LTC4065L/
LTC4065LX/LTC4065L-4.1
APPLICATIONS INFORMATION
Undervoltage Charge Current Limiting (UVCL)
The LTC4065L includes undervoltage charge (ΔV
UVCL1
)
current limiting that prevents full charge current until the
input supply voltage reaches approximately 200mV above
the battery voltage. This feature is particularly useful if the
LTC4065L 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 LTC4065L 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
BAT
+ ΔV
UVCL1
(approximately 220mV above the battery voltage), un-
dervoltage charge current limiting will begin to reduce
the charge current in an attempt to maintain ΔV
UVCL1
between the V
CC
input and the BAT output of the IC. The
LTC4065L 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.
5V WALL
ADAPTER
D1
USB
POWER
4
MP1
1k
USB and Wall Adapter Power
Although the LTC4065L allows 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.
Stability Considerations
The LTC4065L contains two control loops: constant-voltage
and constant-current. The constant-voltage loop is stable
without any compensation when a battery is connected
with low impedance leads. Excessive lead length, however,
may add enough series inductance to require a bypass
capacitor of at least 1μF from BAT to GND. Furthermore,
a 4.7μF capacitor with a 0.2Ω to 1Ω series resistor from
BAT to GND is required to keep ripple voltage low when
the battery is disconnected.
High value capacitors with very low ESR (especially ce-
ramic) may reduce the constant-voltage loop phase margin.
Ceramic capacitors up to 22μF may be used in parallel
with a battery, but larger ceramics should be decoupled
with 0.2Ω to 1Ω of series resistance.
I
BAT
BAT
LTC4065L
V
CC
PROG
3
SYSTEM
LOAD
6
+
800Ω
Li-Ion
BATTERY
4065L F03
Figure 3. Combining Wall Adapter and USB Power
4065lfb
13
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
