LTC4053-4.2
13
4053fa
Regardless of mode, the voltage at the PROG pin is
proportional to the current being delivered to the battery.
Power Dissipation
The conditions that cause the LTC4053 to reduce charge
current due to the thermal protection feedback can be
approximated by considering the power dissipated in the
IC. For high charge currents, the LTC4053 power dissipa-
tion 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 battery
charge current. It is not necessary to perform any worst-
case power dissipation scenarios because the LTC4053
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
CC
– V
BAT
) • I
BAT
•
θ
JA
Example: Consider an LTC4053 operating from a 5V wall
adapter providing 1.2A to a 3.75V Li-Ion battery. The
ambient temperature above which the LTC4053 will begin
to reduce the 1.2A charge current is approximately:
T
A
= 105°C – (5V – 3.75V) • 1.2A • 40°C/W
T
A
= 105°C – 1.5W • 40°C/W = 105°C – 60°C = 45°C
The LTC4053 can be used above 45°C, but the charge
current will be reduced below 1.2A. The approximate
charge current at a given ambient temperature can be
approximated by:
I
CT
VV
BAT
A
CC BAT JA
=
°105 –
(– )•θ
Consider the above example with an ambient temperature
of 55°C. The charge current will be reduced to approxi-
mately:
I
CC
VVCW
C
CA
A
BAT
=
°°
°
=
°
°
=
105 55
5 3 75 40
50
50
1
–
(–. )• / /
APPLICATIO S I FOR ATIO
WUUU
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 LTC4053 applications do
not need to be designed for worst-case thermal conditions
since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately
105°C.
Board Layout Considerations
The ability to deliver maximum charge current under all
conditions require that the exposed metal pad on the
backside of the LTC4053 package be soldered to the PC
board ground. Correctly soldered to a 2500mm
2
double-
sided 1oz. copper board the LTC4053 has a thermal
resistance of approximately 40°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 40°C/W. As an example, a
correctly soldered LTC4053 can deliver over 1250mA 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 capaci-
tors, 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 provided that a battery is
connected. However, a 1µF capacitor with a 1Ω series
resistor to GND is recommended at the BAT pin to keep
ripple voltage low when the battery is disconnected.
In the constant-current mode it is the PROG pin that is in
the feedback loop and not the battery. The constant-
current mode stability is affected by the impedance at the
