LTC4080EMSE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4080EMSE Datasheet PDF : 20 Pages
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LTC4080
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 LTC4080 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
LTC4080 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 LTC4080 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 avail-
able then the peak power dissipation inside the part can
exceed 300mW.
Now consider the same scenario, but with a 5V input sup-
ply with a 200mA current limit. To take advantage of the
supply, it is necessary to program the LTC4080 to charge
at a current greater than 200mA. Assume that the LTC4080
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 collapsed to less than
5V. The power dissipated during this phase of charging
16
is approximately 30mW. That is a ten times improvement
over the non-current limited supply power dissipation.
USB and Wall Adapter Power
Although the LTC4080 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)
D1
1 ICHG
BAT
SYSTEM
LOAD
LTC4080
2
MP1
VCC
4
PROG
+ Li-Ion
BATTERY
MN1 1.33k
1k
2k
4080 F03
Figure 3. Combining Wall Adapter and USB Power
Power Dissipation
The conditions that cause the LTC4080 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 LTC4080 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⎞⎠⎟
4080fb
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