ltc4069edc-4.4 Linear Technology Corporation, ltc4069edc-4.4 Datasheet - Page 13

• Manufacturer Part Number: ltc4069edc-4.4
• Description: Standalone 750ma Li-ion Battery Charger In 2 ? 2 Dfn With Ntc Thermistor Input
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
filter can be used on the PROG pin to measure the average
battery current as shown in Figure 5. A 10K resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.
Power Dissipation
The conditions that cause the LTC4069-4.4 to reduce
charge current through thermal feedback can be approxi-
mated 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 approxi-
mately:
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:
P
T
T
T
T
T
I
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
BAT
is the power dissipated, V
(
CC
V
CC
– V
is the battery voltage and I
115
–
BAT
V
°
D
C T
BAT
CC
U
• θ
) • I
–
– V
)
JA
BAT
A
• θ
BAT
U
JA
) • I
BAT
W
CC
• θ
is the input supply
JA
BAT
is the charge
U
Using the previous example with an ambient temperature
of 73°C, the charge current will be reduced to approxi-
mately:
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 applica-
tions do not need to be designed for worst-case thermal
conditions since the IC will automatically limit power
dissipation when the junction temperature reaches ap-
proximately 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 ther-
mal 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 capaci-
tors, 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
I
BAT
Bypass Capacitor
=
(
5
V
115
– .
3 6
°
C
V
–
)
•
73
60
°
C
°
C W
/
=
LTC4069-4.4
2
84
42
double-sided 1 oz.
°
C A
°
C
/
=
500
13
mA
406944f