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LTC3558
19
3558f
APPLICATIONS INFORMATION
USB and Wall Adapter Power
Although the battery charger is designed to draw power
from a USB port to charge Li-Ion batteries, a wall adapter
can also be used. Figure 5 shows an example of how to
combine wall adapter and USB power inputs. A P-channel
MOSFET, MP1, is used to prevent back conduction into
the USB port when a wall adapter is present and Schottky
diode, D1, is used to prevent USB power loss through the
1k pull-down resistor.
Typically, a wall adapter can supply signifi cantly more
current than the 500mA-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor are
used to increase the maximum charge current to 950mA
when the wall adapter is present.
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC3558 will
automatically reduce the charge current to maintain the
die temperature at approximately 105°C. However, the
approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
T
C P
T
C
V
V
I
A
D JA
A
CC
BAT
BAT
JA
=
°
=
°
(
)
105
105
–
–
–
•
•
θ
θ
Example: Consider an LTC3558 operating from a USB port
providing 500mA to a 3.5V Li-Ion battery. The ambient
temperature above which the LTC3558 will begin to reduce
the 500mA charge current is approximately:
T
C
V
V
mA
C W
T
C
A
A
=
°
(
) (
)
°
=
°
105
5
3 5
500
68
105
0
–
– .
•
•
/
– ..
•
/
–
75
68
105
51
54
W
C W
C
C
T
C
A
°
=
°
°
= °
The LTC3558 can be used above 70°C, but the charge cur-
rent will be reduced from 500mA. The approximate current
at a given ambient temperature can be calculated:
I
C T
V
V
BAT
A
CC
BAT
JA
=
°
(
)
105
–
–
•
θ
Using the previous example with an ambient tem-
perature of 88°C, the charge current will be reduced to
approximately:
I
C
C
V
V
C W
C
C A
BAT
=
°
°
(
)
°
=
°
°
105
88
5
3 5
68
17
102
–
– .
•
/
/
II
mA
BAT
=
167
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
It is important to remember that LTC3558 applications do
not need to be designed for worst-case thermal conditions
since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately
105°C.
V
CC
MP1
MN1
1k
1.74k
1.65k
I
BAT
Li-Ion
BATTERY
3558 F05
BATTERY
CHARGER
BAT
USB
POWER
500mA I
CHG
5V WALL
ADAPTER
950mA I
CHG
PROG
+
D1
Figure 5. Combining Wall Adapter and USB Power
Power Dissipation
The conditions that cause the LTC3558 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. For high
charge currents, the LTC3558 power dissipation is
approximately:
P
V
V
I
D
CC
BAT
BAT
=
(
)
–
•
where P
D
is the power dissipated, V
CC
is the input supply
voltage, V
BAT
is the battery voltage, and I
BAT
is the charge