9

LT1513/LT1513-2

current Schottky diodes have relatively high leakage cur-

rents (5

µA to 500µA) even at room temperature. The latest

very-low-forward devices have especially high leakage cur-

rents. It has been noted that surface mount versions of some

Schottky diodes have as much as ten times the leakage of

their through-hole counterparts. This may be because a low

forward voltage process is used to reduce power dissipation

in the surface mount package. In any case, check leakage

specifications carefully before making a final choice for the

switching diode. Be aware that diode manufacturers want

to specify a maximum leakage current that is ten times

higher than the typical leakage. It is very difficult to get them

to specify a low leakage current in high volume production.

This is an on going problem for all battery charger circuits

and most customers have to settle for a diode whose typical

leakage is adequate, but theoretically has a worst-case

condition of higher than desired battery drain.

Thermal Considerations

Care should be taken to ensure that worst-case conditions

do not cause excessive die temperatures. Typical thermal

resistance is 30

°C/W for the R package but this number will

C1,C3,C5 AND R3

TIED DIRECTLY TO

GROUND PLANE

GROUND PLANE

LT1513 TAB AND GROUND

PIN SOLDERED TO

GROUND PLANE

C1

C1

D1

C5

R3

C3

2 WINDING

INDUCTOR

L1A

L1B

C2

LT1513 • F04

R5

+

++

Figure 4. LT1513 Suggested Partial Layout for Critical Thermal and Electrical Paths

vary depending on the mounting technique (copper area,

airflow, etc.).

Average supply current (including driver current) is:

ImA

VI

V

IN

BAT

CHRG

IN

=+

4

0 024

()(

)( .

)

Switch power dissipation is given by:

P

IR

V

V

V

V

SW

CHRG

SW

BAT

IN

BAT

IN

=

+

() (

)(

)(

)

()

2

2

Total power dissipation of the die is equal to supply current

times supply voltage, plus switch power: