LTC4054-4.2/LTC4054X-4.2
W U U
U
APPLICATIO S I FOR ATIO
V
S
enough to put the LTC4054 into dropout. Figure 4 shows
how this circuit can result in dropout as RCC becomes
large.
R
CC
ThistechniqueworksbestwhenRCC valuesareminimized
tokeepcomponentsizesmallandavoiddropout. Remem-
ber to choose a resistor with adequate power handling
capability.
V
CC
BAT
LTC4054-4.2
PROG
1µF
Li-Ion
CELL
GND
R
PROG
V
CC Bypass Capacitor
405442 F03
Many types of capacitors can be used for input bypassing,
however, caution must be exercised when using multi-
layerceramiccapacitors.Becauseoftheself-resonantand
high Q characteristics of some types of ceramic capaci-
tors, high voltagetransients 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.
Figure 3. A Circuit to Maximize Thermal Mode Charge Current
Solving for IBAT using the quadratic formula2.
IBAT
=
2 4RCC(120°C – TA)
(VS – VBAT )– (VS – VBAT
2RCC
)
θJA
Using RCC = 0.25Ω, VS = 5V, VBAT = 3.75V, TA = 25°C and
θJA = 125°C/W we can calculate the thermally regulated
charge current to be:
Charge Current Soft-Start
The LTC4054 includes a soft-start circuit to minimize the
inrushcurrentatthestartofachargecycle. Whenacharge
cycleisinitiated,thechargecurrentrampsfromzerotothe
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.
IBAT = 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 VCC becomes low
1000
V
= 5V
CONSTANT
CURRENT
S
800
600
400
200
0
V
= 5.5V
S
DROPOUT
V
= 5.25V
S
THERMAL
MODE
V
= 3.75V
BAT
T
= 25°C
A
JA
θ
= 125°C/W
R
= 1.25kΩ
PROG
0
0.5 0.75 1.0 1.25 1.5 1.75
(Ω)
0.25
R
CC
405442 F04
Figure 4. Charge Current vs RCC
Note 2: Large values of RCC will result in no solution for IBAT. This indicates that the LTC4054
will not generate enough heat to require thermal regulation.
405442xf
12
Linear [ Linear ]
