LTC4075 LINER [Linear Technology], LTC4075 Datasheet - Page 13

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LTC4075

Manufacturer Part Number
LTC4075
Description
Dual Input USB/AC Adapter Standalone Li-Ion Battery Chargers
Manufacturer
LINER [Linear Technology]
Datasheet

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APPLICATIO S I FOR ATIO
P
age (either DCIN or USBIN), V
and I
temperature at which the thermal feedback begins to
protect the IC is:
Example: An LTC4075 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 LTC4075 will begin
to reduce the charge current is approximately:
The LTC4075 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:
D
T
T
T
T
T
I
I
I
is the power dissipated, V
BAT
BAT
BAT
A
A
A
A
A
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
=
=
=
is the charge current. The approximate ambient
662
(
(
V
5
IN
V
JA
105
mA
105
– .
is 40°C/W (see Thermal Considerations),
3 3
V
°
D
BAT
C T
°
IN
U
C
V
• θ
– V
)•
)
JA
60
40
A
θ
BAT
JA
U
°
°
C
C W
) • I
/
IN
BAT
BAT
is the input supply volt-
=
is the battery voltage
• θ
W
68
45
JA
°
C A
°
C
/
U
It is important to remember that LTC4075 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 LTC4075 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 LTC4075 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.
Protecting the USB Pin and Wall Adapter Input from
Overvoltage Transients
Caution must be exercised when using ceramic capacitors
to bypass the USBIN pin or the wall adapter inputs. High
voltage transients can be generated when the USB or wall
adapter is hot plugged. When power is supplied via the
USB bus or wall adapter, the cable inductance along with
the self resonant and high Q characteristics of ceramic
capacitors can cause substantial ringing which could
exceed the maximum voltage pin ratings and damage the
LTC4075. Refer to Linear Technology Application Note 88,
entitled “Ceramic Input Capacitors Can Cause Overvoltage
Transients” for a detailed discussion of this problem. The
long cable lengths of most wall adapters and USB cables
2
double sided 1oz copper board, the LTC4075
LTC4075/LTC4075X
13
4075Xf

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