2010 Microchip Technology Inc. DS22051D-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 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 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 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.
