EUP8084
It is not necessary to perform worst-case power
dissipation scenarios because the EUP8084 will
automatically reduce the charge current to maintain the
die temperature at approximately 115°C. However, the
approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
ADP Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using
multi-layer 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
connecting the battery charger input to a live power
source.
o
T
= 115 C − P θ
A
D JA
o
= 115 C −
(
V
−
)
× I
×
θ
T
V
A
ADP
BAT
JA
CHG
SWITCHING REGULATOR
Inductor Selection
if the regulator is off.
The output inductor is selected to limit the ripple current
to some predetermined value, typically 20%~40% of the
full load current at the maximum input voltage. Large
value inductors lower ripple currents. Higher VIN or
VOUT also increases the ripple current as shown in
equation. A reasonable starting point for setting ripple
current is ∆IL=240mA (40% of 600mA).
Example: Consider the extreme case when an EUP8084
is operating from a 6V supply providing 250mA to a 3V
Li-Ion battery, the switching regulator and the LDO are
off. The ambient temperature above which the EUP8084
will begin to reduce the 250mA charge current is
approximately: (Correctly soldered to a 2500mm2
double-sided 1 oz. copper board, the EUP8084 has a
thermal resistance of approximately 43°C/W.)
V
1
o
o
OUT
∆I
=
V
1−
T
= 115 C −
(
6V − 3V
)
×
o
250mA
)
× 43 C / W
L
OUT
A
(f)(L)
V
IN
o
o
o
= 115 C − 0.75W × 43 C / W = 115 C − 32.25 C
T
T
A
A
o
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications
(600mA+120mA). For better efficiency, choose a low
DC-resistance inductor.
= 82.75 C
If there is more power dissipation due to the switching
regulator or the LDO, the thermal regulation will kick in
at a somewhat lower temperature than this. In the above
circumstances, the EUP8084 can be used above 82.75°C,
but the charge current will be reduced from 250mA. The
approximate current at a given ambient temperature can
be calculated:
CIN and COUT Selection
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle VOUT/VIN. The
primary function of the input capacitor is to provide a
low impedance loop for the edges of pulsed current
drawn by the EUP8084. A low ESR input capacitor sized
for the maximum RMS current must be used. The size
required will vary depending on the load, output voltage
and input voltage source impedance characteristics. A
typical value is around 4.7µF.
115o C −
T
A
) × θ
I
=
CHG
(V
− V
ADP
BAT
JA
Using the previous example with an ambient temperature
of 85°C, the charge current will be reduced to approxim-
ately:
The input capacitor RMS current varies with the input
voltage and the output voltage. The equation for the
maximum RMS current in the input capacitor is:
115o C − 85o C
30o C
I
=
=
= 232.6mA
V
V
CHG
6V − 3V
× 43o C / W 129o C / A
O
O
(
)
I
= I
×
× 1 −
RMS
O
V
V
IN
IN
Note: 1V = 1J/C = 1W/A
The output capacitor COUT has a strong effect on loop
stability.
Furthermore, the voltage at the ISET pin will change
proportionally with the charge current as discussed in the
Programming Charge Current section.
The selection of COUT is driven by the required effective
series resistance (ESR).
DS8084 Ver1.0 Apr. 2008
20
EUTECH [ EUTECH MICROELECTRONICS INC ]
