EUP8084
power dissipated during this phase of charging is
approximately 40mW. That is a ten times improvement
over the non-current limited supply power dissipation.
USB and Wall Adapter Power
Although the EUP8084 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batteries.
Figure 4 shows an example of how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pulldown resistor.
Typically a wall adapter can supply significantly more
current than the current-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
can be used to increase the charge current when the wall
adapter is present.
Figure 3. Isolating Capacitive Load on ISET Pin and Filtering
Undervoltage Charge Current Limiting (UVCL)
USB powered systems tend to have highly variable
source impedances (due primarily to cable quality and
length). A transient load combined with such impedance
can easily trip the UVLO threshold and turn the battery
charger off unless undervoltage charge current limiting is
implemented.
Consider a situation where the EUP8084 is operating
under normal conditions and the input supply voltage
begins to sag (e.g. an external load drags the input supply
down). If the input voltage reaches VUVCL (approximately
300mV above the battery voltage, ∆VUVCL), under-
voltage charge current limiting will begin to reduce the
charge current in an attempt to maintain ∆VUVCL between
ADP and BAT. The EUP8084 will continue to operate at
the reduced charge current until the input supply voltage
is increased or voltage mode reduces the charge current
further.
Figure 4. Combining Wall Adapter and USB Power
Operation from Current Limited Wall Adapter
Power Dissipation
By using a current limited wall adapter as the input
supply, the EUP8084 can dissipate significantly less
power when programmed for a current higher than the
limit of the supply.
Consider a situation where an application requires a
200mA charge current for a discharged 800mAh Li-Ion
battery. If a typical 5V (non-current limited) input supply
is available then the peak power dissipation inside the
part can exceed 300mW.
The conditions that cause the EUP8084 battery charger to
reduce charge current through thermal feedback can be
approximated by considering the total power dissipated
in the IC. For high charge currents, the EUP8084 power
dissipation is approximately:
P
=
−
)
×
+
+
V
V
I
P
D _ BUCK
ADP
− V
BAT
CHG
D
(
V
)
× I
Now consider the same scenario, but with a 5V input
supply with a 200mA current limit. To take advantage of
the supply, it is necessary to program the EUP8084 to
charge at a current greater than 200mA. Assume that the
EUP8084 charger is programmed for 300mA (i.e., RISET
= 1.33kΩ) to ensure that part tolerances maintain a
programmed current higher than 200mA. Since the
battery charger will demand a charge current higher than
the current limit of the input supply, the supply voltage
will collapse to the battery voltage plus 200mA times the
on-resistance of the internal PMOSFET. The
on-resistance of the battery charger power device is
approximately 1Ω with a 5V supply. The actual
on-resistance will be slightly higher due to the fact that
the input supply will have collapsed to less than 5V. The
DS8084 Ver1.0 Apr. 2008
INA
OUTA
OUTA
Where PD is the total power dissipated within the IC,
ADP is the input supply voltage, VBAT is the battery
voltage, IBAT is the charge current and PD_BUCK is the
power dissipation due to the regulator. PD_BUCK can be
calculated as:
1
P
= V
× I
−1
D _ BUCK
OUTB OUTB
η
Where VOUTB is the regulated output of the switching
regulator, IOUTB is the regulator load and η is the
regulator efficiency at that particular load.
19
EUTECH [ EUTECH MICROELECTRONICS INC ]
