LT1964
14
1964fb
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + (I
GND
• V
IN(MAX)
)
where,
I
OUT(MAX)
= –100mA
V
IN(MAX)
= –8V
I
GND
at (I
OUT
= –100mA, V
IN
= –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 for the SOT-23 package
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:
T
JMAX
= 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 addition to the normal protection features associated
with monolithic regulators, such as current limiting and
thermal 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 operation,
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 fi xed voltage versions, the output will act like a
large resistor, typically 500k or higher, limiting current fl ow
to less than 40μA. For adjustable versions, the output will
APPLICATIONS INFORMATION
act like an open circuit, no current will fl ow 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 50μA load is required at input-to-output
differential voltages greater than –7V.
When power is fi rst 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 operat-
ing 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.
