MCP73826 datasheet(12/24 Pages) MICROCHIP

12 / 24 page

MCP73826

DS21705A-page 12

 2002 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-chan-

nel pass transistor, Q1, and the ambient cooling air.

The worst-case situation is when the output is shorted.

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 is

crucial to the integrity and reliability of the charging sys-

tem. The following discussion is intended as 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 with-

out degradation to the battery pack performance or life.

The current sense resistor, RSENSE, is calculated by:

Where:

VCS is the current limit threshold voltage

IOUT is the desired peak fast charge current

For the 500 mAH battery pack example, a standard

value 100 m

Ω, 1% resistor provides a typical peak fast

charge current of 530 mA and a maximum peak fast

charge current of 758 mA. Worst case power dissipa-

tion in the sense resistor is:

A Panasonic ERJ-L1WKF100U 100 m

Ω, 1%, 1 W

resistor is more than sufficient for this application.

A larger value sense resistor will decrease the peak

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 peak fast charge current. The selected P-

channel MOSFET must satisfy the thermal and electri-

cal design requirements.

Thermal Considerations

The worst case power dissipation in the external pass

transistor occurs when the input voltage is at the maxi-

mum and the output is shorted. In this case, the power

dissipation is:

Where:

VINMAX is the maximum input voltage

IOUT is the maximum peak fast charge current

K is the foldback current scale factor

Power dissipation with a 5V, +/-10% input voltage

source, 100 m

Ω, 1% sense resistor, and a scale factor

of 0.43 is:

Utilizing a Fairchild NDS8434 or an International Recti-

fier IRF7404 mounted on a 1in2 pad of 2 oz. copper, the

junction temperature rise is 90°C, approximately. This

would allow for a maximum operating ambient temper-

ature of 60°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 main-

taining 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

charge current is at the maximum. The worst case, VGS

is:

Where:

VDRVMAX is the maximum sink voltage at the VDRV

output

RSENSE

VCS

IOUT

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

PowerDissipation

100m

Ω 758mA

57.5mW

PowerDissipation

VINMAX IOUT K

PowerDissipation

5.5V 758mA

0.43

1.8W

VGS

VDRVMAX

VINMIN IOUT RSENSE

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