LTC4063 datasheet(13/18 Pages) LINEAR

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LTC4065L/

LTC4065LX/LTC4065L-4.1

4065lfb

Undervoltage Charge Current Limiting (UVCL)

The LTC4065L includes undervoltage charge (ΔVUVCL1)

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 VBAT + ΔVUVCL1

(approximately 220mV above the battery voltage), un-

dervoltage charge current limiting will begin to reduce

the charge current in an attempt to maintain ΔVUVCL1

between the VCC 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.

APPLICATIONS INFORMATION

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.

VCC

MP1

800

IBAT

Li-Ion

BATTERY

SYSTEM

LOAD

4065L F03

LTC4065L

BAT

USB

POWER

5V WALL

ADAPTER

PROG

Figure 3. Combining Wall Adapter and USB Power

LTC4063 Datasheet(PDF) 13 Page - Linear Integrated Systems

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