MCP73841 datasheet(13/24 Pages) MICROCHIP

13 / 24 page

 2004 Microchip Technology Inc.

DS21823B-page 13

MCP73841/2/3/4

5.0

DETAILED DESCRIPTION

5.1

Analog Circuitry

5.1.1

CHARGE CURRENT SENSE INPUT

(SENSE)

Fast charge current regulation is maintained by the

voltage drop developed across an external sense

resistor (RSENSE) applied to the SENSE input pin. The

following formula calculates the value for RSENSE:

The preconditioning trickle-charge current and the

charge termination current are scaled to approximately

10% and 7% of IREG , respectively.

5.1.2

BATTERY MANAGEMENT INPUT

SUPPLY (VDD)

The VDD input is the input supply to the MCP7384X.

The MCP7384X automatically enters a power-down

mode if the voltage on the VDD input falls below the

undervoltage lockout voltage (VSTOP). This feature

prevents draining the battery pack when the VDD

supply is not present.

5.1.3

CELL TEMPERATURE SENSOR

BIAS (THREF)

A 2.55V voltage reference is provided to bias an

external thermistor for continuous cell temperature

monitoring and pre-qualification. A ratio metric window

comparison is performed at threshold levels of

VTHREF/2 and VTHREF/4. Cell temperature monitoring

is provided by both the MCP73841 and MCP73842.

5.1.4

CELL TEMPERATURE SENSOR

INPUT (THERM)

The MCP73841 and MCP73842 continuously monitor

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 develop 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 0.85V to the THERM input

disables temperature monitoring.

5.1.5

TIMER SET INPUT (TIMER)

The TIMER input programs the period of the safety

timers 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:

The elapsed time termination period:

The preconditioning timer starts after qualification and

resets when the charge cycle transitions to the con-

stant-current, fast charge phase. The fast charge and

elapsed timers start once the MCP7384X transitions

from preconditioning. The fast charge timer resets

when the charge cycle transitions to the constant-volt-

age phase. The elapsed timer will expire and terminate

the charge if the sensed current does not diminish

below the termination threshold.

RSENSE

VFCS

IREG

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

where:

IREG is the desired fast charge current in amps

RT1

2 RCOLD RHOT

RCOLD RHOT

–

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

RT2

2 RCOLD RHOT

RCOLD 3 R

× HOT

–

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

RT1

2 RCOLD RHOT

RHOT RCOLD

–

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

RT2

2 RCOLD RHOT

RHOT 3 R

× COLD

–

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

where:

values at the temperature window of interest.

RCOLD and RHOT are the thermistor resistance

tPRECON

CTIMER

0.1

µF

-------------------1.0Hour

tFAST

CTIMER

0.1

µF

-------------------1.5Hours

tTERM

CTIMER

0.1

µF

-------------------3.0Hours

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