Datasheet

MCP1826/MCP1826S
DS22057B-page 18  2007-2013 Microchip Technology Inc.
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 V
IN
, 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
V
R
+ 0.5V differential applied. The MCP1826/
MCP1826S LDO has a very low dropout voltage
specification of 300 mV (typical) at 1000 mA of output
current. See Section 1.0 “Electrical Characteristics”
for maximum dropout voltage specifications.
The MCP1826/MCP1826S LDO operates across an
input voltage range of 2.3V 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 MCP1826/MCP1826S 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 2.3V, 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 MCP1826/MCP1826S 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 dissipa-
tion, ambient temperature and package thermal
resistance. See Section 5.0 “Application Circuits/
Issues” for more information on LDO power
dissipation and junction temperature.
SHDN
V
OUT
30 µs
70 µs
T
OR
400 ns (typ)