MCP73841 datasheet(16/24 Pages) MICROCHIP

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MCP73841/2/3/4

DS21823B-page 16

 2004 Microchip Technology Inc.

6.1

Application Circuit Design

Due to the low efficiency of linear charging, the most

important factors are thermal design and cost, which are

a direct function of the input voltage, output current and

thermal impedance between the external P-channel

pass transistor and the ambient cooling air. The worst-

case situation occurs when the device has transitioned

from the preconditioning phase to the constant-current

phase. In this situation, the P-channel pass transistor

has to dissipate the maximum power. A trade-off must

be made between the charge current, cost and thermal

requirements of the charger.

6.1.1

COMPONENT SELECTION

Selection of the external components in Figure 6-1 are

crucial to the integrity and reliability of the charging

system. The following discussion is intended to be a

guide for the component selection process.

6.1.1.1

Sense Resistor

The preferred fast charge current for Lithium-Ion cells

is at the 1C rate, with an absolute maximum current at

the 2C rate. For example, a 500 mAh battery pack has

a preferred fast charge current of 500 mA. Charging at

this rate provides the shortest charge cycle times

without degradation to the battery pack performance or

life.

The current sense resistor (RSENSE) is calculated by:

For the 500 mAh battery pack example, a standard

value 220 m

Ω, 1% resistor provides a typical fast

charge current of 500 mA and a maximum fast charge

current of 551 mA. Worst-case power dissipation in the

sense resistor is:

A Panasonic® ERJ-6RQFR22V, 220 mW, 1%, 1/8W

resistor in a standard 0805 package is more than

sufficient for this application.

A larger value sense resistor will decrease the fast

charge current and power dissipation in both the sense

resistor and external pass transistor, but will increase

charge cycle times. Design trade-offs must be

considered to minimize space while maintaining the

desired performance.

6.1.1.2

External Pass Transistor

The external P-channel MOSFET is determined by the

gate-to-source threshold voltage, input voltage, output

voltage and fast charge current. Therefore, the

selected P-channel MOSFET must satisfy the thermal

and electrical design requirements.

Thermal Considerations

The worst-case power dissipation in the external pass

transistor occurs when the input voltage is at the

maximum and the device has transitioned from the

preconditioning phase to the constant-current phase.

In this case, the power dissipation is:

Power dissipation with a 5V, ±10% input voltage

source, 220 m

Ω, 1% sense resistor is:

Utilizing a Fairchild™ NDS8434 or an International

Rectifier IRF7404 mounted on a 1in2 pad of 2 oz.

copper, the junction temperature rise is 75°C,

approximately. This would allow for a maximum

operating ambient temperature of 75°C.

By increasing the size of the copper pad, a higher

ambient temperature can be realized, or a lower value

sense resistor could be utilized.

Alternatively, different package options can be utilized

for more or less power dissipation. Again, design trade-

offs should be considered to minimize size while

maintaining the desired performance.

Electrical Considerations

The gate-to-source threshold voltage and RDSON of the

external P-channel MOSFET must be considered in the

design phase.

The worst-case VGS provided by the controller occurs

when the input voltage is at the minimum and the fast

charge current regulation threshold is at the maximum.

The worst-case VGS is:

RSENSE

VFCS

IREG

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

Where:

IREG is the desired fast charge current.

PowerDissipation

220m

Ω 551mA

66.8mW

PowerDissipation

VDDMAX VPTHMIN

–

() IREGMAX

Where:

VDDMAX is the maximum input voltage.

IREGMAX is the maximum fast charge current.

VPTHMIN is the minimum transition threshold voltage.

PowerDissipation

5.5V 2.75V

–

() 551mA

1.52W

VGS

VDRVMAX

VDDMIN VFCSMAX )

–

(

–

Where:

VDRVMAX is the maximum sink voltage at the

VDRV output

VDDMIN is the minimum input voltage source

VFCSMAX is the maximum fast charge current

regulation threshold

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