MCP73837-AMISLASHMF datasheet(16/36 Pages) MICROCHIP

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MCP73837/8

DS20002071C-page 16

 2007-2015 Microchip Technology Inc.

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. The UVLO circuitry has a built-in

hysteresis of 75 mV for the USB port and 55 mV for the

AC adapter.

In the event a battery is present when the input power

is applied, the input supply must rise 100 mV above the

battery

voltage

before

MCP73837/8

becomes

operational.

The UVLO circuit places the device in shutdown mode

if the input supply falls to within +100 mV of the battery

voltage.

The UVLO circuit is always active. If, at any time, the

input supply is below the UVLO threshold or within

+100 mV of the voltage at the VBAT pin, the

MCP73837/8 is placed in a Shutdown mode.

During any UVLO condition, the battery reverse

discharge current is less than 2 µA.

4.2

Autonomous Power Source

Selection

The MCP73837/8 devices are designed to select the

USB port or AC adapter as the power source

automatically. If the AC adapter input is not present, the

USB port is selected. If both inputs are available, the

AC adapter has first priority.

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 con-

nected from PROG1 to VSS. If the PROG1 or PROG2

pin are open or floating, the MCP73837/8 is disabled

and the battery reverse discharge current is less than

2 µA. In this manner, the PROG1 pin acts as a charge

enable and can be used as a manual shutdown.

4.4

Preconditioning

If the voltage at the VBAT pin is less than the

preconditioning threshold, the MCP73837/8 enters a

preconditioning mode. The preconditioning threshold is

factory set.

Refer to Section 1.0 “Electrical Characteristics” for pre-

conditioning threshold options.

In this mode, the MCP73837/8 supplies a percentage

of the charge current (established with the value of the

resistor connected to the PROG1 pin for AC mode,

established by PROG2 level for USB mode) to the bat-

tery. The percentage or ratio of the current is factory

set. Refer to Section 1.0 “Electrical Characteristics” for

preconditioning current options.

When the voltage at the VBAT pin rises above the

preconditioning threshold, the MCP73837/8 enters the

Constant Current or Fast Charge mode.

4.5

Constant Current Mode – Fast

Charge

During Constant Current mode, the programmed (AC

adapter) or selected (USB port) charge current is sup-

plied to the battery or load.

For AC adapter, the charge current is established

using a single resistor from PROG1 to VSS. The

program resistor and the charge current are calculated

using the Equation 4-1.

EQUATION 4-1:

When charging from a USB port, the host

microcontroller has the option of selecting either a

one-unit-load or a five-unit-loads charge rate based on

the PROG2 input. A logic Low selects a one-unit-load

charge rate, a High selects a five-unit-loads charge

rate, and high impedance input suspends or disables

charging.

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.

Note:

If the input power is switched during a

charge cycle, the power path switch-over

will be a break-before-make connection.

As a result, the charge current can

momentarily go to zero. The charge cycle

timer will remain continuous.

Note:

USB Specification Rev. 2.0 defines the

maximum absolute current for one unit

load is 100 mA. This value is not an

average over time and cannot be

exceeded.

REG

1000V

PROG

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

Where:

RPROG =

kilohm (k



IREG =

milliampere (mA)

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