MCP1801 datasheet(13/26 Pages) MICROCHIP

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DS22051C-page 13

MCP1801

4.0

DETAILED DESCRIPTION

4.1

Output Regulation

A portion of the LDO output voltage is fed back to the

internal error amplifier and compared with the precision

internal bandgap reference. The error amplifier output

will adjust the amount of current that flows through the

P-Channel pass transistor, thus regulating the output

voltage to the desired value. Any changes in input

voltage or output current will cause the error amplifier

to respond and adjust the output voltage to the target

voltage (refer to Figure 4-1).

4.2

Overcurrent

The MCP1801 internal circuitry monitors the amount of

current flowing through the P-Channel pass transistor.

In the event that the load current reaches the current

limiter level of 300 mA (typical), the current limiter

circuit will operate and the output voltage will drop. As

the output voltage drops, the internal current foldback

circuit will further reduce the output voltage causing the

output current to decrease. When the output is shorted,

a typical output current of 50 mA flows.

4.3

Shutdown

The SHDN input is used to turn the LDO output voltage

on and off. When the SHDN input is at a logic-high

level, the LDO output voltage is enabled. When the

SHDN input is pulled to a logic-low level, the LDO

output voltage is disabled and the LDO enters a low

quiescent current shutdown state where the typical

quiescent current is 0.01 µA. The SHDN pin does not

have

internal

pull-up

pull-down

resistor.

Therefore, the SHDN pin must be pulled either high or

low to prevent the device from becoming unstable. The

internal device current will increase when the device is

operational and current flows through the pull-up or

pull-down resistor to the SHDN pin internal logic. The

SHDN pin internal logic is equivalent to an inverter

input.

4.4

Output Capacitor

The MCP1801 requires a minimum output capacitance

of 1 µF for output voltage stability. Ceramic capacitors

are recommended because of their size, cost, and

environmental robustness qualities.

Aluminum-electrolytic and tantalum capacitors can be

used on the LDO output as well. The output capacitor

should be located as close to the LDO output as is

practical. Ceramic materials X7R and X5R have low

temperature coefficients and are well within the

acceptable ESR range required. A typical 1 µF X7R

0805 capacitor has an ESR of 50 milli-ohms.

Larger LDO output capacitors can be used with the

MCP1801 to improve dynamic performance and power

supply

ripple

rejection

performance.

Aluminum-

electrolytic capacitors are not recommended for low

temperature applications of

≤ 25°C.

4.5

Input Capacitor

Low input source impedance is necessary for the LDO

output to operate properly. When operating from

batteries, or in applications with long lead length

(> 10 inches) between the input source and the LDO,

some input capacitance is recommended. A minimum

of 0.1 µF to 4.7 µF is recommended for most

applications.

For

applications

that

have

output

step

load

requirements, the input capacitance of the LDO is very

important. The input capacitance provides the LDO

with a good local low-impedance source to pull the

transient currents from in order to respond quickly to

the output load step. For good step response

performance,

the

input

capacitor

should

equivalent (or higher) value than the output capacitor.

The capacitor should be placed as close to the input of

the LDO as is practical. Larger input capacitors will also

help reduce any high-frequency noise on the input and

output of the LDO and reduce the effects of any

inductance that exists between the input source

voltage and the input capacitance of the LDO.

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