LTC4054X-4.2_15 Datasheet, PDF (12/16 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger with Thermal Regulation in ThinSOT

LTC4054-4.2/LTC4054X-4.2
APPLICATIO S I FOR ATIO
V_S
R_CC
V_CC
BAT
1µF ^LTC4054-4.2
PROG
GND
R_PROG
Li-Ion
CELL
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Figure 3. A Circuit to Maximize Thermal Mode Charge Current
Solving for I_BAT using the quadratic formula².
I_BAT =
(V_S – V_BAT ) –


(V_S
–
V_BAT )²
4R_CC (120°C
θ_JA
–
T_A
)

2R_CC
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
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.
1000
V_S = 5V
800
CONSTANT
CURRENT
600
V_S = 5.5V
V_S = 5.25V
400 _THERMAL
MODE
DROPOUT
200
0
0
V_BAT = 3.75V
T_A = 25°C
θ_JA = 125°C/W
R_PROG = 1.25kΩ
0.25 0.5 0.75 1.0 1.25 1.5 1.75
R_CC (Ω)
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Figure 4. Charge Current vs R_CC
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.
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