LTC4069-4.4 Linear Technology, LTC4069-4.4 Datasheet - Page 13

• Manufacturer Part Number: LTC4069-4.4
• Description: Standalone 750mA Li-Ion Battery Charger in 2 x 2 DFN
• Manufacturer: Linear Technology

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:
where P
voltage, V
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:
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:
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:
Using the previous example with an ambient temperature
of 73°C, the charge current will be reduced to
approximately:
P
T
T
T
T
T
I
I
BAT
BAT
A
A
A
A
A
D
= 115°C – P
= 115°C – (V
= 115°C – (5V – 3.6V) • (750mA) • 60°C/W
= 115°C – (1.05W • 60°C/W) = 115°C – 63°C
= 52°C
= (V
=
=
D
is the power dissipated, V
BAT
(
(
CC
5
V
CC
V
– V
is the battery voltage and I
115
115
– .
–
3 6
BAT
V
D
°
°
CC
C T
BAT
C
• θ
) • I
V
–
–
)
– V
JA
)
•
73
BAT
A
• θ
60
BAT
°
JA
C
°
C W
) • I
/
BAT
=
84
CC
• θ
42
°
is the input supply
JA
C A
°
BAT
C
/
=
is the charge
500
mA
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
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
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.
CC
Bypass Capacitor
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LTC4069-4.4
2
double-sided 1 oz.
13
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