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LTC4065/LTC4065A

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Undervoltage Charge Current Limiting (UVCL)

The LTC4065/LTC4065A includes undervoltage charge

(

rent until the input supply voltage reaches approximately

200mV above the battery voltage. This feature is particu-

larly useful if the LTC4065 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 is operating

under normal conditions and the input supply voltage

begins to droop (e.g., an external load drags the input

(approximately 220mV above the battery voltage),

undervoltage charge current limiting will begin to reduce

the charge current in an attempt to maintain

LTC4065 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 dissipates significantly less power

when programmed for a current higher than the limit of

the supply as compared to using a non-current 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 (non-current 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 to

charge at a current above 600mA. Assume that the LTC4065

that part tolerances maintain a programmed current higher

than 600mA. Since the LTC4065 will demand a charge

current higher than the current limit of the voltage 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 power device is approxi-

mately 450m

Ω with a 5V supply. The actual “on” resis-

tance 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 240mW. That is

a 76% improvement over the non-current limited supply

power dissipation.

USB and Wall Adapter Power

Although the LTC4065/LTC4065A 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.

Figure 3. Combining Wall Adapter and USB Power

MP1

MN1

1k

2k

4.02k

3

4

6

Li-Ion

BATTERY

SYSTEM

LOAD

4065 F03

LTC4065

BAT

USB

POWER

500mA

5V WALL

ADAPTER

750mA

PROG

+

D1

APPLICATIO S I FOR ATIO

Stability Considerations

The LTC4065/LTC4065A contain two control loops: con-

stant-voltage and constant-current. The constant-voltage

loop is stable without any compensation when a battery is

connected with low impedance leads. Excessive lead