LTC4069EDC-TRPBF View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4069EDC-TRPBF

Standalone 750mA Li-Ion Battery Charger in 2 × 2 DFN with NTC Thermistor Input

Linear Technology 

LTC4069

Applications Information

Undervoltage Charge Current Limiting (UVCL)

The LTC4069 includes undervoltage charge (∆VUVCL1)

current limiting that prevents full charge current until the

input supply voltage exceeds approximately 220mV above

the battery voltage. This feature is particularly useful if the

LTC4069 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

impedance can easily trip the UVLO threshold and turn the

charger off unless undervoltage charge current limiting

is implemented.

Consider a situation where the LTC4069 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 + ∆VUVCL1 (approximately

220mV above the battery voltage), undervoltage charge

current limiting will begin to reduce the charge current

in an attempt to maintain ∆VUVCL1 between the VCC input

and the BAT output of the IC. The LTC4069 will continue

to operate at the reduced charge current until the input

supply voltage is increased or constant-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 LTC4069 dissipates significantly less power when pro-

grammed 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 used, the

charger’s peak power dissipation can exceed 1W.

Now consider the same scenario, but with a 5V input

supply with a 600mA current limit. To take advantage

of the current limited supply, it is necessary to pro-

gram the LTC4069 to charge at a current above 600mA.

Assume that the LTC4069 is programmed for 750mA

(i.e., RPROG = 1.33k) to ensure that part tolerances main-

tain a programmed current higher than 600mA. Since the

LTC4069 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

LTC4069 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 240mW. That is a 76% improvement over the

non-current limited supply power dissipation.

USB and Wall Adapter Power

Although the LTC4069 allows charging from a USB port,

a wall adapter can also be used to charge Li-Ion batteries.

Figure 4 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.

Stability Considerations

The LTC4069 contains two control loops: constant-voltage

and constant-current. The constant-voltage loop is stable

without any compensation when a battery is connected

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For more information www.linear.com/LTC4069

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