LTC4096_15 Datasheet, PDF (13/16 Pages) Linear Technology – Dual Input Standalone Li-Ion Battery Chargers

LTC4096/LTC4096X
APPLICATIONS INFORMATION
WALL
ADAPTER
LTC4096
DCIN
BAT
500mA
USB
PORT
1µF
1µF
USBIN
IUSB
+
4.2V
1-CELL
Li-Ion
IDC
ITERM
R_ISET
GND
2k
R_ITERM
1k
BATTERY
1%
1%
4096 F03
Figure 3. Dual Input Charger Circuit. The
Wall Adapter Charge Current and USB Charge
Current are Both Programmed to be 500mA
Power Dissipation
When designing the battery charger circuit, it is not neces-
sary to design for worst-case power dissipation scenarios
because the LTC4096 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4096 to reduce charge current through
thermal feedback can be approximated by considering the
power dissipated in the IC. Most of the power dissipation
is generated from the internal MOSFET pass device. Thus,
the power dissipation is calculated to be:
P_D = (V_IN – V_BAT) • I_BAT
PD is the power dissipated, V_IN is the input supply volt-
age (either DCIN or USBIN), V_BAT is the battery voltage
and I_BAT is the charge current. The approximate ambient
temperature at which the thermal feedback begins to
protect the IC is:
T_A = 115°C – P_D • θ_JA
T_A = 115°C – (V_IN – V_BAT) • I_BAT • θ_JA
Example: An LTC4096 operating from a 5V wall adapter (on
the DCIN input) is programmed to supply 800mA full-scale
current to a discharged Li-Ion battery with a voltage of 3.3V.
Assuming θ_JA is 40°C/W (see Thermal Considerations),
the ambient temperature at which the LTC4096 will begin
to reduce the charge current is approximately:
WALL
ADAPTER
USB
PORT
LTC4096
DCIN
BAT
USBIN
1µF
1µF
PWR
R_IUSB
2k
1%
R_IDC
1.24k
1%
IUSB CHRG
IDC
ITERM
GND
800mA (WALL)
500mA (USB)
1k
+
R_ITERM
1k
1%
4.2V
1-CELL
Li-Ion
BATTERY
4096 F04
Figure 4. Full Featured Dual Input Charger Circuit
T_A = 115°C – (5V – 3.3V) • (800mA) • 40°C/W
T_A = 115°C – 1.36W • 40°C/W = 115°C – 54.4°C
T_A = 60.6°C
The LTC4096 can be used above 60.6°C ambient, but
the charge current will be reduced from 800mA. The ap-
proximate current at a given ambient temperature can be
approximated by:
I_BAT
=
105°C – T_A
(V_IN – V_BAT) • θ_JA
Using the previous example with an ambient temperature
of 70°C, the charge current will be reduced to approxi-
mately:
I_BAT
=
105°C – 60°C
(5V – 3.3V)• 40°C
/W
=
45°C
68°C / A
I_BAT = 662mA
It is important to remember that LTC4096 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
115°C. Moreover a thermal shut down protection circuit
around 150°C safely prevents any damage putting LTC4096
into shut down mode.
4096xf
13