LTC4078/LTC4078X
13
4078xfb
APPLICATIONS INFORMATION
Assuming θ
JA
is 40°C/W (see Thermal Considerations), the
ambient temperature at which the LTC4078/LTC4078X will
begin to reduce the charge current is approximately:
T
A
= 120°C – (5V – 3.3V) • (800mA) • 40°C/W
T
A
= 120°C – 1.36W • 40°C/W = 120°C – 54.4°C
T
A
= 65.6°C
The LTC4078/LTC4078X can be used above 70.6°C ambi-
ent, but the charge current will be reduced from 800mA.
The approximate current at a given ambient temperature
can be approximated by:
I
BAT
=
120°C–T
A
(V
IN
–V
BAT
•
JA
Using the previous example with an ambient temperature
of 75°C, the charge current will be reduced to approxi-
mately:
I
BAT
=
120°C–75°C
(5V – 3.3V) • 40°C/W
=
45°C
68°C/A
I
BAT
= 662mA
It is important to remember that LTC4078/LTC4078X
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 120°C.
Thermal Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC4078/LTC4078X DFN package is
properly soldered to the PC board ground. When cor-
rectly soldered to a 2500mm
2
double sided 1oz copper
board, the LTC4078/LTC4078X 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
Figure 4. Input Soft Connect Circuit
LTC4078/LTC4078X can deliver over 800mA to a battery
from a 5V supply at room temperature. Without a good
backside thermal connection, this number would drop to
much less than 500mA.
Input Capacitor Selection
When an input supply is connected to a portable product,
the inductance of the cable and the high-Q ceramic input
capacitor form an L-C resonant circuit. While the LTC4078/
LTC4078X are capable of withstanding input voltages as
high as 22V, if the input cable does not have adequate
mutual coupling or if there is not much impedance in
the cable, it is possible for the voltage at the input of the
product to reach as high as 2x the input voltage before it
settles out. To prevent excessive voltage from damaging
the LTC4078/LTC4078X during a hot insertion, it is best to
have a low voltage coeffi cient capacitor at the input pins
to the LTC4078/LTC4078X. This is achievable by select-
ing an X5R or X7R ceramic capacitor that has a higher
voltage rating than that required for the application. For
example, if the maximum expected input voltage is 15V,
a 25V X5R 1μF capacitor would be a better choice than
the smaller 16V X5R capacitor. Note that no charging will
occur with 15V in.
Using a tantalum capacitor or an aluminum electrolytic
capacitor for input bypassing, or paralleling with a ceramic
capacitor will also reduce voltage overshoot during a hot
insertion. Ceramic capacitors with Y5V or Z5U dielectrics
are not recommeded.
Alternatively, the following soft connect circuit can be
employed (as shown in Figure 4).
LTC4078
DCIN/USBIN
INPUT CABLE
+15V
INPUT
C1
1μF
C2
100nF
MN1
4078X F04
GND
R1
40k
