MCP73871-2CCISLASHML datasheet(20/38 Pages) MICROCHIP

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MCP73871

DS22090B-page 20

4.1

UnderVoltage Lockout (UVLO)

An internal undervoltage lockout (UVLO) circuit

monitors the input voltage and keeps the charger in

shutdown mode until the input supply rises above the

UVLO threshold.

In the event a battery is present when the input power

is applied, the input supply must rise approximately

100 mV

above

the

battery

voltage

before

the

MCP73871 device become operational.

The UVLO circuit places the device in shutdown mode

if the input supply falls to approximately 100 mV of the

battery voltage.

The UVLO circuit is always active. At any time, the

input

supply

below

the

UVLO

threshold

approximately 100 mV of the voltage at the VBAT pin,

the MCP73871 device is placed in a shutdown mode.

During any UVLO condition, the battery reverse

discharge current shall be less than 2 µA.

4.2

System Load Sharing

The system load sharing feature gives the system

priority on input power, allowing the system to

power-up with deeply depleted battery packs.

With the SEL input active Low, the MCP73871 device

is designed to provide system power and Li-Ion battery

charging from a USB input while adhering to the current

limits governed by the USB specification.

With the SEL input active High, the MCP73871 device

limits the total supply current to 1.8A (system power

and charge current combined).

FIGURE 4-2:

System Load Sharing

Diagram.

4.3

Charge Qualification

For a charge cycle to begin, all UVLO conditions must

be met and a battery or output load must be present.

A charge current programming resistor must be

connected from PROG1 to VSS when SEL = High.

When SEL = Low, PROG2 needs to tie to High or Low

for proper operation.

4.4

Preconditioning

If the voltage at the VBAT pin is less than the

preconditioning threshold, the MCP73871 device

enters a preconditioning mode. The preconditioning

threshold

factory

set.

Refer

Section 1.0

“Electrical

Characteristics”

for

preconditioning

threshold options.

In this mode, the MCP73871 device supplies 10% of

the fast charge current (established with the value of

the resistor connected to the PROG1 pin) to the

battery.

When the voltage at the VBAT pin rises above the

preconditioning threshold, the MCP73871 device

enters the constant current (fast charge) mode.

4.5

Constant Current Mode - Fast

Charge

During the constant current mode, the programmed

charge current is supplied to the battery or load. The

charge current is established using a single resistor

from PROG1 to VSS. The program resistor and the

charge current are calculated using the following

equation:

EQUATION 4-1:

Constant current mode is maintained until the voltage

at the VBAT pin reaches the regulation voltage, VREG.

When constant current mode is invoked, the internal

timer is reset.

4.5.1

TIMER EXPIRED DURING

CONSTANT CURRENT - FAST

CHARGE MODE

If the internal timer expires before the recharge voltage

threshold is reached, a timer fault is indicated and the

charge cycle terminates. The MCP73871 device

remains in this condition until the battery is removed. If

the battery is removed, the MCP73871 device enters

the Stand-by mode where it remains until a battery is

reinserted.

4.6

Constant Voltage Mode

When the voltage at the VBAT pin reaches the

regulation voltage, VREG, constant voltage regulation

begins. The regulation voltage is factory set to 4.10V

or 4.20V with a tolerance of ±0.5%.

0.2

Ideal

Diode,

Synchronous

Switch

Direction

Control

0.2

Current

Limit

Direction

Control

Charge

FET

System

Power

FET

VBAT

OUT

Charge

Control

REG

1000V

PROG1

--------------------

Where:

RPROG

kilo-ohms (k

Ω)

IREG

milliampere (mA)

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