LTC4069-4.4
13
406944fa
APPLICATIONS INFORMATION
Power Dissipation
The conditions that cause the LTC4069-4.4 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. For high
charge currents, the LTC4069-4.4 power dissipation is
approximately:
P
D
= (V
CC
– V
BAT
) • I
BAT
where P
D
is the power dissipated, V
CC
is the input supply
voltage, V
BAT
is the battery voltage and I
BAT
is the charge
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC4069-4.4
will automatically reduce the charge current to maintain
the die temperature at approximately 115°C. However, 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
CC
– V
BAT
) • I
BAT
• θ
JA
Example: Consider an LTC4069-4.4 operating from a 5V
wall adapter providing 750mA to a 3.6V Li-Ion battery.
The ambient temperature above which the LTC4069-
4.4 will begin to reduce the 750mA charge current is
approximately:
T
A
= 115°C – (5V – 3.6V) • (750mA) • 60°C/W
T
A
= 115°C – (1.05W • 60°C/W) = 115°C – 63°C
T
A
= 52°C
The LTC4069-4.4 can be used above 70°C, but the
charge current will be reduced from 750mA. The
approximate current at a given ambient temperature can
be calculated:
I
CT
VV
BAT
A
CC BAT JA
=
°
()
115 –
–•θ
Using the previous example with an ambient temperature
of 73°C, the charge current will be reduced to
approximately:
I
CC
VV CW
C
CA
mA
BAT
=
°°
()
°
=
°
°
=
115 73
536 60
42
84
500
–
–. • / /
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
It is important to remember that LTC4069-4.4 applications
do not need to be designed for worst-case thermal
conditions since the IC will automatically limit power
dissipation when the junction temperature reaches
approximately 115°C.
Board Layout Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC4069-4.4 package is soldered to
the PC board copper and extending out to relatively large
copper areas or internal copper layers connected using
vias. Correctly soldered to a 2500mm
2
double-sided 1 oz.
copper board the LTC4069-4.4 has a thermal resistance
of approximately 60°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 60°C/W. As an example, a correctly soldered
LTC4069-4.4 can deliver over 750mA to a battery from
a 5V supply at room temperature. Without a backside
thermal connection, this number could drop to less than
500mA.
V
CC
Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multi-layer
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 live
power source. For more information, refer to Application
Note 88.
