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LT1121 Datasheet(PDF) 11 Page - Linear Technology

Part # LT1121
Description  Low Dropout Negative Micropower Regulator in ThinSOT
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1121 Datasheet(HTML) 11 Page - Linear Technology

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LT1964
11
1964f
Calculating Junction Temperature
Example: Given an output voltage of –5V, an input voltage
range of –6V to –8V, an output current range of 0mA to
–100mA, and a maximum ambient temperature of 50
°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX))
where,
IOUT(MAX) = –100mA
VIN(MAX) = –8V
IGND at (IOUT = –100mA, VIN = –8V) = –2mA
so,
P = –100mA • (–8V + 5V) + (–2mA • –8V) = 0.32W
The thermal resistance (junction to ambient) will be in the
range of 125
°C/W to 150°C/W depending on the copper
area. So the junction temperature rise above ambient will
be approximately equal to:
0.32W • 140
°C/W = 44.2°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
TJMAX = 50°C + 44.2°C = 94.2°C
Protection Features
The LT1964 incorporates several protection features which
make it ideal for use in battery-powered circuits. In addi-
tion to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device is protected against reverse input
voltages and reverse output voltages.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125
°C.
The output of the LT1964 can be pulled above ground
without damaging the device. If the input is left open circuit
or grounded, the output can be pulled above ground by
20V. For fixed voltage versions, the output will act like a
large resistor, typically 500k
Ω or higher, limiting current
flow to less than 40
µA. For adjustable versions, the output
will act like an open circuit, no current will flow into the pin.
If the input is powered by a voltage source, the output will
sink the short-circuit current of the device and will protect
itself by thermal limiting. In this case, grounding the SHDN
pin will turn off the device and stop the output from sinking
the short-circuit current.
Like many IC power regulators, the LT1964 series have
safe operating area protection. The safe area protection
activates at input-to-output differential voltages greater
than –7V. The safe area protection decreases the current
limit as the input-to-output differential voltage increases
and keeps the power transistor inside a safe operating
region for all values of forward input to-output voltage.
The protection is designed to provide some output current
at all values of input-to-output voltage up to the device
breakdown. A 50
µA load is required at input-to-output
differential voltages greater than –7V.
When power is first turned on, as the input voltage rises,
the output follows the input, allowing the regulator to start
up into very heavy loads. During start-up, as the input
voltage is rising, the input-to-output voltage differential is
small, allowing the regulator to supply large output
currents. With a high input voltage, a problem can occur
wherein removal of an output short will not allow the
output voltage to fully recover. Other regulators, such as
the LT1175, also exhibit this phenomenon, so it is not
unique to the LT1964 series.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low.
Common situations are immediately after the removal of
a short-circuit or when the SHDN 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. If this happens, there are two stable operating
points for the regulator. With this double intersection, the
input supply may need to be cycled down to zero and
brought up again to make the output recover.
APPLICATIO S I FOR ATIO
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.


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