MCP1804_13 datasheet(17/36 Pages) MICROCHIP

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DS20002200D-page 17

MCP1804

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 MCP1804 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 200 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 an internal pull-up or 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 MCP1804 requires a minimum output capacitance

of 0.1 µF to 1.0 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.

Larger LDO output capacitors can be used with the

MCP1804 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 1.0 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 be of

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.

4.6

Thermal Shutdown

The MCP1804 thermal shutdown circuitry protects the

device when the internal junction temperature reaches

the typical thermal limit value of +150°C. The thermal

limit shuts off the output drive transistor. Device output

will resume when the internal junction temperature falls

below the thermal limit value by an amount equal to the

thermal limit hysteresis value of +25°C.

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