MCP73855T datasheet(13/24 Pages) MICROCHIP

13 / 24 page

 2004 Microchip Technology Inc.

DS21915A-page 13

MCP73853/55

5.0

DETAILED DESCRIPTION

5.1

Analog Circuitry

5.1.1

BATTERY MANAGEMENT INPUT

SUPPLY (VDD1, VDD2)

The VDD input is the input supply to the MCP7385X

devices. The MCP7385X devices automatically enter a

power-down mode if the voltage on the VDD input falls

below the UVLO voltage (VSTOP). This feature prevents

draining the battery pack when the VDD supply is not

present.

5.1.2

PROG INPUT

Fast charge current regulation can be scaled by placing

a programming resistor (RPROG) from the PROG input

to VSS. Connecting the PROG input to VSS allows for a

maximum fast charge current of 400 mA, typically. The

minimum fast charge current is 85 mA (Typ) and is set

by letting the PROG input float. Equation 5-1 calculates

the value for RPROG.

EQUATION 5-1:

The preconditioning trickle charge current and the

charge termination current are scaled to approximately

10% and 7% of IREG, respectively.

5.1.3

CELL TEMPERATURE SENSOR

BIAS (THREF)

A 2.55V voltage reference is provided to bias an exter-

nal thermistor for continuous cell temperature monitor-

ing

and

prequalification.

ratio-metric

window

comparison is performed at threshold levels of

VTHREF/2 and VTHREF/4.

5.1.4

CELL TEMPERATURE SENSOR

INPUT (THERM)

The MCP73853 continuously monitors temperature by

comparing the voltage between the THERM input and

VSS with the upper and lower temperature thresholds.

A negative or positive temperature coefficient, NTC or

PTC thermistor and an external voltage divider typically

develops this voltage. The temperature-sensing circuit

has its own reference to which it performs a ratio-metric

comparison. Therefore, it is immune to fluctuations in

the supply input (VDD). The temperature-sensing circuit

is removed from the system when VDD is not applied,

eliminating additional discharge of the battery pack.

Figure 6-1 depicts a typical application circuit with

connection of the THERM input. The resistor values of

RT1 and RT2 are calculated with the following

equations.

For NTC thermistors:

For PTC thermistors:

Applying a voltage equal to VTHREF/3 to the THERM

input disables temperature monitoring.

5.1.5

TIMER SET INPUT (TIMER)

The TIMER input programs the period of the safety tim-

ers by placing a timing capacitor (CTIMER) between the

TIMER input pin and VSS. Three safety timers are

programmed via the timing capacitor.

The preconditioning safety timer period:

The fast charge safety timer period:

And, the elapsed time termination period:

The preconditioning timer starts after qualification and

resets when the charge cycle transitions to the

constant-current, fast charge phase. The fast charge

timer and the elapsed timer start after the MCP7385X

devices transition from preconditioning. The fast

charge timer resets when the charge cycle transitions

to the Constant-voltage mode. The elapsed timer will

expire and terminate the charge if the sensed current

does not diminish below the termination threshold.

During thermal regulation, the timer is slowed down

proportional to the charge current.

PROG

13.32

33.3

REG

–

14.1

REG

1.2

–

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

Where:

IREG is the desired fast charge current in

amps

RPROG is in kilo-ohms.

COLD

HOT

COLD

HOT

–

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

COLD

HOT

COLD

HOT

–

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

COLD

HOT

COLD

–

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

COLD

HOT

COLD

–

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

Where:

RCOLD and

RHOT are

the

thermistor

resistance values at the temperature window

of interest.

PREC ON

TIMER

0.1

µF

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

1.0Hour

FAST

TIMER

0.1

µF

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

1.5Hours

TERM

TIME R

0.1

µF

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

3.0Hours

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