MCP73837AMIMF datasheet(18/30 Pages) MICROCHIP

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MCP73837/8

DS22071A-page 18

5.0

DETAILED DESCRIPTION

5.1

Analog Circuitry

5.1.1

BATTERY MANAGEMENT INPUT

SUPPLY (VDD)

The VDD input is the input supply to the MCP73837/8.

The MCP73837/8 can be supplied by either AC-

Adapter (VAC) or USB-Port (VUSB) with autonomous

source selection. The MCP73837/8 automatically

enters a Power-down mode if the voltage on the VDD

input falls to within +100 mV of the battery voltage or

below the UVLO voltage (VSTOP). This feature prevents

draining the battery pack when both the VAC and VUSB

supplies are not present.

5.1.2

AC-ADAPTER CURRENT

REGULATION SET (PROG1)

For the MCP73837/8, the charge current regulation

can be scaled by placing a programming resistor

(RPROG) from the PROG input to VSS. The program

resistor and the charge current are calculated using

the following equation:

EQUATION 5-1:

The

preconditioning

current

and

the

charge

termination current are ratiometric to the fast charge

current based on the selected device options.

5.1.3

BATTERY CHARGE CONTROL

OUTPUT (VBAT)

The battery charge control output is the drain terminal

of an internal P-channel MOSFET. The MCP73837/8

provides constant current and voltage regulation to the

battery pack by controlling this MOSFET in the linear

region. The battery charge control output should be

connected to the positive terminal of the battery pack.

5.1.4

TEMPERATURE QUALIFICATION

(THERM)

The MCP73837/8 continuously monitors battery

temperature during a charge cycle by measuring the

voltage between the THERM and the VSS pins. An

internal 50 µA current source provides the bias for the

most common 10 k

Ω negative-temperature coefficient

(NTC) or positive-temperature coefficient (PTC)

thermistors. The current source is controlled, avoiding

measurement sensitivity to fluctuations in the supply

voltage (VDD). The MCP73837/8 compares the voltage

at the THERM pin to factory set thresholds of 1.20V

and 0.25V, typically. Once a voltage outside the

thresholds is detected during a charge cycle, the

MCP73837/8 immediately suspends the charge cycle.

The MCP73837/8 suspends charge by turning off the

pass transistor and holding the timer value. The charge

cycle resumes when the voltage at the THERM pin

returns to the normal range.

If temperature monitoring is not required, place a

standard 10 k

Ω resistor from THERM to VSS.

5.1.5

SYSTEM TEST (LDO) MODE

The MCP73837/8 can be placed in a system test mode.

In this mode, the MCP73837/8 operates as a low drop-

out linear regulator (LDO). The output voltage is

regulated to the factory set voltage regulation option.

The available output current is limited to the pro-

grammed fast charge current. For stability, the VBAT

output must be bypassed to VSS with a minimum

capacitance of 1 µF for output currents up to 250 mA.

A minimum capacitance of 4.7 µF is required for output

currents above 250 mA.

The system test mode is entered by driving the THERM

input greater than (VDD - 100 mV) with no battery

connected to the output. In this mode, the MCP73837/

8 can be used to power the system without a battery

being present.

5.2

Digital Circuitry

5.2.1

STATUS INDICATORS AND POWER

GOOD (PG) OPTION

The charge status outputs have two different states:

Low (L), and High Impedance (Hi-Z). The charge status

outputs can be used to illuminate LEDs. Optionally, the

charge status outputs can be used as an interface to a

host microcontroller. Table 5-1 summarizes the state of

the status outputs during a charge cycle.

REG

1000V

PROG

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

Where:

RPROG

kilo-ohms (k

Ω)

IREG

milli-ampere (mA

)

Note 1: ITHERM is disabled during shutdown,

stand-by, and system test modes.

2: A pull-down current source on the

THERM input is active only in stand-by

and system test modes.

3: During system test mode, the PROG

input sets the available output current

limit.

4: System test mode shall be exited by

releasing the THERM input or cycling

input power.

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