LTC4096/LTC4096X
13
4096xf
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:
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
CT
VV
BAT
A
IN BAT JA
=
°105 –
(– •
)
θ
Using the previous example with an ambient temperature
of 70°C, the charge current will be reduced to approxi-
mately:
I
CC
VVCW
C
C
ImA
BAT
BAT
=
°°
°
=
°
°
=
105 60
53340
45
68
662
–
(–.)• / /
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.
APPLICATIONS INFORMATION
LTC4096
DCIN
USBIN
IUSB
IDC
BAT
ITERM
R
ISET
2k
1%
R
ITERM
1k
1%
WALL
ADAPTER
USB
PORT
1µF1µF
+
500mA
4096 F03
GND
4.2V
1-CELL
Li-Ion
BATTERY
LTC4096
DCIN
USBIN
IUSB
IDC
R
IDC
1.24k
1%
WALL
ADAPTER
USB
PORT
1µF1µF
+
800mA (WALL)
500mA (USB)
4096 F04
GND
R
IUSB
2k
1%
R
ITERM
1k
1%
4.2V
1-CELL
Li-Ion
BATTERY
BAT
ITERM
1k
PWR
CHRG
Figure 3. Dual Input Charger Circuit. The
Wall Adapter Charge Current and USB Charge
Current are Both Programmed to be 500mA
Figure 4. Full Featured Dual Input Charger Circuit
