Preliminary Datasheet

LP28056

LP28056 – 01 Ver. 1.1 Datasheet

FEB.-2012

www.lowpowersemi.com

Page 7 of 9

Operation

The LP28056 is a single cell lithium-ion battery charger

using a constant-current/constant-voltage algorithm. It can

deliver up to 1200mA of charge current (using a good

thermal PCB layout) with a final float voltage accuracy of

±1%. The LP28056 includes an internal P-channel power

MOSFET and thermal regulation circuitry. No blocking

diode or external current sense resistor is required; thus, the

basic charger circuit requires only two external components.

Furthermore, the LP28056 is capable of operating from a

USB power source.

Normal Charge Cycle

A charge cycle begins when the voltage at the V

above the UVLO threshold level and a 1% ISET ram

resistor is connected from the ISET pin to ground or when a

battery is connected to the charger output. If the BAT pin is

less than 2.9V, the charger enters trickle charge mode. In

this mode, the LP28056 supplies approximately 1/10 the

ISET rammed charge current to bring the battery voltage up

to a safe level for full current charging. (Note: The

LP28056 does not include this trickle charge feature).

When the BAT pin voltage rises above 2.9V, the charger

enters constant-current mode, where the ISET rammed

charge current is supplied to the battery. When the BAT pin

approaches the final float voltage (4.2V), the LP28056

enters constant-voltage mode and the charge current begins

to decrease. When the charge current drops to 1/10 of the

ISET rammed value, the charge cycle ends.

ISET ramming Charge Current

The charge current is ISET rammed using a single resistor

from the ISET pin to ground. The battery charge current is

1000 times the current out of the ISET pin. The ISET ram

resistor and the charge current are calculated using the

following equations:

The charge current out of the BAT pin can be determined at

any time by monitoring the ISET pin voltage using the

following equation:

Charge Termination

A charge cycle is terminated when the charge current falls

to 1/10th the ISET rammed value after the final float

voltage is reached. This condition is detected by using an

internal, filtered comparator to monitor the ISET pin. When

the ISET pin voltage falls below 100mV

1

for longer than

t

current is latched off and the LP28056 enters standby mode,

where the input supply current drops to 200µA. (Note: C/10

termination is disabled in trickle charging and thermal

limiting modes).

When charging, transient loads on the BAT pin can cause

the ISET pin to fall below 100mV for short periods of time

before the DC charge current has dropped to 1/10th the

ISET rammed value. The 1ms filter time (t

termination comparator ensures that transient loads of this

nature do not result in premature charge cycle termination.

Once the average charge current drops below 1/10th the

ISET rammed value, the LP28056 terminates the charge

cycle and ceases to provide any current through the BAT

pin. In this state, all loads on the BAT pin must be supplied

by the battery.

The LP28056 constantly monitors the BAT pin voltage in

standby mode. If this voltage drops below the 4.05V

recharge threshold (V

and current is once again supplied to the battery. To

manually restart a charge cycle when in standby mode, the

input voltage must be removed and reapplied, or the

charger must be shut down and restarted using the ISET pin.

Figure 1 shows the state diagram of a typical charge cycle.

Charge Status Indicator (STAT)

The charge status output has three different states: strong

pull-down (~10mA), weak pull-down (~20µA) and high

impedance. The strong pull-down state indicates that the

LP28056 is in a charge cycle. Once the charge cycle has

terminated, the pin state is determined by undervoltage

lockout conditions. A weak pull-down indicates that V

CC

meets the UVLO conditions and the LP28056 is ready to

charge. High impedance indicates that the LP28056 is in

undervoltage lockout mode: either V

above the BAT pin voltage or insufficient voltage is applied

to the V

between these three states—this method is discussed in the

Applications Information section.

Thermal Limiting