Datasheet

LM2941, LM2941C
www.ti.com SNVS770G –JUNE 1999–REVISED APRIL 2013
ON/OFF
The ON/OFF pin has no internal pull-up or pull-down to establish a default condition and, as a result, this pin
must be terminated externally, either actively or passively.
The ON/OFF pin requires a low level to enable the output, and a high level to disable the output. To ensure
reliable operation, the ON/OFF pin voltage must rise above the maximum ON/OFF
(OFF)
voltage threshold (2.00V)
to disable the output, and must fall below the minimum ON/OFF
(ON)
voltage threshold (0.80V) to enable the
output. If the ON/OFF function is not needed this pin can be connected directly to Ground.
If the ON/OFF pin is being pulled to a high state through a series resistor, an allowance must be made for the
ON/OFF pin current that will cause a voltage drop across the pull-up resistor.
Thermal Overload Protection
The LM2941 incorporates a linear form of thermal protection that limits the junction temperature (T
J
) to typically
155°C.
Should the LM2941 see a fault condition that results in excessive power dissipation and the junction temperature
approaches 155°C, the device will respond by reducing the output current (which reduces the power dissipation)
to hold the junction temperature at 155°C.
Thermal Overload protection is not an ensured operating condition. Operating at, or near to, the Thermal
Overload condition for any extended period of time is not encouraged, or recommended, as this may shorten the
lifetime of the device.
Power Dissipation
Consideration should be given to the maximum power dissipation (P
D(MAX)
) which is limited by the maximum
operating junction temperature (T
J(MAX)
) of 125°C, the maximum operating ambient temperature (T
A(MAX
)) of the
application, and the thermal resistance (θ
JA
) of the package. Under all possible conditions, the junction
temperature (T
J
) must be within the range specified in the Operating Ratings. The total power dissipation of the
device is given by:
P
D
= ( (V
IN
− V
OUT
) x I
OUT
) + (V
IN
x I
GND
) (3)
where I
GND
is the operating ground pin current of the device (specified under Electrical Characteristics).
The maximum allowable junction temperature rise (ΔT
J
) depends on the maximum expected ambient
temperature (T
A(MAX)
) of the application, and the maximum allowable junction temperature (T
J(MAX)
):
ΔT
J
= T
J(MAX)
− T
A(MAX)
(4)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, required to keep the junction
temperature, T
J
, from exceeding maximum allowed can be calculated using the formula:
θ
JA
= ΔT
J
/ P
D(MAX)
(5)
The maximum allowable power dissipation, P
D(MAX)
, required allowed for a specific ambient temperature can be
calculated using the formula:
P
D(MAX)
= ΔT
J
/ θ
JA
(6)
Additional information for thermal performance of surface mount packages can be found in AN-1520: A Guide to
Board Layout for Best Thermal Resistance for Exposed Packages (literature number SNVA183), AN-1187:
Leadless Leadframe Package (LLP) (literature number SNOA401), and AN-2020: Thermal Design By Insight, Not
Hindsight (literature number SNVA419).
TO-263 Mounting
The thermal dissipation of the TO-263 package is directly related to the printed circuit board construction and the
amount of additional copper area connected to the TAB.
The TAB on the bottom of the TO-263 package is connected to the die substrate via a conductive die attach
adhesive, and to device pin 3. As such, it is strongly recommend that the TAB area be connected to copper area
directly under the TAB that is extended into the ground plane via multiple thermal vias. Alternately, but not
recommended, the TAB may be left floating (i.e. no direct electrical connection). The TAB must not be connected
to any potential other than ground.
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