LT1129/LT1129-3.3/LT1129-5
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PPLICATI
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S I FOR ATIO
so,
P =500mA × (5.5V – 3.3V) + (25mA × 5.5V)
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.
= 1.24W
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:
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.
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:
T
JMAX = 50°C + 37.2°C = 87.2°C
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
following information will help optimize battery life. Table
3 summarizes the following information.
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.
Output current will be minimized if the input pin of the
LT1129 is pulled to ground when the output is held high.
Figure 3 shows reverse output current as a function of
output voltage with the input pin pulled to ground. This
currentflowsthroughthedevicetoground. Thiscurvewill
hold as long as the input pin is pulled below about 0.8V or
if the impedance from the input pin to ground is less than
50kΩ. The state of the shutdown pin will have no effect on
output current when the input pin is pulled to ground.
Protection Features
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.
Insomeapplicationsitmaybenecessarytoleavetheinput
to the LT1129 unconnected when the output is held high.
This can happen when the LT1129 is powered from a
rectified AC source. If the AC source is removed, then the
input of the LT1129 is effectively left floating. In this
configuration the reverse output current is slightly higher.
It is roughly equal to the normal quiescent current. Note
that in this configuration the state of the shutdown pin has
a significant effect on the output current. Pulling the
shutdown pin to ground will minimize the output current
in this configuration. Figure 4 shows output current as a
function of output voltage with the input pin floating (open
circuit or connected to an input bypass capacitor) and the
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.
The input of the device will withstand reverse voltages of
30V.Currentflowintothedevicewillbelimitedtolessthan
1mA (typically less than 100µA) and no negative voltage
10
Linear [ Linear ]
