EUP8054/8054X
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.
Figure 9. A Circuit to Maximize Thermal Mode
Charge Current
Solving for IBAT using the quadratic formaula2
I
=
BAT
Increasing Thermal Regulation Current
4R
(
120°C − T
)
2
CC
A
(
V − V
)
−
(
V − V
)
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
component, such as a resistor or diode.
S
BAT
S
BAT
θ
JA
2R
CC
Using RCC = 0.25Ω, VS = 5V, VBAT = 3.75V, TA = 25℃
and θJA = 125℃/W we can calculate the thermally
regulated charge current to be:
Example: An EUP8054 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.
I
= 708.4mA
BAT
Assuming θ is 125℃/W, the approximate charge
JA
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 VCC becomes
low enough to put the EUP8054 into dropout.
current at an ambient temperature of 25℃is:
120°C − 25°C
)
5V − 3.75V •125°C / W
I
=
= 608mA
BAT
(
This technique works best when RCC values are
minimized to keep component size small and avoid
dropout. Remember to choose a resistor with adequate
power handling capability.
By dropping voltage across a resistor in series with a 5V
wall adapter (shown in Figure 9), the on-chip power
dissipation can be decreased, thus increasing the
thermally regulated charge current
VCC Bypass Capacitor
Many types of capacitors can be used for input bypassing,
however, caution must be exercised when using
multilayer ceramic capacitors. Because of the
self-resonant and high Q characteristics of some types of
ceramic capacitors, high voltage transients can be
generated under some start-up conditions, such as
120°C − 25°C
I
=
BAT
(
V − I
R
− V
)
• θ
S
BAT CC
BAT
JA
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.
DS8054 Ver1.1 Jan. 2007
14
EUTECH [ EUTECH MICROELECTRONICS INC ]
