LT1513/LT1513-2
U
W U U
APPLICATIONS INFORMATION
vary depending on the mounting technique (copper area,
airflow, etc.).
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.Ithasbeennotedthatsurfacemountversionsofsome
Schottky diodes have as much as ten times the leakage of
their through-hole counterparts. This may be because a low
forwardvoltageprocessisusedtoreducepowerdissipation
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.
Average supply current (including driver current) is:
(V )(I
)(0.024)
BAT CHRG
I = 4mA +
IN
V
IN
Switch power dissipation is given by:
2
(I
) (R )(V
+ V )(V
)
CHRG
SW BAT
IN BAT
P
=
SW
2
(V )
IN
RSW = Output switch ON resistance
Total power dissipation of the die is equal to supply current
times supply voltage, plus switch power:
PD(TOTAL) = (IIN)(VIN) + PSW
For VIN = 10V, VBAT = 8.2V, ICHRG = 1.2A, RSW = 0.3Ω,
IIN = 4mA + 24mA = 28mA
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
PSW = 0.64W
PD = (10)(0.028) + 0.64 = 0.92W
GROUND PLANE
V
BAT
+
+
C1
C1
D1
LT1513 TAB AND GROUND
PIN SOLDERED TO
GROUND PLANE
C2
C5
2 WINDING
INDUCTOR
L1A
L1B
+
R5
C3
R3
C1,C3,C5 AND R3
TIED DIRECTLY TO
GROUND PLANE
LT1513 • F04
V
IN
Figure 4. LT1513 Suggested Partial Layout for Critical Thermal and Electrical Paths
9
Linear [ Linear ]
