LM317AMDT National Semiconductor, LM317AMDT Datasheet - Page 9

• Manufacturer Part Number: LM317AMDT
• Description: IC, ADJ LINEAR REG, 1.2V TO 37V, TO252-3
• Manufacturer: National Semiconductor

Specifications of LM317AMDT

• Primary Input Voltage: 40V
• Dropout Voltage Vdo: 2V
• No. Of Outputs: 1
• No. Of Pins: 3
• Output Current: 500mA
• Operating Temperature Range: -40°C To +125°C
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Hints
when either the input, or the output, is shorted. Internal to the
LM117 is a 50Ω resistor which limits the peak discharge
current. No protection is needed for output voltages of 25V
or less and 10 µF capacitance. Figure 3 shows an LM117
with protection diodes included for use with outputs greater
than 25V and high values of output capacitance.
D1 protects against C1
D2 protects against C2
HEATSINK REQUIREMENTS
The LM317 regulators have internal thermal shutdown to
protect the device from over-heating. Under all operating
conditions, the junction temperature of the LM317 should not
exceed the rated maximum junction temperature (T
150˚C for the LM117, or 125˚C for the LM317A and LM317.
A heatsink may be required depending on the maximum
device power dissipation and the maximum ambient tem-
perature of the application. To determine if a heatsink is
needed, the power dissipated by the regulator, P
calculated:
Figure 4 shows the voltage and currents which are present in
the circuit.
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
where T
ture (150˚C for the LM117, or 125˚C for the LM317A/LM317),
and T
be encountered in the application.
Using the calculated values for T
mum allowable value for the junction-to-ambient thermal
resistance (θ
A(MAX)
FIGURE 3. Regulator with Protection Diodes
J(MAX)
P
is the maximum ambient temperature which will
JA
D
is the maximum allowable junction tempera-
) can be calculated:
= ((V
T
R(MAX)
θ
IN
JA
− V
= (T
= T
OUT
J(MAX)
R(MAX)
) x I
(Continued)
R(MAX)
L
− T
) + (V
R(MAX)
/ P
A(MAX)
D
)
IN
:
and P
x I
G
00906307
)
D
, the maxi-
D
, must be
J
) of
(2)
(3)
(4)
(5)
9
If the calculated maximum allowable thermal resistance is
higher than the actual package rating, then no additional
work is needed. If the calculated maximum allowable ther-
mal resistance is lower than the actual package rating either
the power dissipation (P
mum ambient temperature T
thermal resistance (θ
sink, or some combination of these.
If a heatsink is needed, the value can be calculated from the
formula:
where (θ
between the device case and the heatsink surface, and θ
is thermal resistance from the junction of the die to surface of
the package case.
When a value for θ
a heatsink must be selected that has a value that is less
than, or equal to, this number.
The θ
manufacturer in the catalog, or shown in a curve that plots
temperature rise vs power dissipation for the heatsink.
HEATSINKING SURFACE MOUNT PACKAGES
The TO-263 (S), SOT-223 (EMP) and TO-252 (MDT) pack-
ages 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 package to the plane.
HEATSINKING THE SOT-223 PACKAGE
Figure 5 and Figure 6 show the information for the SOT-223
package. Figure 6 assumes a θ
copper and 51˚C/W for 2 ounce copper and a maximum
junction temperature of 125˚C. Please see AN-1028 for ther-
mal enhancement techniques to be used with SOT-223 and
TO-252 packages.
(H−A)
CH
FIGURE 4. Power Dissipation Diagram
rating is specified numerically by the heatsink
is the thermal resistance of the contact area
θ
HA
(H−A)
JA
≤ (θ
) must be lowered by adding a heat-
is found using the equation shown,
D
JA
) needs to be reduced, the maxi-
A(MAX)
- (θ
CH
(J−A)
needs to be reduced, the
+ θ
of 74˚C/W for 1 ounce
JC
))
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