LTC4057 Datasheet, PDF (8/12 Pages) Linear Technology – Linear Li-Ion Battery Charger with Thermal Regulation in ThinSOT

LTC4057- 4.2
APPLICATIO S I FOR ATIO
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I_BAT
=
120°C – 60°C
(4.5V – 3.7V)•150°C
/
W
=
60°C
120°C / A
I_BAT = 500mA
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 LTC4057 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.
Table 1 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 one ounce copper.
Table 1. Measured Thermal Resistance
COPPER AREA
TOPSIDE*
BACKSIDE
2500mm²
2500mm²
1000mm²
225mm²
2500mm²
2500mm²
100mm²
50mm²
2500mm²
2500mm²
*Device is mounted on topside.
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
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 LTC4057-4.2 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_BAT
=
(5V
120°C – 25°C
– 3.75V)•125°C
/
W
=
608mA
By dropping voltage across a resistor in series with a 5V
wall adapter (shown in Figure 2), the on-chip power
dissipation can be decreased, thus increasing the ther-
mally regulated charge current.
I_BAT
=
(V_S
120°C – 25°C
– I_BATR_CC – V_BAT )• θ_JA
4057f
8