8

LT1512

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 0.5A, and this is

handled with a 22

µF,25VcapacitorshowninFigure1.Thisis

an AVX type TPS or Sprague type 593D surface mount solid

tantalum unit 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 0.5A at full charg-

ing 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 2.2

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 1A. A 1A diode is recommended for most applica-

tions, although smaller devices could be used at reduced

1

2

3

4

R5

C4

R3

R4

S/S

1512 F04b

C5

R2

C1

D1

C2

C3

R1

+

+

GND

L1B

L1A

R4

b. Single-Sided Altenative Layout

2 WINDING

INDUCTOR

L1A

1512 F04a

L1B

R4

R1

1

4

3

2

C4

R2

D1

GND

GND

R3

C2A

C2B

C3

C5

C1

U1

R5

S/S

a. Double-Sided (Vias Connect to the Backside of Ground Plane.

Dash Lines Indicate Interconnects on Backside. Demo Board

Uses This Layout, Except that R5 Has Been Added to Increase

Phase Margin)

Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths