06-11-2012, 05:51 PM
LM341/LM78MXX Series 3-Terminal Positive Voltage Regulators
3-Terminal Positive.pdf (Size: 735.38 KB / Downloads: 15)
General Description
The LM341 and LM78MXX series of three-terminal positive
voltage regulators employ built-in current limiting, thermal
shutdown, and safe-operating area protection which makes
them virtually immune to damage from output overloads.
With adequate heatsinking, they can deliver in excess of
0.5A output current. Typical applications would include local
(on-card) regulators which can eliminate the noise and degraded
performance associated with single-point regulation.
Features
Output current in excess of 0.5A
No external components
Internal thermal overload protection
Internal short circuit current-limiting
Output transistor safe-area compensation
Available in TO-220, TO-39, and TO-252 D-PAK
packages
Output voltages of 5V, 12V, and 15V
Design Considerations
The LM78MXX/LM341XX fixed voltage regulator series has
built-in thermal overload protection which prevents the device
from being damaged due to excessive junction temperature.
The regulators also contain internal short-circuit protection
which limits the maximum output current, and safe-area
protection for the pass transistor which reduces the shortcircuit
current as the voltage across the pass transistor is
increased.
Although the internal power dissipation is automatically limited,
the maximum junction temperature of the device must
be kept below +125°C in order to meet data sheet specifications.
An adequate heatsink should be provided to assure
this limit is not exceeded under worst-case operating conditions
(maximum input voltage and load current) if reliable
performance is to be obtained).
HEATSINK CONSIDERATIONS
When an integrated circuit operates with appreciable current,
its junction temperature is elevated. It is important to
quantify its thermal limits in order to achieve acceptable
performance and reliability. This limit is determined by summing
the individual parts consisting of a series of temperature
rises from the semiconductor junction to the operating
environment. A one-dimension steady-state model of conduction
heat transfer is demonstrated in The heat generated
at the device junction flows through the die to the die attach
pad, through the lead frame to the surrounding case material,
to the printed circuit board, and eventually to the ambient
environment. Below is a list of variables that may affect
the thermal resistance and in turn the need for a heatsink.