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LT1964

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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 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:

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 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 ﬁxed voltage versions, the output will act like a

large resistor, typically 500k or higher, limiting current ﬂow

to less than 40μA. For adjustable versions, the output will

APPLICATIONS INFORMATION

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 50μA 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 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.

LT1964 Datasheet(PDF) 14 Page - Linear Technology

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