MIC79050 datasheet(15/28 Pages) MICROCHIP

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DS20005771B-page 15

MIC79050

constant-current mode. When the current through the

resistor drops and the battery gets closer to full charge,

the output of the op-amp rises and allows the internal

feedback network of the regulator take over, regulating

the output to 4.2V.

FIGURE 5-10:

Constant-Current,

Constant-Voltage Charger.

5.9

Simple Charging

The MIC79050 is available in a three-terminal package,

allowing for extremely simple battery charging. When

used with a current-limited, low-power input supply, the

MIC79050-4.2YS

completes

very

simple,

low-charge-rate, battery-charger circuit. It provides the

accuracy

required

for

termination,

while

current-limited input supply offers the constant-current

portion of the algorithm.

5.10 Thermal Considerations

The MIC79050 is offered in three packages for the

various applications. The SOT-223 is most thermally

efficient of the three packages, with the power SOIC-8

and the power MSOP-8 following suit.

5.10.1

POWER SOIC-8 THERMAL

CHARACTERISTICS

One of the secrets of the MIC79050’s performance is

its power SOIC-8 package that features half the

thermal resistance of a standard SOIC-8 package.

Lower thermal resistance means more output current

or higher input voltage for a given package size.

Lower thermal resistance is achieved by joining the

four ground leads with the die attach paddle to create a

single-piece electrical and thermal conductor. This

concept has been used by MOSFET manufacturers for

years, proving very reliable and cost effective for the

user.

Thermal resistance consists of two main elements, θJC,

or thermal resistance junction to case and θCA, thermal

resistance case to ambient (Figure 5-11). θJC is the

resistance from the die to the leads of the package. θCA

is the resistance from the leads to the ambient air and

it includes θCS, thermal resistance case to sink, and

θSA, thermal resistance sink to ambient. Using the

power SOIC-8 reduces the θJC dramatically and allows

the user to reduce θCA. The total thermal resistance,

θJA, junction to ambient thermal resistance, is the

limiting factor in calculating the maximum power

dissipation capability of the device. Typically, the power

SOIC-8 has a θJC of 20°C/W, this is significantly lower

than the standard SOIC-8, which is typically 75°C/W.

θCA is reduced because pins 5-8 can now be soldered

directly to a ground plane, which significantly reduces

the case to sink thermal resistance and sink to ambient

thermal resistance.

FIGURE 5-11:

Thermal Resistance.

The MIC79050 is rated to a maximum junction

temperature of +125°C. It is important not to exceed

this maximum junction temperature during operation of

the device. To prevent this maximum junction

temperature from being exceeded, the appropriate

ground plane heat sink must be used.

Figure 5-12 shows curves of copper area versus power

dissipation, each trace corresponding to different

temperature rises above ambient. From these curves,

the minimum area of copper necessary for the part to

operate safely can be determined. The maximum

allowable temperature rise must be calculated to

determine operation along which curve.

FIGURE 5-12:

Copper Area vs. Power

SOIC Power Dissipation (ΔTJA).

BAT

GND

MIC79050-4.2YMM

SD101

4.7μF

0.01μF

MIC7300

LM4041CIM3-1.2

16.2k

221k

10k

Printed Circuit Board

Ground Plane

Heat Sink Area

SOIC-8

AMBIENT

100

200

300

400

500

600

700

800

900

0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

JA =

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