Datasheet

LM2936
www.ti.com SNOSC48N –JUNE 2000–REVISED MARCH 2013
APPLICATIONS INFORMATION
Unlike other PNP low dropout regulators, the LM2936 remains fully operational to 40V. Owing to power
dissipation characteristics of the available packages, full output current cannot be ensured for all combinations of
ambient temperature and input voltage. As an example, consider an LM2936Z–5.0 operating at 25°C ambient.
Using the formula for maximum allowable power dissipation given in
(1)
, we find that P
Dmax
= 641 mW at 25°C.
Including the small contribution of the quiescent current to total power dissipation the maximum input voltage
(while still delivering 50 mA output current) is 17.3V. The LM2936Z–5.0 will go into thermal shutdown if it
attempts to deliver full output current with an input voltage of more than 17.3V. Similarly, at 40V input and 25°C
ambient the LM2936Z–5.0 can deliver 18 mA maximum.
Under conditions of higher ambient temperatures, the voltage and current calculated in the previous examples
will drop. For instance, at the maximum ambient of 125°C the LM2936Z–5.0 can only dissipate 128 mW, limiting
the input voltage to 7.34V for a 50 mA load, or 3.5 mA output current for a 40V input.
The junction to ambient thermal resistance θ
JA
rating has two distinct components: the junction to case thermal
resistance rating θ
JC
; and the case to ambient thermal resistance rating θ
CA
. The relationship is defined as: θ
JA
=
θ
JC
+ θ
CA
.
For the SOIC-8 and PFM surface mount packages the θ
JA
rating can be improved by using the copper mounting
pads on the printed circuit board as a thermal conductive path to extract heat from the package.
On the SOIC-8 package the four ground pins are thermally connected to the backside of the die. Adding
approximately 0.04 square inches of 2 oz. copper pad area to these four pins will improve the θ
JA
rating to
approximately 110°C/W. If this extra pad are is placed directly beneath the package there should not be any
impact on board density.
On the PFM package the ground tab is thermally connected to the backside of the die. Adding 1 square inch of 2
oz. copper pad area directly under the ground tab will improve the θ
JA
rating to approximately 50°C/W.
While the LM2936 has an internally set thermal shutdown point of typically 160°C, this is intended as a safety
feature only. Continuous operation near the thermal shutdown temperature should be avoided as it may have a
negative affect on the life of the device.
While the LM2936 maintains regulation to 60V, it will not withstand a short circuit above 40V because of safe
operating area limitations in the internal PNP pass device. Above 60V the LM2936 will break down with
catastrophic effects on the regulator and possibly the load as well. Do not use this device in a design where the
input operating voltage may exceed 40V, or where transients are likely to exceed 60V.
SHUTDOWN PIN
The LM2936BM has a pin for shutting down the regulator output. Applying a Logic Level High (>2.0V) to the
Shutdown pin will cause the output to turn off. Leaving the Shutdown pin open, connecting it to Ground, or
applying a Logic Level Low (<0.6V) will allow the regulator output to turn on.
(1) The maximum power dissipation is a function of T
Jmax
, θ
JA
, and T
A
. The maximum allowable power dissipation at any ambient
temperature is P
D
= (T
Jmax
− T
A
)/θ
JA
. If this dissipation is exceeded, the die temperature will rise above 150°C and the LM2936 will go
into thermal shutdown.
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