LTC4076
13
4076fa
Power Dissipation
When designing the battery charger circuit, it is not neces-
sary to design for worst-case power dissipation scenarios
because the LTC4076 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4076 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
P
D
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
= 105°C – P
D
• θ
JA
T
A
= 105°C – (V
IN
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4076 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 LTC4076 will begin
to reduce the charge current is approximately:
T
A
= 105°C – (5V – 3.3V) • (800mA) • 40°C/W
T
A
= 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
T
A
= 50.6°C
The LTC4076 can be used above 50.6°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
C T
V V
BAT
A
IN BAT JA
=
°105 –
( –
•
)
θ
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I
C C
V V C W
C
C A
I m
A
BAT
BAT
=
° °
°
=
°
°
=
105 60
5 3
3 4
0
45
68
662
–
( –
. )
• /
/
It is important to remember that LTC4076 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.
Thermal Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4076 package is properly soldered
to the PC board ground. When correctly soldered to a
2500mm
2
double sided 1oz copper board, the LTC4076
has a thermal resistance of approximately 40°C/W. Failure
to make thermal contact between the exposed pad on the
backside of the package and the copper board will result in
thermal resistances far greater than 40°C/W. As an example,
a correctly soldered LTC4076 can deliver over 800mA to
a battery from a 5V supply at room temperature. Without
a good backside thermal connection, this number would
drop to much less than 500mA.
APPLICATIO S I FOR ATIO
W UU
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