Preliminary Datasheet

LP28400

LP28400-00 Version 1.0 May.-2013

www.lowpowersemi.com

Page 14 of 17

Operation Application

The LP28400 is a constant current, constant voltage 1-cell and

2-cell Li-Ion battery charger controller that uses a current mode

PWM step-down (buck) switching architecture. The charge

current is set by an external sense resistor across the SRP and

SRN pins. Voltage amplifier and the resistor divider provide

regulation with ±1% accuracy.

Normal Charge Cycle

The LP28400 series offers a high accuracy voltage regulator on

for the charging voltage. The LP28400 uses CELL pin to select

number of cells with a fixed 4.2V/cell. Connecting CELL to

AGND or floating CELL sets 2 cell output, and connecting to

VREF sets 1 cell output

CELL PIN

VOLTAGE ULATION

GND or Floating

8.4V

VREF

4.2V

Normal Charge Cycle

A charge cycle begins when the voltage at the V

above the UVLO threshold level and when a battery is

connected to the charger output. If the BAT pin is less than

2.9V/5.8V, the charger enters trickle charge mode. In this mode,

the LP28400 supplies approximately 1/10 the ISET rammed

charge current to bring the battery voltage up to a safe level for

full current charging.

When the BAT pin voltage rises above 2.9V/5.8V, the charger

goes into the full- scale constant current charge mode. In

constant current mode The charge current is set by an external

sense resistor

across the SRP and SRN charge current is

supplied to the battery. When the BAT pin approaches the final

float voltage (4.2V/8.4V), LP28400 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.

Charge Current Regulation

The ISET input sets the maximum charging current. Battery

current is sensed by current sensing resistor RSR connected

between SRP and SRN. The full-scale differential voltage

between SRP and SRN is 75mV max.

Under high ambient temperature, the charge current will fold

back to keep IC temperature not exceeding 125°C

Input Current Regulation

The total input current from an AC adapter or other DC sources

is a function of the system supply current and the battery

charging current. System current normally fluctuated as

portions of the systems are powered up or down. Without

Dynamic Power Management (DPM), the source must be able

to supply the maximum system current and the maximum

available charger input current simultaneously. By using DPM,

the input current regulator reduces the charging current when

the summation of system power and charge power exceeds the

maximum input power. Therefore, the current capability of the

AC adapter can be lowered, reducing system cost., the sense

voltage between ACP and ACN is 75mV typ.

The ACP and ACN pins are used to sense across RAC with

default value of 20mΩ. However, resistors of other values can

also be used. A larger sense resistor will give a larger sense

voltage and higher regulation accuracy, at the expense of

higher conduction loss.

Charge Termination

A charge cycle is terminated when the charge current falls to

1/10th the SRP and SRN rammed value after the final float

voltage is reached. This condition is detected by using an

internal, filtered comparator to monitor the SRP and SEN pin.

When the SRP and SEN pin voltage falls below 1/10 for longer

than TTERM_DEG, charging is terminated. The charge current

is latched off and the LP28400 enters standby mode, where the

input supply current drops to 1.2mA. (Note: C/10 termination is

disabled in trickle charging and thermal limiting modes).

When charging, transient loads on the VSRP-SRN can cause

the VSRP-SRN to fall below 1/10 for short periods of time

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

VSRP-SRN

rammed

value.

The

100ms

filter

time

(TTERM_DEG) on the termination comparator ensures that

transient loads of this nature do not result in premature charge

cycle termination. Once the average charge current drops