LTC4080X View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4080X
Linear Technology
LTC4080X Datasheet PDF : 20 Pages
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LTC4080X
APPLICATIO S I FOR ATIO
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 LTC4080X 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, ΔVUVCL), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain ΔVUVCL between VCC and BAT. The
LTC4080X will continue to operate at the reduced charge
current until the input supply voltage is increased or volt-
age mode reduces the charge current further.
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input sup-
ply, the LTC4080X can dissipate significantly less power
when programmed for a current higher than the limit of
the wall adapter.
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 avail-
able then the peak power dissipation inside the part can
exceed 300mW.
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 LTC4080X
to charge at a current greater than 200mA. Assume that
the LTC4080X charger is programmed for 300mA (i.e.,
RPROG = 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 PFET. The on-resistance of
the battery charger power device is approximately 0.75Ω
with a 5V supply. The actual on-resistance will be slightly
higher due to the fact that the input supply will have col-
lapsed to less than 5V. The power dissipated during this
phase of charging is approximately 30mW. That is a ten
16
times improvement over the non-current limited supply
power dissipation.
USB and Wall Adapter Power
Although the LTC4080X allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batter-
ies. Figure 3 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.
5V WALL
ADAPTER
(300mA)
USB
POWER
(200mA)
1 ICHG
SYSTEM
D1
BAT
LOAD
LTC4080X
2
MP1
VCC
4
PROG
+ Li-Ion
BATTERY
MN1 1.33k
1k
2k
4080X F03
Figure 3. Combining Wall Adapter and USB Power
Power Dissipation
The conditions that cause the LTC4080X 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 LTC4080X power
dissipation is approximately:
( ) PD = VCC − VBAT • IBAT + PD_BUCK
Where PD is the total power dissipated within the IC, VCC
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:
PD _ BUCK
= VOUT
•
IOUT
⎛
⎝⎜
1
η
−
1⎞⎠⎟
4080Xf
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