ltc4076 Linear Technology Corporation, ltc4076 Datasheet - Page 13

• Manufacturer Part Number: ltc4076
• Description: Dual Input Standalone Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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
age (either DCIN or USBIN), V
and I
temperature at which the thermal feedback begins to
protect the IC is:
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
the ambient temperature at which the LTC4076 will begin
to reduce the charge current is approximately:
D
P
T
T
T
T
T
is the power dissipated, V
A
A
A
A
A
D
BAT
= 105°C – P
= 105°C – (V
= 105°C – (5V – 3.3V) • (800mA) • 40°C/W
= 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
= 50.6°C
= (V
is the charge current. The approximate ambient
IN
JA
– V
is 40°C/W (see Thermal Considerations),
BAT
D
IN
U
) • I
• θ
– V
JA
BAT
BAT
U
) • I
IN
BAT
BAT
is the input supply volt-
is the battery voltage
• θ
W
JA
U
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:
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
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
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.
I
I
I
BAT
BAT
BAT
=
=
=
2
662
(
(
V
5
double sided 1oz copper board, the LTC4076
IN
V
105
mA
105
– .
–
3 3
V
°
BAT
C T
°
C
V
–
)•
)
–
•
60
40
A
θ
JA
°
°
C
C W
/
=
68
45
°
LTC4076
C A
°
C
/
13
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