MCP73213 datasheet(22/34 Pages) MICROCHIP

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MCP73213

DS22190A-page 22

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

transitioned

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 Li-Ion / Li-Poly

cells is below the 1C rate, with an absolute maximum

current at the 2C rate.

The recommended fast charge

current

should

obtained

from

battery

manufacturer. For example, a 500 mAh battery pack

with 0.7C preferred fast charge current has a charge

current of 350 mA. Charging at this rate provides the

shortest charge cycle times without degradation to the

battery pack performance or life.

6.1.1.2

Thermal Considerations

The worst-case power dissipation in the battery

charger occurs when the input voltage is at the

maximum and the device has transitioned from the

Preconditioning mode to the Constant-current mode. In

this case, the power dissipation is:

EQUATION 6-1:

Power dissipation with a 9V, ±10% input voltage

source, 500 mA ±10% and preconditioning threshold

voltage at 6V is:

EQUATION 6-2:

This power dissipation with the battery charger in the

DFN-10 package will result approximately 92

°C above

room temperature.

6.1.1.3

External Capacitors

The MCP73213 is stable with or without a battery load.

In order to maintain good AC stability in the Constant-

voltage mode, a minimum capacitance of 1 µF is

recommended to bypass the VBAT pin to VSS. This

capacitance provides compensation when there is no

battery

load.

addition,

the

battery

and

interconnections appear inductive at high frequencies.

These elements are in the control feedback loop during

Constant-voltage

mode.

Therefore,

the

bypass

capacitance may be necessary to compensate for the

inductive nature of the battery pack.

A minimum of 16V rated 1 µF, is recommended to apply

for output capacitor and a minimum of 25V rated 1 µF,

is recommended to apply for input capacitor for typical

applications.

TABLE 6-1:

MLCC CAPACITOR EXAMPLE

Virtually any good quality output filter capacitor can be

used,

independent

the

capacitor’s

minimum

Effective Series Resistance (ESR) value. The actual

value of the capacitor (and its associated ESR)

depends on the output load current. A 1 µF ceramic,

tantalum or aluminum electrolytic capacitor at the

output is usually sufficient to ensure stability.

6.1.1.4

Reverse-Blocking Protection

The MCP73213 provides protection from a faulted or

shorted input. Without the protection, a faulted or

shorted input would discharge the battery pack through

the body diode of the internal pass transistor.

Note:

Please consult with your battery supplier

or refer to battery data sheet for preferred

charge rate.

PowerDissipation

DDMAX

PTHMIN

–

() I

REGMAX

Where:

VDDMAX = the maximum input voltage

IREGMAX = the maximum fast charge current

VPTHMIN = the minimum transition threshold

voltage

MLCC

Capacitors

Temperature

Range

Tolerance

X7R

-55

°C to +125°C

±15%

X5R

-55

°C to +85°C

±15%

PowerDissipation

9.9V 6.0V

–

() 550mA

2.15W

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