LT1965
APPLICATIONS INFORMATION
Other regulators, such as the LT1083/LT1084/LT1085
family, also exhibit this phenomenon, so it is not unique
to the LT1965.
Thermal Considerations
The LT1965’s maximum rated junction temperature of
125°C limits its power handling capability. Two compon-
ents comprise the power dissipated by the device:
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon situations include immediately after the removal of a
short-circuit or if the shutdown pin is pulled high after the
input voltage has already been turned on. The load line for
such a load may intersect the output current curve at two
points.Ifthishappens,therearetwostableoutputoperating
points for the regulator. With this double intersection, the
input power supply may need to be cycled down to zero
and brought up again to make the output recover.
1. Output current multiplied by the input/output voltage
differential: I
• (V – V ), and
OUT
IN OUT
2. GND pin current multiplied by the input voltage:
• V
I
GND
IN
GND pin current is determined using the GND Pin Current
curvesintheTypicalPerformanceCharacteristicssection.
Power dissipation equals the sum of the two components
listed.
The LT1965 regulator has internal thermal limiting that
protects the device during overload conditions. For con-
tinuous normal conditions, do not exceed the maximum
junction temperature rating of 125°C. Carefully consider
all sources of thermal resistance from junction to ambi-
ent including other heat sources mounted in proximity to
the LT1965.
Output Voltage Noise
TheLT1965regulator’sdesignprovideslowoutputvoltage
noise over the 10Hz to 100kHz bandwidth while operating
atfullload.Outputvoltagenoiseisapproximately80nV/√Hz
over this frequency bandwidth for the LT1965. For higher
output voltages (generated by using a resistor divider),
the output voltage noise gains up accordingly.
The underside of the LT1965 DFN package has exposed
Higher values of output voltage noise may be measured
if care is not exercised with regard to circuit layout and
testing.Crosstalkfromnearbytracescaninduceunwanted
noiseontotheLT1965’soutput. Powersupplyripplerejec-
tion must also be considered; the LT1965 regulator does
not have unlimited power supply rejection and will pass a
small portion of the input noise through to the output.
2
metal (4mm ) from the lead frame to the die attachment.
The underside of the LT1965 MSOP package also has ex-
2
posed metal (2mm ). Both packages allow heat to directly
transfer from the die junction to the printed circuit board
metaltocontrolmaximumoperatingjunctiontemperature.
The dual-in-line pin arrangement allows metal to extend
beyondtheendsofthepackageonthetopside(component
side) of a PCB. Connect this metal to GND on the PCB.
The multiple IN and OUT pins of the LT1965 also assist
in spreading heat to the PCB.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
1965f
10
Linear [ Linear ]
