10
LTC4068-4.2/LTC4068X-4.2
406842fa
The LTC4068 can be used above 52°C ambient but the
charge current will be reduced from the programmed
800mA. The approximate current at a given ambient
temperature can be approximated by:
I
CT
VV
BAT
A
CC BAT JA
=
°
()
120 –
–•θ
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to
approximately:
I
CC
VV CW
C
CA
ImA
BAT
BAT
=
°°
()
°
=
°
°
=
120 60
533 50
60
85
706
–
–. • /
/
Moreover, when thermal feedback reduces the charge cur-
rent the voltage at the PROG pin is also reduced proportion-
ally as discussed in the Operation section. It is important
to remember that LTC4068 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
I
n order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4068 package is soldered to the PC
board ground. Correctly soldered to a 2500mm
2
double-
sided 1oz copper board, the LTC4068 has a thermal
resistance of approximately 40°C/W. Failure to make
thermal contact between the exposed pad on the back-
side of the package and the copper board will result in
thermal resistances far greater than 40°C/W. As an
example, a correctly soldered LTC4068 can deliver over
800mA to a battery from a 5V supply at room tempera-
ture. Without a good backside thermal connection, this
number will drop considerably.
V
CC
Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multilayer
APPLICATIO S I FOR ATIO
WUUU
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. Adding a 1.5Ω resistor in series
with an X5R ceramic capacitor will minimize start-up
voltage transients. For more information, see Application
Note 88.
Charge Current Soft-Start
The LTC4068 includes a soft-start circuit to minimize the
inrush current at the start of a charge cycle. When a charge
cycle is initiated, the charge current ramps from zero to
full-scale current over a period of approximately 100µs.
This has the effect of minimizing the transient current load
on the power supply during start-up.
USB and Wall Adapter Power
The LTC4068 allows charging from both a wall adapter
and a USB port. Figure 3 shows how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB port
when a wall adapter is present and a Schottky diode, D1,
is used to prevent USB power loss through the 1k pull-
down resistor.
Typically a wall adapter can supply more current than
the 500mA-limited USB port. Therefore, an N-channel
MOSFET, MN1, and an extra 3.3k program resistor are
used to increase the charge current to 800mA when the
wall adapter is present. The charge termination threshold
remains fixed at 80mA.
+
LTC4068-4.2
BAT
ITERM
PROG
V
CC
GND
D1
5V WALL
ADAPTER
800mA I
CHG
USB POWER
500mA I
CHG
I
CHG
SYSTEM
LOAD
Li-Ion
BATTERY
MP1
1k
3.3k
2k
MN1
6
4, 9
2
1
5
406842 F03
1.25k
Figure 3. Combining Wall Adapter and USB Power
