MCP73837-AMISLASHMF datasheet(21/36 Pages) MICROCHIP

21 / 36 page

 2007-2015 Microchip Technology Inc.

DS20002071C-page 21

MCP73837/8

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 battery charger

and the ambient cooling air.

The worst-case situation is when the device has transi-

tioned from the Preconditioning mode to the Constant

Current mode. In this situation, the battery charger 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

system. The following discussion is intended as a guide

for the component selection process.

6.1.1.1

Charge Current

The preferred fast charge current for Lithium-Ion cells

should always follow references and guidance from

battery manufacturers. For example, programming

700 mA fast charge current for a 1000 mAh Li-Ion

battery pack if its preferred fast charge rate is 0.7C.

This will result in the shortest charge cycle time without

degradation of a battery's life and performance.

6.1.1.2

Input Over-Voltage Protection

Input over-voltage protection must be used when the

input power source is hot-pluggable. This includes USB

cables and wall-type power supplies. The cabling of

these supplies acts as an inductor. When the supplies

are connected/ disconnected from the system, large

voltage transients are created which may damage the

system circuitry. These transients should be snubbed

out. A TransZorb® diode (unidirectional or bidirec-

tional), connected from the VAC and VUSB inputs to 0V

ground reference, will snub the transients. An example

of this can be shown in Figure 6-4.

FIGURE 6-4:

Input Over-Voltage Protection Example.

STAT1

VAC

VSS

/PG

VBAT

Single

Li-Ion

Cell

MCP73837

STAT2

THERM

VUSB

PROG1

PROG2

USB Port

Low

Thermistor

RPROG

1 kΩ

REGULATED

5V WALL CUBE

CIN1

CIN2

COUT

SMAJ5.0A/AC

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