MCP73853_13 datasheet(13/32 Pages) MICROCHIP

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DS21915C-page 13

MCP73853/55

4.0

DEVICE OVERVIEW

The MCP7385X devices are highly-advanced, linear

charge management controllers. For more information,

refer to the “Functional Block Diagram” on page 2.

Figure 4-2 depicts the operational flow algorithm from

charge initiation to completion and automatic recharge.

4.1

Charge Qualification and

Preconditioning

Upon insertion of a battery or application of an external

supply, the MCP7385X devices automatically perform a

series of safety checks to qualify the charge. The input

source voltage must be above the Undervoltage Lock-

out (UVLO) threshold, the enable pin must be above the

logic high level, and the cell temperature monitor must

be within the upper and lower thresholds (MCP73853

only). The qualification parameters are continuously

monitored, with any deviation beyond the limits automat-

ically suspending or terminating the charge cycle. The

input voltage must deviate below the UVLO stop

threshold for at least one clock period to be considered

valid.

Once the qualification parameters have been met, the

MCP7385X devices initiate a charge cycle. The charge

status output is pulled low throughout the charge cycle

(see Table 5-1 and Table 5-2 for charge status out-

puts). If the battery voltage is below the preconditioning

threshold (VPTH), the MCP7385X devices precondition

the battery with a trickle charge. The preconditioning

current is set to approximately 10% of the fast charge

regulation current. The preconditioning trickle charge

safely replenishes deeply depleted cells and minimizes

heat dissipation during the initial charge cycle. If the

battery voltage has not exceeded the preconditioning

threshold before the preconditioning timer has expired,

a fault is indicated and the charge cycle is terminated.

4.2

Constant Current Regulation –

Fast Charge

Preconditioning ends and fast charging begins when

the battery voltage exceeds the preconditioning thresh-

old. Fast charge regulates to a constant current (IREG),

which is set via an external resistor connected to the

PROG pin. Fast charge continues until either the

battery voltage reaches the regulation voltage (VREG)

or the fast charge timer expires; in which case, a fault

is indicated and the charge cycle is terminated.

4.3

Constant Voltage Regulation

When the battery voltage reaches the regulation volt-

age (VREG), constant voltage regulation begins. The

MCP7385X devices monitor the battery voltage at the

VBAT pin. This input is tied directly to the positive termi-

nal of the battery. The MCP7385X devices select the

voltage regulation value based on the state of VSET.

With VSET tied to VSS, the MCP7385X devices regulate

to 4.1V or with VSET tied to VDD, the MCP7385X

devices regulate to 4.2V.

4.4

Charge Cycle Completion and

Automatic Recharge

The MCP7385X devices monitor the charging current

during the Constant-voltage Regulation mode. The

charge cycle is considered complete when either the

charge current has diminished below approximately

7% of the regulation current (IREG) or the elapsed timer

has expired.

Assuming all the qualification parameters are met, the

MCP7385X devices automatically begin a new charge

cycle when the battery voltage falls below the recharge

threshold (VRTH).

4.5

Thermal Regulation

The MCP7385X devices limit the charge current based

on the die temperature. Thermal regulation optimizes

the charge cycle time while maintaining device reliabil-

ity. If thermal regulation is entered, the timer is automat-

ically slowed down to ensure that a charge cycle does

not terminate prematurely. Figure 4-1 depicts the

thermal regulation.

FIGURE 4-1:

Typical Maximum Charge

Current vs. Junction Temperature.

4.6

Thermal Shutdown

The MCP7385X devices suspend charge if the die

temperature exceeds 155°C. Charging resumes when

the die temperature has cooled by approximately 10°C.

The thermal shutdown is a secondary safety feature in

the event that there is a failure within the thermal

regulation circuitry.

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