MCP73861_11 datasheet(17/34 Pages) MICROCHIP

Part #	MCP73861

Description	Advanced Single or Dual Cell, Fully Integrated Li-Ion/Li-Polymer Charge Management

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Manufacturer	MICROCHIP [Microchip Technology]

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 2011 Microchip Technology Inc.

DS21893E-page 17

MCP73861/2/3/4

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 MCP7386X.

The MCP7386X automatically enters 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 1.2A, typically. The

minimum fast charge current is 100 mA, set by letting

the PROG input float. The following formula calculates

the value for RPROG:

The preconditioning trickle-charge current and the

charge termination current are scaled to approximately

10% and 8% of IREG, respectively.

5.1.3

CELL TEMPERATURE SENSOR

BIAS (THREF)

A 2.5V voltage reference is provided to bias an external

thermistor for continuous cell temperature monitoring

and prequalification. A ratio metric window comparison

is performed at threshold levels of VTHREF/2 and

VTHREF/4.

5.1.4

CELL TEMPERATURE SENSOR

INPUT (THERM)

The MCP73861/2/3/4 continuously monitors tempera-

ture by comparing the voltage between the THERM

input and VSS with the upper and lower temperature

thresholds. A negative or positive temperature coeffi-

cient, NTC or PTC thermistor and an external voltage-

divider typically develop this voltage. The temperature

sensing circuit has its own reference to which it per-

forms a ratio metric comparison. Therefore, it is

immune to fluctuations in the supply input (VDD). The

temperature-sensing circuit is removed from the sys-

tem when VDD is not applied, eliminating additional

discharge of the battery pack.

Figure 6-1 depicts a typical application circuit with con-

nection 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:

The elapsed time termination period:

The preconditioning timer starts after qualification and

resets when the charge cycle transitions to the fast

charge, Constant-current mode. The fast charge timer

and the elapsed timer start once the MCP7386X

transitions from preconditioning. The fast charge timer

resets when the charge cycle transitions to the Con-

stant-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.

R

PROG

13.2

11

I

REG



–

12

I

REG



1.2

–

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

=

Where:

IREG = the desired fast charge current in amps.

RPROG = measured in kΩ

R

T1

2

R

COLD

R

HOT



R

COLD

R

HOT

–

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

=

R

T2

2

R

COLD

R

HOT



R

COLD

3

R



HOT

–

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

=

R

T1

2

R

COLD

R

HOT



R

HOT

R

COLD

–

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

=

R

T2

2

R

COLD

R

HOT



R

HOT

3

R



COLD

–

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

=

Where:

RCOLD and RHOT are the thermistor resis-

tance values at the temperature window of

interest.

t

PRECON

C

TIMER

0.1

F

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

1.0Hour



s

=

t

FAST

C

TIMER

0.1

F

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

1.5Hours



=

t

TERM

C

TIMER

0.1

F

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

3.0Hours



=

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