LT1129/LT1129-3.3/LT1129-5
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APPLICATIO S I FOR ATIO
Calculating Junction Temperature
Protection Features
Example: Given an output voltage of 3.3V, an input voltage
range of 4.5V to 5.5V, an output current range of 0mA to
500mA, and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
TheLT1129incorporatesseveralprotectionfeatureswhich
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 voltages from output to input. For
fixed voltage devices the output and sense pins are tied
together at the output.
The power dissipated by the device will be equal to:
IOUT MAX • (VIN MAX – VOUT) + (IGND • VIN MAX)
where, IOUT MAX = 500mA
VIN MAX = 5.5V
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
IGND at (IOUT = 500mA, VIN = 5.5V) = 25mA
so,
P = 500mA • (5.5V – 3.3V) + (25mA • 5.5V)
= 1.24W
The input of the device will withstand reverse voltages of
30V.Currentflowintothedevicewillbelimitedtolessthan
1mA (typically less than 100µA) and no negative voltage
will appear at the output. The device will protect both itself
and the load. This provides protection against batteries
that can be plugged in backwards.
If we use a DD package, then the thermal resistance will be
in the range of 25°C/W to 35°C/W depending on copper
area. So the junction temperature rise above ambient will
be approximately equal to:
1.24W • 30°C/W = 37.2°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
For fixed voltage versions of the device, the sense pin is
internally clamped to one diode drop below ground. For
the adjustable version of the device, the output pin is
internally clamped at one diode drop below ground. If the
output pin of an adjustable device, or the sense pin of a
fixedvoltagedevice, ispulledbelowground, withtheinput
open or grounded, current must be limited to less than
5mA.
TJMAX = 50°C + 37.2°C = 87.2°C
Output Capacitance and Transient Performance
The LT1129 is designed to be stable with a wide range of
output capacitors. The minimum recommended value is
3.3µF with an ESR of 2Ω or less. The LT1129 is a
micropower device and output transient response will be
a function of output capacitance. See the Transient Re-
sponsecurvesintheTypicalPerformanceCharacteristics.
Larger values of output capacitance will decrease the peak
deviations and provide improved output transient re-
sponse. Bypass capacitors, used to decouple individual
components powered by the LT1129, will increase the
effective value of the output capacitor.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to
ground, pulled to some intermediate voltage, or is left
open circuit. Current flow back into the output will vary
depending on the conditions. Many battery-powered cir-
cuits incorporate some form of power management. The
followinginformationwillhelpoptimizebatterylife.Table 3
summarizes the following information.
The reverse output current will follow the curve in Figure 3
when the input pin is pulled to ground. This current flows
through the output pin to ground. The state of the shut-
down pin will have no effect on output current when the
input pin is pulled to ground.
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Linear Systems [ Linear Systems ]
