Datasheet

9
LT1513/LT1513-2
sn1513 1513fas
APPLICATIONS INFORMATION
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current Schottky diodes have relatively high leakage cur-
rents (5µA 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
V
BAT
C1 C1
D1
C5
R3
C3
2 WINDING
INDUCTOR
L1A L1B
V
IN
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
IRVVV
V
SW
CHRG SW BAT IN BAT
IN
=
+()()( )()
()
2
2
R
SW
= Output switch ON resistance
Total power dissipation of the die is equal to supply current
times supply voltage, plus switch power:
P
D(TOTAL)
= (I
IN
)(V
IN
) + P
SW
For V
IN
= 10V, V
BAT
= 8.2V, I
CHRG
= 1.2A, R
SW
= 0.3Ω,
I
IN
= 4mA + 24mA = 28mA
P
SW
= 0.64W
P
D
= (10)(0.028) + 0.64 = 0.92W