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

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

APPLICATIO S I FOR ATIO
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 to reduce charge
current through thermal feedback can be approximated by
considering the power dissipated in the IC. For high charge
currents, the LTC4069 power dissipation is approximately:
where P
voltage, V
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC4069 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 operating from a 5V wall
adapter providing 750mA to a 3.6V Li-Ion battery. The
ambient temperature above which the LTC4069 will begin
to reduce the 750mA charge current is approximately:
The LTC4069 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 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 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 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 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 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
/
=
2
84
42
double-sided 1 oz.
°
C A
°
C
/
LTC4069
=
500
13
mA
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