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Thermal-Overload Protection

Thermal-overload protection limits total power dissipation

in the MAX1658/MAX1659. When the junction temperature

exceeds TJ = +165°C, the pass transistor deactivates,

allowing the IC to cool. Once it has cooled by 10°C, the

control logic enables operation. Under thermal overload,

the output of the device pulses as the die heats up and

then cools to operational levels. Prolonged operation

under these conditions is not recommended.

Operating Region and Power Dissipation

Maximum power dissipation of the MAX1658/MAX1659

depends on the thermal resistance of the package and

circuit board, the temperature difference between the

die and ambient air, and the rate of air flow. The power

dissipation by the device is P = IOUT (VIN - VOUT). The

maximum power dissipation is:

(

)

(

)

MAX





−

= 







+θ





where (TJ - TA) is the temperature difference between

MAX1658/MAX1659 die junction and the surrounding air,

θJB is the thermal resistance of the package, and θBA is

the thermal resistance through the printed circuit board,

copper traces, and other materials to the surrounding

air. The 8-pin SO package for the MAX1658/MAX1659

features a special lead frame with a lower thermal

resistance and higher allowable power dissipation than

a standard 8-pin SO package. The thermal resistance

of this package is θJB = 69°C/W, compared with θJB =

170°C/W for a standard 8-pin SO package.

The IN pins of the MAX1658/MAX1659 package perform

the dual function of providing an electrical connection to

IN and channeling heat away. Connect all IN pins to the

input voltage using a large pad or power plane on the

surface. Where this is impossible, connect to a copper

plane on an adjacent layer. The pad should meet the

dimensions specified in Figure 4.

Figure 4 assumes the IC is soldered directly to the pad,

has a +125°C maximum junction temperature and a +25°C

ambient air temperature, and has no other heat sources.

Use larger pad sizes for lower junction temperatures,

higher ambient temperatures, or conditions where the IC

is not soldered directly to a heat-sinking IN pad.

The MAX1658/MAX1659 can regulate currents up to

350mA and operate with input voltages up to 16.5V,

but not simultaneously. High output currents can only

be sustained when input-output differential voltage is

low, as shown in the following equation. Maximum

power dissipation depends on packaging, board layout,

temperature, and air flow. The maximum output current is:

(

)

(

)

MAX

OUT(MAX)

OUT

125 C T

100 C

° −

−

×°

where PMAX is derived from the TJ = 125°C curve of

Figure 4.

Reverse-Battery Protection

The MAX1658/MAX1659 feature reverse-battery

protection. Under normal operation, a p-channel MOSFET

connects the substrate of the device to IN. When the input

voltage falls below ground (implying reverse-battery

conditions), the p-channel switch turns off and

disconnects the substrate from IN, disabling the device.

The maximum reverse-battery voltage allowed is -17V.

SHDN also withstands reverse-battery conditions and

can be connected directly to IN with no loss of protection.

Polarized input bypass capacitors become damaged

under reverse-battery conditions. To ensure circuit

reliability, use a nonpolarized capacitor at the input.

The MAX1658/MAX1659 do not provide reverse-current

protection. If VOUT is greater than VIN by more than

300mV, reverse current flows. Reverse-current protection

can be added by connecting a Schottky diode in series

with IN.

Figure 4. Typical Maximum Power Dissipation vs. Ground Pad

Area

1600

1400

1200

1000

800

600

400

0.1

0.65

6.5

COPPER GROUND PAD AREA

10 (in2)

65 (cm2)

Tj = +125°C

Tj = +85°C

SINGLE-SIDED 1oz. COPPER

TA = +25°C, STILL AIR

MAX1658/MAX1659

350mA, 16.5V Input,

Low-Dropout Linear Regulators

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