LP78084 datasheet(14/20 Pages) POWER

14 / 20 page

Preliminary Datasheet

LP78084

LP78084 – 02 Ver. 1.1 Datasheet

Nov.-2007

Page 14 of 22

Operation

The LP78084 is a full-featured linear battery charger with

an integrated synchronous buck converter, and a linear

regulator designed primarily for handheld applications. The

battery charger is capable of charging single-cell 4.2V

Li-Ion batteries. The buck converter is powered from the

VINB pin and has a programmable output voltage providing

a maximum load current of 600mA. The linear regulator is

design

for

low

noise,

low-dropout

application.

The

converter, the linear regulator and the battery charger can

run simultaneously or independently of each other.

BATTERY CHARGER OPERATION

Featuring an internal P-channel power MOSFET, MP1, the

battery charger uses a constant-current/constant-voltage

charge

algorithm with

programmable

current.

Charge

current can be programmed up to 500mA with a final float

voltage of 4.2V ± 1%. The STAT open-drain status output

indicates when C/10 has been reached. No blocking diode or

external sense resistor is required; thus, the basic charger

circuit requires only two external components. An internal

termination timer adheres to battery manufacturer safety

guidelines. Furthermore, the LP78084 battery charger is

capable of operating form a USB power source.

A charge cycle begins when the voltage at the ADP pin rises

above 3.6V and approximately 1 10mV above the BAT pin

voltage, a 1% program resistor is connected form the ISET

pin to ground, and the EN_BAT pin is pulled above the

enable threshold (VIH ). If the battery voltage is less than

2.95V, the battery charger begins trickle charging at 10% of

the programmed charge current.

When the BAT pin approaches the final float voltage of 4.2V,

the battery charger enters constant-voltage mode and the

charge current begins to decrease. When the current drops to

10% of the full-scale charge current, an internal comparator

turns off the N-channel MOSFET driving the STAT pin, and

the pin becomes high impedance.

An internal thermal limit reduces the programmed charge

current if the die temperature attempts to rise above a preset

value of approximately 115℃. This feature protects the

LP78084 from excessive temperature and allows the user to

push the limits of the power handling capability of a given

circuit board without the risk of damaging the LP78084 or

external components. Another benefit of the thermal limit is

that charge current can be set according to typical, rather

than

worst-case,

ambient

temperatures

for

given

application with the assurance that the battery charger will

automatically reduce the current in worst-case conditions.

An internal timer sets the total charge time, tTI M E R

(typically 4.5 hours). When this time elapses, the charge

cycle terminates and the STAT pin assumes a high

impedance state even if C/10 has not yet been reached. To

restart the charge cycle, remove the input-voltage and

reapply it or momentarily force the EN_BAT pin below VIL .

A new charge cycle will automatically restart if the BAT pin

voltage falls below VBAT_EOC (typically 4.05V).

Constant-Current / Constant-Voltage /

Constant- Temperature

The LP78084 battery charger uses a unique architecture to

charge a battery in a constant-current, constant-voltage and

constant-temperature fashion. Figure 2 shows a Simplified

Block Diagram of the LP78084. Three of the amplifier

feedback loops shown control the constant-current, CA,

constant-voltage, VA, and constant-temperature, TA modes.

A fourth amplifier feedback loop, MA, is used to increase

the output impedance of the current source pair, MP 1 and

MP3 (note that MP1 is the internal P-channel power

MOSFET). It ensures that the drain current of MP 1 is

exactly 400 times the drain current of MP3.

Amplifiers CA and VA are used in separate feedback loops

to force the charger into constant-current or constant

voltage mode, respectively. Diodes D1 and D2 provide

priority to either the constant-current or constant-voltage

loop, whichever is trying to reduce the charge current the

most. The output of the other amplifier saturates low which

effectively removes its loop from the system. When in

constant-current mode, CA servos the voltage at the ISET

pin to be precisely 1V. VA servos its non-inverting input to

1.22V when in constant-voltage mode and the internal

resistor divider made up of R1 and R2 ensures that the

battery voltage is maintained at 4.2V. The ISET pin voltage

gives an indication of the charge current anytime in the

charge cycle, as discussed in “Programming Charge

Current” in the Applications Information section.

If the die temperature starts to creep up above 115°C due to

internal power dissipation, the transconductance amplifier,

TA, limits the die temperature to approximately 115°C by

reducing the charge current. Diode D3 ensures that TA does

not affect the charge current when the die temperature is

below 115°C. In thermal regulation, the ISET pin voltage

continues to give an indication of the charge current.

typical

operation,

the

charge

cycle

begins

constant-current mode with the current delivered to the

battery equal to 400V/RISET. If the power dissipation of the

EUP8 084 results in the junction temperature approaching

115°C, the amplifier (TA) will begin decreasing the charge

current to limit the die temperature to approximately 115°C.

As the battery voltage rises, the LP78084 either returns to

constant-current mode or enters constant-voltage mode

straight from constant-temperature mode.

Battery Charger Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the input

voltage and keeps the battery charger off until ADP rises

above 3 .6V and approximately 11 0mV above the BAT pin

voltage. The 3.6V UVLO circuit has a built-in hysteresis of

approximately 0.6V, and the 110mV automatic shutdown

threshold has a built-in hysteresis of approximately 65 mV.

During undervoltage lockout conditions, maximum battery

drain current is 5 uA and maximum supply current is 10 uA.

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