11
LTC4054-4.2/LTC4054X-4.2
405442xf
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 BOARD THERMAL RESISTANCE
TOPSIDE BACKSIDE AREA JUNCTION-TO-AMBIENT
2500mm
2
2500mm
2
2500mm
2
125°C/W
1000mm
2
2500mm
2
2500mm
2
125°C/W
225mm
2
2500mm
2
2500mm
2
130°C/W
100mm
2
2500mm
2
2500mm
2
135°C/W
50mm
2
2500mm
2
2500mm
2
150°C/W
Table 2. Measured Thermal Resistance (4-Layer Board**)
COPPER AREA BOARD THERMAL RESISTANCE
(EACH SIDE) AREA JUNCTION-TO-AMBIENT
2500mm
2***
2500mm
2
80°C/W
Increasing Thermal Regulation Current
Reducing the voltage drop across the internal MOSFET
can significantly decrease the power dissipation in the IC.
This has the effect of increasing the current delivered to
the battery during thermal regulation. One method is by
dissipating some of the power through an external compo-
nent, such as a resistor or diode.
Example: An LTC4054 operating from a 5V wall adapter is
programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V. Assum-
ing θ
JA
is 125°C/W, the approximate charge current at an
ambient temperature of 25°C is:
I
CC
VVCW
mA
BAT
=
°°
°
=
120 25
5 3 75 125
608
–
(–. )• /
By dropping voltage across a resistor in series with a 5V
wall adapter (shown in Figure 3), the on-chip power
dissipation can be decreased, thus increasing the ther-
mally regulated charge current
I
CC
VIR V
BAT
S BAT CC BAT JA
=
°°120 25–
(– – )•θ
APPLICATIO S I FOR ATIO
WUUU
The LTC4054 can be used above 45°C ambient, but the
charge current will be reduced from 400mA. The approxi-
mate current at a given ambient temperature can be
approximated by:
I
CT
VV
BAT
A
CC BAT JA
=
°
()
120 –
–•θ
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I
CC
VV CW
C
CA
ImA
BAT
BAT
=
°°
()
°
=
°
°
=
120 60
5 3 75 150
60
187 5
320
–
–. • /
./
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 LTC4054 applications do
not need to be designed for worst-case thermal conditions
since the IC will automatically reduce power dissipation
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.
*Each layer uses one ounce copper
*Top and bottom layers use two ounce copper, inner layers use one ounce copper.
**10,000mm
2
total copper area
