LTC4065-4.4 Linear Technology, LTC4065-4.4 Datasheet - Page 12

• Manufacturer Part Number: LTC4065-4.4
• Description: Standalone 750mA Li-Ion Battery Charge
• Manufacturer: Linear Technology

w w w . D a t a S h e e t 4 U . c o m
APPLICATIO S I FOR ATIO
Undervoltage Charge Current Limiting (UVCL)
The LTC4065-4.4 includes undervoltage charge (∆V
current limiting that prevents full charge current until the
input supply voltage reaches approximately 220mV above
the battery voltage. This feature is particularly useful if the
LTC4065-4.4 is powered from a supply with long leads (or
any relatively high output impedance).
For example, USB-powered systems tend to have highly
variable source impedances (due primarily to cable quality
and length). A transient load combined with such imped-
ance can easily trip the UVLO threshold and turn the
charger off unless undervoltage charge current limiting is
implemented.
Consider a situation where the LTC4065-4.4 is operating
under normal conditions and the input supply voltage
begins to droop (e.g., an external load drags the input
supply down). If the input voltage reaches V
(approximately 220mV above the battery voltage),
undervoltage charge current limiting will begin to reduce
the charge current in an attempt to maintain ∆V
between the V
LTC4065-4.4 will continue to operate at the reduced
charge current until the input supply voltage is increased
or voltage mode reduces the charge current further.
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input
supply, the LTC4065-4.4 dissipates significantly less
power when programmed for a current higher than the
limit of the supply as compared to using a noncurrent
limited supply at the same charge current.
Consider a situation where an application demands a
600mA charge current for an 800mAh Li-Ion battery. If a
typical 5V (noncurrent limited) input supply is available
then the peak power dissipation inside the part can
exceed 1W.
Now consider the same scenario, but with a 5V input
supply with a 600mA current limit. To take advantage of
the supply, it is necessary to program the LTC4065-4.4 to
LTC4065-4.4
12
CC
input and the BAT output of the IC. The
U
U
W
BAT
U
+ ∆V
UVCL1
UVCL1
UVCL1
)
charge at a current above 600mA. Assume that the
LTC4065-4.4 is programmed for 750mA (i.e., R
1.33k) to ensure that part tolerances maintain a pro-
grammed current higher than 600mA. Since the LTC4065-
4.4 will demand a charge current higher than the current
limit of the input supply, the supply voltage will drop to the
battery voltage plus 600mA times the on-resistance of the
internal PFET. The on-resistance of the LTC4065-4.4 power
device is approximately 450mΩ with a 5V supply. The
actual on-resistance will be slightly higher due to the fact
that the input supply will drop to less than 5V. The power
dissipated during this phase of charging is less than
180mW. That is an 82% improvement over the noncurrent
limited supply power dissipation.
USB and Wall Adapter Power
Although the LTC4065-4.4 allow charging from a USB
port, a wall adapter can also be used to charge Li-Ion
batteries. 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 pull-down
resistor.
Typically a wall adapter can supply significantly more
current than the 500mA-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
are used to increase the charge current to 750mA when the
wall adapter is present.
ADAPTER
5V WALL
POWER
750mA
500mA
Figure 3. Combining Wall Adapter and USB Power
I
USB
I
CHG
CHG
MP1
1k
D1
4
MN1
LTC4065-4.4
V
CC
4.02k
PROG
BAT
3
6
I
2k
CHG
+
4065 F03
Li-Ion
BATTERY
SYSTEM
LOAD
PROG
406544f
=