Datasheet

LTC4085-3/LTC4085-4
21
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Where, P
D
is the power dissipated, V
OUT
is the supply
voltage, V
BAT
is the battery voltage, and I
BAT
is the battery
charge current. It is not necessary to perform any worst-
case power dissipation scenarios because the LTC4085
will automatically reduce the charge current to maintain
the die temperature at approximately 105°C. However, 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
OUT
– V
BAT
) • I
BAT
• θ
JA
Example: Consider an LTC4085 operating from a wall
adapter with 5V at V
OUT
providing 0.8A to a 3V Li-Ion battery.
The ambient temperature above which the LTC4085 will
begin to reduce the 0.8A charge current, is approximately
T
A
= 105°C – (5V – 3V) • 0.8A • 43°C/W
T
A
= 105°C – 1.6W • 43°C/W = 105°C – 69°C = 36°C
The LTC4085 can be used above 36°C, but the charge
current will be reduced below 0.8A. The charge current
at a given ambient temperature can be approximated by:
I
BAT
=
105°C – T
A
V
OUT
–V
BAT
()
• θ
JA
Consider the above example with an ambient temperature of
55°C. The charge current will be reduced to approximately:
I
BAT
=
105°C – 55°C
5V – 3V
()
• 43°C/W
=
50°C
86°C/A
= 0.58A
APPLICATIONS INFORMATION
Board Layout Considerations
In order to be able to deliver maximum charge current
under all conditions, it is critical that the Exposed Pad on
the backside of the LTC4085 package is soldered to the
board. Correctly soldered to a 2500mm
2
double-sided
1oz. copper board the LTC4085 has a thermal resistance
of approximately 43°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 43°C/W. As an example, a correctly soldered
LTC4085 can deliver over 1A to a battery from a 5V supply
at room temperature. Without a backside thermal connec-
tion, this number could drop to less than 500mA.
V
IN
and Wall Adapter Bypass Capacitor
Many types of capacitors can be used for input bypassing.
However, caution must be exercised when using multilayer
ceramic capacitors. Because of the self resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions, such as connecting the charger input to a hot
power source. For more information, refer to Application
Note 88.
Stability
The constant-voltage mode feedback loop is stable without
any compensation when a battery is connected. However,
a 4.7µF capacitor with a 1 series resistor to GND is
recommended at the BAT pin to keep ripple voltage low
when the battery is disconnected.