LM7818CK 查看數據表（PDF） - International Cmos Technology

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LM7818CK

3-Terminal Positive Regulators

International Cmos Technology 

Application Hints (Continued)

Regulator Floating Ground (Figure 2): When the ground

pin alone becomes disconnected, the output approaches the

unregulated input, causing possible damage to other circuits

connected to VOUT. If ground is reconnected with power

“ON”, damage may also occur to the regulator. This fault is

most likely to occur when plugging in regulators or modules

with on card regulators into powered up sockets. Power

should be turned off first, thermal limit ceases operating, or

ground should be connected first if power must be left on.

Transient Voltages: If transients exceed the maximum

rated input voltage of the device, or reach more than 0.8V

below ground and have sufficient energy, they will damage

the regulator. The solution is to use a large input capacitor, a

series input breakdown diode, a choke, a transient suppres-

sor or a combination of these.

FIGURE 1. Input Short

00778108

00778110

FIGURE 3. Transients

When a value for θ(H–A) is found using the equation shown,

a heatsink must be selected that has a value that is less than

or equal to this number.

θ(H–A) is specified numerically by the heatsink manufacturer

in this catalog, or shown in a curve that plots temperature

rise vs power dissipation for the heatsink.

HEATSINKING TO-263 AND SOT-223 PACKAGE PARTS

Both the TO-263 (“S”) and SOT-223 (“MP”) packages use a

copper plane on the PCB and the PCB itself as a heatsink.

To optimize the heat sinking ability of the plane and PCB,

solder the tab of the plane.

shows for the TO-263 the measured values of θ(J–A) for

different copper area sizes using a typical PCB with 1 ounce

copper and no solder mask over the copper area used for

heatsinking.

00778109

FIGURE 2. Regulator Floating Ground

00778139

FIGURE 4. θ(J–A) vs Copper (1 ounce)

Area for the TO-263 Package

As shown in the figure, increasing the copper area beyond 1

square inch produces very little improvement. It should also

be observed that the minimum value of θ(J–A) for the TO-263

package mounted to a PCB is 32˚C/W.

As a design aid, Figure 5 shows the maximum allowable

power dissipation compared to ambient temperature for the

TO-263 device (assuming θ(J–A) is 35˚C/W and the maxi-

mum junction temperature is 125˚C).

www.national.com

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