MCP73828 datasheet(10/24 Pages) MICROCHIP

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MCP73828

DS21706A-page 10

 2002 Microchip Technology Inc.

5.0

DETAILED DESCRIPTION

Refer to the typical application circuit, Figure 6-1.

5.1

Analog Circuitry

5.1.1

CELL TEMPERATURE MONITOR (THERM)

The cell temperature monitor, THERM, input is used to

inhibit charging when the battery temperature exceeds

a predetermined temperature range. This temperature

range is programmed externally with either a single

Thermistor or a resistor/Thermistor network. An exam-

ple of this type of network is illustrated in Figure 6-1.

The MCP73828 internally generates a current source

out of the THERM pin (shown in the Functional Block

Diagram). The nominal value of the current source

(ITHERM) is 25 µA. This current flows through the ther-

mistor network to ground. The factory programmed

voltage range of the THERM input (VTH) is 113 mV

(typ) to 839 mV (typ). Dependent on the type of Ther-

mistor used and the resistive network, the temperature

trip points can be controlled. If the THERM pin is lower

that 113 mV or higher than 839 mV the device will shut-

down operation. This condition can be corrected by

bringing the THERM pin back between these threshold

voltages.

As an application example, if a 10 k

Ω NTC Thermistor

with a sensitivity index (b) of 3982 is connected from

THERM to ground, the operational temperature range

is from –0.5°C to 44.2°C. See Section 6.1.1.6 for more

details concerning using the resistive network.

Alternatively, a positive temperature coefficient, PTC,

thermistor can be utilized. Connect the thermistor from

the THERM input to GND. If temperature monitoring is

not required, replace the thermistor with a standard

10 k

Ω resistor.

5.1.2

CELL VOLTAGE MONITOR INPUT (VBAT)

The MCP73828 monitors the cell voltage at the VBAT

pin. This input is tied directly to the positive terminal of

the battery. The MCP73828 is offered in two fixed-volt-

age versions for single cells with either coke or graphite

anodes:

4.1V

(MCP73828-4.1)

and

4.2V

(MCP73828-4.2).

5.1.3

GATE DRIVE OUTPUT (VDRV)

The MCP73828 controls the gate drive to an external

P-channel MOSFET, Q1. The P-channel MOSFET is

controlled in the linear region, regulating current and

voltage supplied to the cell. The drive output is auto-

matically turned off when the input supply falls below

the voltage sensed on the VBAT input.

5.1.4

CURRENT SENSE INPUT (VSNS)

Fast charge current regulation is maintained by the

voltage drop developed across an external sense resis-

tor, RSENSE, applied to the VSNS input pin. The follow-

ing formula calculates the value for RSENSE:

Where:

VCS is the current limit threshold.

IOUT is the desired peak fast charge current in

amps. The preconditioning current is scaled to

approximately 43% of IOUT.

5.1.5

SUPPLY VOLTAGE (VIN)

The VIN input is the input supply to the MCP73828. The

MCP73828 automatically enters a power-down mode if

the voltage on the VIN input falls below the voltage on

the VBAT pin. This feature prevents draining the battery

pack when the VIN supply is not present.

5.2

Digital Circuitry

5.2.1

SHUTDOWN INPUT (SHDN)

The shutdown input pin, SHDN, can be used to termi-

nate a charge anytime during the charge cycle, initiate

a charge cycle, or initiate a recharge cycle.

Applying a logic High input signal to the SHDN pin, or

tying it to the input source, enables the device. Apply-

ing a logic Low input signal disables the device and ter-

minates a charge cycle. In shutdown mode, the

device’s supply current is reduced to 0.7 µA, typically.

5.2.2

CHARGE COMPLETE OUTPUT (CD10)

A charge complete indicator, CD10, provides informa-

tion on the state of charge. The open-drain output can

be used to illuminate an external LED. Optionally, a

pull-up resistor can be used on the output for commu-

nication with a microcontroller. Table 5-1 summarizes

the state of this output during a charge cycle.

TABLE 5-1:

Charge Complete Output.

Charge Cycle State

Mode

Qualification

OFF

Preconditioning

Controlled Current Fast Charge

Constant Voltage

Charge Complete

OFF

Temperature Monitor Invalid

OFF

Disabled - Sleep mode

OFF

Battery Disconnected

OFF

RSENSE

VCS

IOUT

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