10

LT1129/LT1129-3.3/LT1129-5

S

APPLICATI

I FOR ATIO

so,

P = 500mA

× (5.5V – 3.3V) + (25mA × 5.5V)

= 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:

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:

Output Capacitance and Transient Performance

The LT1129 is designed to be stable with a wide range of

output capacitors. The minimum recommended value is

3.3

µF 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-

sponse curves in the Typical Performance Characteristics.

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.

Protection Features

The LT1129 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 voltages from output to input. For

fixed voltage devices the output and sense pins are tied

together at the output.

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 input of the device will withstand reverse voltages of

30V. Current flow into the device will be limited to less than

1mA (typically less than 100

µA) 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.

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

fixed voltage device, is pulled below ground, with the input

open or grounded, current must be limited to less than

5mA.

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 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

current flows through the device to ground. This curve will

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

50k

Ω. The state of the shutdown pin will have no effect on

output current when the input pin is pulled to ground.

In some applications it may be necessary to leave the input

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