LM340-12MDC National Semiconductor Corporation, LM340-12MDC Datasheet - Page 10

• Manufacturer Part Number: LM340-12MDC
• Manufacturer: National Semiconductor Corporation

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Application Hints
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 V
“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 2. Regulator Floating Ground
OUT
FIGURE 1. Input Short
FIGURE 3. Transients
. If ground is reconnected with power
(Continued)
00778109
00778110
00778108
10
When a value for θ
a heatsink must be selected that has a value that is less than
or equal to this number.
θ
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
different copper area sizes using a typical PCB with 1 ounce
copper and no solder mask over the copper area used for
heatsinking.
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
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
mum junction temperature is 125˚C).
(H–A)
is specified numerically by the heatsink manufacturer
FIGURE 5. Maximum Power Dissipation vs
FIGURE 4. θ
T
Area for the TO-263 Package
AMB
(H–A)
for the TO-263 Package
(J–A)
is found using the equation shown,
vs Copper (1 ounce)
(J–A)
is 35˚C/W and the maxi-
(J–A)
00778139
for the TO-263
00778140
(J–A)
for