Datasheet

12
LTC4054-4.2/LTC4054X-4.2
405442xf
APPLICATIO S I FOR ATIO
WUUU
Solving for I
BAT
using the quadratic formula
2
.
I
VV VV
RCT
R
BAT
S BAT S BAT
CC A
JA
CC
=
°






(– )– (– )
(–)
2
4 120
2
θ
Using R
CC
= 0.25Ω, V
S
= 5V, V
BAT
= 3.75V, T
A
= 25°C and
θ
JA
= 125°C/W we can calculate the thermally regulated
charge current to be:
I
BAT
= 708.4mA
While this application delivers more energy to the battery
and reduces charge time in thermal mode, it may actually
lengthen charge time in voltage mode if V
CC
becomes low
R
CC
(Ω)
0
CHARGE CURRENT (mA)
1000
800
600
400
200
0
0.5
1.0
1.25
405442 F04
0.25
0.75
1.5
1.75
CONSTANT
CURRENT
V
BAT
= 3.75V
T
A
= 25°C
θ
JA
= 125°C/W
R
PROG
= 1.25kΩ
THERMAL
MODE
DROPOUT
V
S
= 5.25V
V
S
= 5.5V
V
S
= 5V
Figure 4. Charge Current vs R
CC
enough to put the LTC4054 into dropout. Figure 4 shows
how this circuit can result in dropout as R
CC
becomes
large.
This technique works best when R
CC
values are minimized
to keep component size small and avoid dropout. Remem-
ber to choose a resistor with adequate power handling
capability.
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, refer to Applica-
tion Note 88.
Charge Current Soft-Start
The LTC4054 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.
This has the effect of minimizing the transient current load
on the power supply during start-up.
Note 2: Large values of R
CC
will result in no solution for I
BAT
. This indicates that the LTC4054
will not generate enough heat to require thermal regulation.
Figure 3. A Circuit to Maximize Thermal Mode Charge Current
V
CC
R
PROG
R
CC
Li-Ion
CELL
405442 F03
LTC4054-4.2
1µF
V
S
BAT
PROG
GND