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.
V
PWRGD_TH
V
OUT
T
PG
V
OH
T
VDET_PWRGD
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.
PWRGD
V
OL
FIGURE 4-2:
Power Good Timing.
V
IN
T
OR
70 µs
30 µs
SHDN
4.5
Power Good Output (PWRGD)
T
PG
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.
V
OUT
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.
PWRGD
FIGURE 4-3:
Shutdown.
Power Good Timing from
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.
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.
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 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.
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).
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
© 2007 Microchip Technology Inc.
DS22057A-page 17
MICROCHIP [ MICROCHIP ]
