Datasheet
SLUS215B − OCTOBER 1998 − REVISED FEBRUARY 2004
9
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
thermal design
The Package Information section of the Power Supply Control Products Data Book, Volume 3 (Literature
No. SLUD003) contains reference material for the thermal ratings of various packages. The section also
includes an excellent article Thermal Characteristics of Surface Mount Packages, that is the basis of the
following discussion.
Thermal design for the UCC383 family of linear regulators includes two modes of operation, normal and pulsed
mode. In normal operation, the linear regulator and heat sink must dissipate power equal to the maximum
forward voltage drop multiplied by the maximum load current. Assuming a constant current load, the expected
heat rise at the regulator’s junction can be calculated as follows:
t
RISE
(q) + P
DISS
ǒ
q
jc
) q
ca
Ǔ
°C
Where theta, (θ) is thermal resistance and P
DISS
is the power dissipated. The thermal resistance of both the
TO−220 and TO−263 packages (junction to case) is 3°C per Watt. In order to prevent the regulator from going
into thermal shutdown, the case to ambient theta must keep the junction temperature below 150°C. If the LDO
is mounted on a 5-square inch pad of 1-ounce copper, for example, the thermal resistance from junction to
ambient becomes 60°C per Watt. If a lower thermal resistance is required by the application, the device heat
sinking would need to be improved.
When the UCC383 regulator is in pulsed mode due to an overload or short-circuit in the application, the
maximum average power dissipation is calculated as follows:
P
PULSE(avg)
+
ǒ
V
IN
* V
OUT
Ǔ
I
CL
t
ON
33 t
ON
Watts
As seen in Equation 6, the average power during a fault is reduced dramatically by the duty cycle, allowing the
heat sink to be sized for normal operation. Although the peak power in the regulator during the t
ON
period can
be significant, the thermal mass of the package generally keeps the junction temperature from rising unless the
t
ON
period is increased to tens of milliseconds.
ripple rejection
Even though the UCC383 family of linear regulators are not optimized for fast transient applications (Refer to
the UC182 Fast LDO Linear Regulator), they do offer significant power supply rejection at lower frequencies.
Figure 6 depicts ripple rejection performance in a typical application. The performance can be improved with
additional filtering.
(5)
(6)