LTC4062
16
4062fb
APPLICATIO S I FOR ATIO
WUUU
Example: An LTC4062 operating from a 5V wall adapter is
programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.3V. Assuming
θ
JA
is 40°C/W (see Thermal Considerations), the ambient
temperature at which the LTC4062 will begin to reduce the
charge current is approximately:
T
A
= 105°C – (5V – 3.3V) • (800mA) • 40°C/W
T
A
= 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
T
A
= 50.6°C
The LTC4062 can be used above 50.6°C ambient, but the
charge current will be reduced from 800mA. The approxi-
mate current at a given ambient temperature can be
approximated by:
I
CT
VV
BAT
A
CC BAT JA
=
°105 –
(– )•θ
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to
approximately:
I
CC
VVCW
C
CA
ImA
BAT
BAT
=
°°
°
=
°
°
=
105 60
53340
45
68
662
–
(–.)• / /
It is important to remember that LTC4062 applications do
not need to be designed for worst-case thermal condi-
tions, since the IC will automatically reduce power dissi-
pation if the junction temperature reaches approximately
105°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 LTC4062 package is properly soldered to
the PC board ground. Correctly soldered to a 2500mm
2
double sided 1oz copper board, the LTC4062 has a ther-
mal 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 LTC4062 can deliver over 800mA to a
battery from a 5V supply at room temperature. Without a
good 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 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 and Soft-Stop
The LTC4062 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 the
full-scale current over a period of approximately 100µs.
Likewise, internal circuitry slowly ramps the charge cur-
rent from full-scale to zero when the charger is shut off or
self terminates. This has the effect of minimizing the
transient current load on the power supply during start-up
and charge termination.
Reverse Polarity Input Voltage Protection
In some applications, protection from reverse polarity on
V
CC
is desired. If the supply voltage is high enough, a
series blocking diode can be used. In other cases, where
the diode voltage drop must be kept low, a P-channel
MOSFET can be used (as shown in Figure 7).
Figure 7. Low Loss Input Reverse Polarity Protection
V
CC
V
IN
4062 F07
LTC4062
DRAIN-BULK
DIODE OF FET
