Datasheet

LTC4081
17
4081f
V
CC
MP1
MN1
1k
4k
1k
1
I
CHG
2
D1
4
Li-Ion
BATTERY
SYSTEM
LOAD
4081 F04
LTC4081
BAT
USB
POWER
(100mA)
5V WALL
ADAPTER
(500mA)
PROG
+
Figure 4. Combining Wall Adapter and USB Power
APPLICATIO S I FOR ATIO
WUU
U
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input sup-
ply, the LTC4081 can dissipate signifi cantly less power
when programmed for a current higher than the limit of
the wall adapter.
Consider a situation where an application requires a 200mA
charge current for a discharged 800mAh Li-Ion battery.
If a typical 5V (non-current limited) input supply is avail-
able then the peak power dissipation inside the part can
exceed 300mW.
Now consider the same scenario, but with a 5V input
supply with a 200mA current limit. To take advantage
of the supply, it is necessary to program the LTC4081
to charge at a current greater than 200mA. Assume that
the LTC4081 charger is programmed for 300mA (i.e.,
R
PROG
= 1.33kΩ) to ensure that part tolerances maintain
a programmed current higher than 200mA. Since the
battery charger will demand a charge current higher than
the current limit of the input supply, the supply voltage
will collapse to the battery voltage plus 200mA times the
on-resistance of the internal PFET. The on-resistance of
the battery charger power device is approximately 0.7Ω
with a 5V supply. The actual on-resistance will be slightly
higher due to the fact that the input supply will have col-
lapsed to less than 5V. The power dissipated during this
phase of charging is approximately 30mW. That is a ten
times improvement over the non-current limited supply
power dissipation.
USB and Wall Adapter Power
Although the LTC4081 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batter-
ies. Figure 4 shows an example of how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pulldown resistor.
Typically a wall adapter can supply signifi cantly more
current than the current-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
can be used to increase the charge current when the wall
adapter is present.
Power Dissipation
The conditions that cause the LTC4081 battery charger to
reduce charge current through thermal feedback can be
approximated by considering the total power dissipated
in the IC. For high charge currents, the LTC4081 power
dissipation is approximately:
PVV I P
D CC BAT BAT D BUCK
=−
()
+•
_
Where P
D
is the total power dissipated within the IC, V
CC
is the input supply voltage, V
BAT
is the battery voltage, I
BAT
is the charge current and P
D_BUCK
is the power dissipation
due to the regulator. P
D_BUCK
can be calculated as:
PVI
D BUCK OUT OUT_
•=−
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1
η