MCP73853_13 datasheet(15/32 Pages) MICROCHIP

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 2004-2013 Microchip Technology Inc.

DS21915C-page 15

MCP73853/55

5.0

DETAILED DESCRIPTION

5.1

Analog Circuitry

5.1.1

BATTERY MANAGEMENT INPUT

SUPPLY (VDD1, VDD2)

The VDD pin is the input supply pin for 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 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

external thermistor for continuous cell temperature

monitoring and prequalification. A ratiometric 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 develop this voltage. The temperature-

sensing circuit has its own reference, to which it

performs a ratiometric 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

expires and terminates 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.

RPROG

13.32 33.3 IREG



–

14.1 IREG



1.2

–

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

Where:

IREG is the desired fast charge current in

amps

RPROG is in kilohms.

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:

RCOLD and RHOT are the thermistor

resistance values at the temperature window

of interest.

tPRECON

CTIMER

0.1

F

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



tFAST

CTIMER

0.1

F

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



tTERM

CTIMER

0.1

F

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



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