8

LT1513/LT1513-2

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 M

recommended. Series resistance should be less than 0.04

Ω

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 = 10

µH and less than 0.5A with

L = 5

µH.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

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 4.7

µFceramiccapacitorsuch

as the one used for the coupling capacitor can also be used.

These capacitors do not have a turn-on surge limitation. The

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 22

µF,25Vcapacitorsshown

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

IV

V

V

COUP RMS

CHRG

IN

BAT

IN

()

()( . )

()

=

+

11

2

(1.1 is a fudge factor to account for inductor ripple current

and other losses)

The recommended capacitor is a 4.7

µF 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 22

µF 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 M

µ is a registered trademark of Magnetics, Inc.

Metglas is a registered trademark of AlliedSignal Inc.