MCP1826ST-0802ESLASHEB datasheet(17/36 Pages) MICROCHIP

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DS22057A-page 17

MCP1826/MCP1826S

4.4

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 1.0 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 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.

4.5

Power Good Output (PWRGD)

The PWRGD output is used to indicate when the output

voltage of the LDO is within 92% (typical value, see

Section 1.0 “Electrical Characteristics” for Minimum

and Maximum specifications) of its nominal regulation

value.

As the output voltage of the LDO rises, the PWRGD

output will be held low until the output voltage has

exceeded the power good threshold plus the hysteresis

value. Once this threshold has been exceeded, the

power good time delay is started (shown as TPG in the

Electrical Characteristics table). The power good time

delay is fixed at 200 µs (typical). After the time delay

period, the PWRGD output will go high, indicating that

the output voltage is stable and within regulation limits.

If the output voltage of the LDO falls below the power

good threshold, the power good output will transition

low. The power good circuitry has a 170 µs delay when

detecting a falling output voltage, which helps to

increase noise immunity of the power good output and

avoid false triggering of the power good output during

fast output transients. See Figure 4-2 for power good

timing characteristics.

When the LDO is put into Shutdown mode using the

SHDN input, the power good output is pulled low

immediately, indicating that the output voltage will be

out of regulation. The timing diagram for the power

good output when using the shutdown input is shown in

Figure 4-3.

The power good output is an open-drain output that can

be pulled up to any voltage that is equal to or less than

the LDO input voltage. This output is capable of sinking

1.2 mA (VPWRGD < 0.4V maximum).

FIGURE 4-2:

Power Good Timing.

FIGURE 4-3:

Power Good Timing from

Shutdown.

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

TPG

TVDET_PWRGD

VPWRGD_TH

VOUT

PWRGD

VOL

VOH

VIN

SHDN

VOUT

30 µs

70 µs

TOR

PWRGD

TPG

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