ltc4096 Linear Technology Corporation, ltc4096 Datasheet - Page 13

• Manufacturer Part Number: ltc4096
• Description: Dual Input Standalone Li-ion Battery Chargers
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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:
PD is the power dissipated, V
age (either DCIN or USBIN), V
and I
temperature at which the thermal feedback begins to
protect the IC is:
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
the ambient temperature at which the LTC4096 will begin
to reduce the charge current is approximately:
ADAPTER
P
T
T
WALL
PORT
A
A
D
USB
BAT
= 115°C – P
= 115°C – (V
= (V
Figure 3. Dual Input Charger Circuit. The
Wall Adapter Charge Current and USB Charge
Current are Both Programmed to be 500mA
is the charge current. The approximate ambient
IN
1µF
JA
– V
is 40°C/W (see Thermal Considerations),
BAT
D
IN
) • I
1µF
• θ
– V
JA
BAT
R
2k
1%
ISET
BAT
) • I
DCIN
USBIN
IUSB
IDC
IN
LTC4096
BAT
BAT
GND
is the input supply volt-
ITERM
is the battery voltage
• θ
BAT
JA
R
1k
1%
ITERM
500mA
+
4096 F03
4.2V
1-CELL
Li-Ion
BATTERY
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:
Using the previous example with an ambient temperature
of 70°C, the charge current will be reduced to approxi-
mately:
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.
ADAPTER
T
T
T
I
I
I
WALL
PORT
BAT
BAT
BAT
A
A
A
USB
= 115°C – (5V – 3.3V) • (800mA) • 40°C/W
= 115°C – 1.36W • 40°C/W = 115°C – 54.4°C
= 60.6°C
Figure 4. Full Featured Dual Input Charger Circuit
=
=
=
R
IUSB
662
1%
(
(
2k
V
5
IN
V
105
1µF
mA
105
– .
–
1.24k
R
3 3
1%
IDC
V
°
LTC4096/LTC4096X
BAT
C T
°
C
V
–
1µF
)•
)
–
•
60
40
A
θ
DCIN
USBIN
IUSB
IDC
JA
°
°
LTC4096
C
C W
GND
/
ITERM
CHRG
PWR
BAT
=
68
45
1k
°
C A
°
R
1k
1%
C
ITERM
/
800mA (WALL)
500mA (USB)
+
13
4096 F04
4.2V
1-CELL
Li-Ion
BATTERY
4096xf