the maximum charging current graph. Higher
inductance values give slightly higher maximum charging
current,butarelargerandmoreexpensive.Alowlosstoroid
core such as Kool Mµ®, Molypermalloy or Metglas® is
recommended.Seriesresistanceshouldbelessthan0.04Ω
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.
in Figure 1. These are AVX type TPS or Sprague type 593D
surface mount solid tantalum units intended for switching
applications. Donot substituteothertypeswithout 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:
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. AlowESR22µF, 25Vsolidtantalumcapacitor(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
currentsuchaswouldbegeneratedifalowimpedanceinput
source were “hot switched” to the charger input. If this
condition can occur, the input capacitor should have the
highestpossiblevoltagerating,atleasttwicethesurgeinput
voltage if possible. Consult with the capacitor manufacturer
beforeafinalchoiceismade.A4.7µFceramiccapacitorsuch
as the one used for the coupling capacitor can also be used.
Thesecapacitorsdonothaveaturn-onsurgelimitation. The
inputcapacitormustbeconnecteddirectlytotheVINpinand
the ground plane close to the LT1513.
I
(V + V )(1.1)
BAT
CHRG IN
I
=
COUP(RMS)
2(V )
IN
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
WithICHRG =1.2A, VIN =15VandVBAT =8.2V, ICOUP =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
tantalumunitscanbesubstitutediftheirripplecurrentrating
is adequate, but typical values will increase to 22µF or more
to meet the ripple current requirements.
Diode Selection
Output Capacitor
The switching diode should be a Schottky type to minimize
both forward and reverse recovery losses. Average diode
currentisthesameasoutputchargingcurrent,sothiswillbe
under2A.A3Adiodeisrecommendedformostapplications,
although smaller devices could be used at reduced charging
current. Maximum diode reverse voltage will be equal to
input voltage plus battery voltage.
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. Ferritebeadsorlinechokesareofteninsertedinseries
with the battery leads to eliminate high frequency currents
that could create EMI problems. This forces all the ripple
currentintotheoutputcapacitor. TotalRMScurrentintothe
capacitor has a maximum value of about 1A, and this is
handledwiththetwoparalleled22µF,25Vcapacitorsshown
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.
8
Linear [ Linear ]
