ltc4054x-4.2 Linear Technology Corporation, ltc4054x-4.2 Datasheet - Page 11

• Manufacturer Part Number: ltc4054x-4.2
• Description: Standalone Linear Li-ion Battery Charger With Thermal Regulation In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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:
Using the previous example with an ambient temperature
of 60 C, the charge current will be reduced to approxi-
mately:
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.
I
I
I
BAT
BAT
BAT
320
V
5
CC
V
120
mA
– .
120
–
3 75
V
C T
BAT
U
C
–
V
–
•
•
A
60
150
U
JA
C
C W
/
W
187 5
60
.
C
C A
U
/
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*)
*Each layer uses one ounce copper
Table 2. Measured Thermal Resistance (4-Layer Board**)
*Top and bottom layers use two ounce copper, inner layers use one ounce copper.
**10,000mm
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
ambient temperature of 25 C is:
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
2500mm
1000mm
TOPSIDE
225mm
100mm
50mm
I
I
BAT
BAT
JA
COPPER AREA
COPPER AREA
(EACH SIDE)
2500mm
LTC4054-4.2/LTC4054X-4.2
is 125 C/W, the approximate charge current at an
2
2
2
2
2
2
total copper area
(
(
5
V
V
S
BACKSIDE
2500mm
2500mm
2500mm
2500mm
2500mm
2***
–
– .
120
I
3 75
BAT CC
120
2
2
2
2
2
R
C
V
C
)•
–
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
–
25
125
BOARD
BOARD
–
AREA
AREA
25
V
BAT
C
C W
C
2
2
2
2
2
2
)•
/
JUNCTION-TO-AMBIENT
JUNCTION-TO-AMBIENT
THERMAL RESISTANCE
THERMAL RESISTANCE
JA
608
125 C/W
125 C/W
130 C/W
135 C/W
150 C/W
80 C/W
mA
11
405442xf