Applications Section

GENERAL INFORMATION

The LP8340 is a low-dropout, low quiescent current linear

regulator. As shown in Figure 1 it consists of a 1.25V refer-

ence, error amplifier, MOSFET driver, PMOS pass transistor

and for the fixed output versions, an internal feedback net-

work (R

load by a thermal shutdown circuit and a foldback current

limit circuit

The 1.25V reference is connected to the inverting input of

the error amplifier. Regulation of the output voltage is

achieved by means of negative feedback to the non-

inverting input of the error amplifier. Feedback resistors R

1

and R

ing on whether it is a fixed voltage version or the adjustable

version. The negative feedback and high open loop gain of

the error amplifier cause the two inputs of the error amp to be

virtually equal in voltage. If the output voltage changes due

to load changes, the error amplifier and MOSFET driver

provide the appropriate drive to the pass transistor to main-

tain the error amplifier’s inputs as virtually equal.

EXTERNAL CAPACITOR

An Input capacitor of 1µF or greater is required between the

LP8340 V

bypassing of the V

(i.e. 10µF) can provide improved bypassing of power supply

noise.

Stable operation can be achieved with an output capacitor of

1µF or greater, either ceramic X7R dielectric or aluminum/

tantalum electrolytic. While the minimum capacitor value is

1µF, the typical output capacitor values selected range from

1µF to 10µF. The larger values provide improved load-

transient response, power supply rejection and stability.

OUTPUT VOLTAGE SETTING (ADJ VERSION ONLY)

The output voltage is set according to the amount of nega-

tive feedback (Note that the pass transistor inverts the feed-

back signal). This feedback is determined by R

the resulting output voltage represented by the following

equation:

Use the following equation to determine the values of R

R

Ω is recommended).

MINIMUM LOAD CURRENT

A minimum load of 100µA is required for regulation and

stability over the entire operating temperature range. If ac-

tual load current fall below 100µA it is recommended that a

resistor of value R

ground.

20060903

FIGURE 1. LP8340 Functional Block Diagram

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