LTC4054L-4.2 Datasheet, PDF (12/16 Pages) Linear Technology – 150mA Standalone Linear Li-Ion Battery Charger in ThinSOT

LTC4054L-4.2
APPLICATIO S I FOR ATIO
Example: An LTC4054L operating from a 6V wall adapter
is programmed to supply 150mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V. Assum-
ing θ_JA is 200°C/W, the ambient temperature at which the
LTC4054L will begin to reduce the charge current is
approximately:
T_A = 120°C – (6V – 3.75V) • (150mA) • 200°C/W
T_A = 120°C – 0.3375W • 200°C/W = 120°C – 67.5°C
T_A = 52.5°C
The LTC4054L can be used above 52.5°C, but the charge
current will be reduced from 150mA. The approximate
current at a given ambient temperature can be approxi-
mated by:
( ) ^{IBAT =}
120°C – T_A
V_CC – V_BAT • θ_JA
Using the previous example with an ambient tempera-
ture of 60°C, the charge current will be reduced to
approximately:
I_BAT
=
(^6V
120°C – 60°C
– 3.75V₎ • 200°C/W
=
60°C
450°C/A
I_BAT = 133mA
Moreover, when thermal feedback reduces the charge
current, the voltage at the PROG pin is also reduced
proportionally as discussed in the Operation section.
It is important to remember that LTC4054L applications
do not need to be designed for worst-case thermal condi-
tions since the IC will automatically reduce power dissipa-
tion when the junction temperature reaches approximately
120°C.
Thermal Considerations
Because of the small size of the ThinSOT package, it is very
important to use a good thermal PC board layout to
maximize the available charge current. The thermal path
for the heat generated by the IC is from the die to the
copper lead frame, through the package leads, (especially
the ground lead) to the PC board copper. The PC board
copper is the heat sink. The footprint copper pads should
be as wide as possible and expand out to larger copper
areas to spread and dissipate the heat to the surrounding
ambient. Feedthrough vias to inner or backside copper
layers are also useful in improving the overall thermal
performance of the charger. Other heat sources on the
board, not related to the charger, must also be considered
when designing a PC board layout because they will affect
overall temperature rise and the maximum charge current.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with the device
mounted on topside.
Table 1. Measured Thermal Resistance (2-Layer Board*)
COPPER AREA
TOPSIDE BACKSIDE
2500mm² 2500mm²
1000mm² 2500mm²
225mm² 2500mm²
100mm² 2500mm²
50mm² 2500mm²
BOARD
AREA
2500mm²
2500mm²
2500mm²
2500mm²
2500mm²
THERMAL RESISTANCE
JUNCTION-TO-AMBIENT
125°C/W
125°C/W
130°C/W
135°C/W
150°C/W
*Each layer uses one ounce copper
Table 2. Measured Thermal Resistance (4-Layer Board**)
COPPER AREA
(EACH SIDE)
BOARD
AREA
THERMAL RESISTANCE
JUNCTION-TO-AMBIENT
2500mm^2***
2500mm²
80°C/W
**Top and bottom layers use two ounce copper, inner layers use one
ounce copper.
***10,000mm² total copper area
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