MCP1824 datasheet(20/34 Pages) MICROCHIP

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MCP1824/MCP1824S

DS22070A-page 20

4.6

Shutdown Input (SHDN)

The SHDN input is an active-low input signal that turns

the LDO on and off. The SHDN threshold is a

percentage of the input voltage. The typical value of

this shutdown threshold is 30% of VIN, with minimum

and

maximum

limits

over

the

entire

operating

temperature range of 45% and 15%, respectively.

The SHDN input will ignore low-going pulses (pulses

meant to shut down the LDO) that are up to 400 ns in

pulse width. If the shutdown input is pulled low for more

than 400 ns, the LDO will enter Shutdown mode. This

small bit of filtering helps to reject any system noise

spikes on the shutdown input signal.

On the rising edge of the SHDN input, the shutdown

circuitry has a 30 µs delay before allowing the LDO

output to turn on. This delay helps to reject any false

turn-on signals or noise on the SHDN input signal. After

the 30 µs delay, the LDO output enters its soft-start

period as it rises from 0V to its final regulation value. If

the SHDN input signal is pulled low during the 30 µs

delay period, the timer will be reset and the delay time

will start over again on the next rising edge of the

SHDN input. The total time from the SHDN input going

high (turn-on) to the LDO output being in regulation is

typically 100 µs. See Figure 4-4 for a timing diagram of

the SHDN input.

FIGURE 4-4:

Shutdown Input Timing

Diagram.

4.7

Dropout Voltage and Undervoltage

Lockout

Dropout voltage is defined as the input-to-output

voltage differential at which the output voltage drops

2% below the nominal value that was measured with a

VR + 0.5V differential applied. The MCP1824/

MCP1824S LDO has a very low dropout voltage

specification of 210 mV (typical) at 300 mA of output

current. See Section 1.0 “Electrical Characteristics”

for maximum dropout voltage specifications.

The MCP1824/MCP1824S LDO operates across an

input voltage range of 2.1V to 6.0V and incorporates

input Undervoltage Lockout (UVLO) circuitry that keeps

the LDO output voltage off until the input voltage

reaches a minimum of 2.00V (typical) on the rising

edge of the input voltage. As the input voltage falls, the

LDO output will remain on until the input voltage level

reaches 1.82V (typical).

Since the MCP1824/MCP1824S LDO undervoltage

lockout activates at 1.82V as the input voltage is falling,

the dropout voltage specification does not apply for

output voltages that are less than 1.8V.

For high-current applications, voltage drops across the

PCB traces must be taken into account. The trace

resistances can cause significant voltage drops

between the input voltage source and the LDO. For

applications with input voltages near 2.1V, these PCB

trace voltage drops can sometimes lower the input

voltage enough to trigger

a shutdown due to

undervoltage lockout.

4.8

Overtemperature Protection

The MCP1824/MCP1824S LDO has temperature-

sensing circuitry to prevent the junction temperature

from exceeding approximately 150°C. If the LDO

junction temperature does reach 150°C, the LDO

output will be turned off until the junction temperature

cools to approximately 140°C, at which point the LDO

output will automatically resume normal operation. If

the internal power dissipation continues to be

excessive, the device will again shut off. The junction

temperature of the die is a function of power

dissipation, ambient temperature and package thermal

resistance. See Section 5.0 “Application Circuits/

Issues”

for

information

LDO

power

dissipation and junction temperature.

SHDN

VOUT

30 µs

70 µs

TOR

400 ns (typ)

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