LTC3558_15 Datasheet, PDF (19/32 Pages) Linear Technology – Linear USB Battery Charger with Buck and Buck-Boost Regulators

LTC3558
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 significantly 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.
5V WALL
ADAPTER
950mA I_CHG
USB
POWER
500mA I_CHG
BAT
D1 _BATTERY
CHARGER
V_CC
MP1
PROG
MN1 ^1.65k
1k
I_BAT
+ Li-Ion
BATTERY
1.74k
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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_D ^{= V}_CC ^{– V}_BAT ^•I_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
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_A = 105°C – P_Dθ_JA
( ) ^T_A ^{= 105°C – V}_CC ^{– V}_BAT ^•I_BAT ^{• θ}_JA
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_A = 105°C – (5V – 3.5V) • (500mA) • 68°C / W
T_A = 105°C – 0.75W • 68°C / W = 105°C – 51°C
T_A = 54°C
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:
( ) ^{IBAT =}
105°C – T_A
V_CC – V_BAT • θ_JA
Using the previous example with an ambient tem-
perature of 88°C, the charge current will be reduced to
approximately:
I_BAT
=
(5V
105°C – 88°C
– 3.5V) • 68°C
/
W
=
17°C
102°C /
A
I_BAT = 167mA
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.
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