MIC5249_12 Datasheet, PDF (5/7 Pages) Micrel Semiconductor – 300mA μCap LDO with Programmable Power-On Reset Delay

Micrel, Inc.
Application Information
Enable/Shutdown
The MIC5249 comes with an active-high enable pin that
allows the regulator to be disabled. Forcing the enable
pin low disables the regulator and sends it into a “zero”
off-mode-current state. In this state, current consumed
by the regulator goes nearly to zero. Forcing the enable
pin high enables the output voltage. This part is CMOS
and the enable pin cannot be left floating; a floating
enable pin may cause an indeterminate state on the
output.
Input Capacitor
The MIC5249 is a high-performance, high-bandwidth
device. Therefore, it requires a well-bypassed input
supply for optimal performance. A 1.0µF capacitor is
required from the input to ground to provide stability.
Low-ESR ceramic capacitors provide optimal
performance at a minimum of space. Additional high-
frequency capacitors such as small valued NPO
dielectric type capacitors help to filter out high frequency
noise and are good practice in any RF-based circuit.
Output Capacitor
The MIC5249 requires an output capacitor of 2.2µF or
greater to maintain stability. The design is optimized for
use with low-ESR ceramic chip capacitors. High-ESR
capacitors may cause high frequency oscillation. The
maximum recommended ESR is 300mΩ. The output
capacitor can be increased, but performance has been
optimized for a 2.2µF ceramic output capacitor and does
not improve significantly with larger capacitance.
X7R/X5R dielectric-type ceramic capacitors are
recommended because of their temperature
performance. X7R-type capacitors change capacitance
by 15% over their operating temperature range and are
the most stable type of ceramic capacitors. Z5U and
Y5V dielectric capacitors change value by as much as
50% and 60%, respectively, over their operating
temperature ranges. To use a ceramic chip capacitor
with Y5V dielectric, the value must be much higher than
anX7R ceramic capacitor to ensure the same minimum
capacitance over the equivalent operating temperature
range.
DELAY Pin Input
The power-on-reset (POR) function can be implemented
on the MIC5249 by adding an external capacitor from
the DELAY pin to ground. This external capacitor sets
the delay time (T_DELAY) of the RESET output.
MIC5249
The capacitor value required can be easily calculated
using the formula:
C _DELAY
=
⎜⎜⎝⎛
T_DELAY × I_DELAY
V_DELAY
⎟⎟⎠⎞
where I_DELAY is 0.55µA and the V_DELAY is 1.4V. When no
capacitor is used, there will be no delay and the POR
output acts like a standard error FLAG output.
RESET Output
The RESET output of the MIC5249 provides the power-
on reset signal based on the capacitor from the DELAY
pin to ground when input power is applied to the
regulator. The reset signal stays low for a preset time
period after power is applied to the regulator, and then
goes high.
The reset output is an active-low, open-drain output that
drives low when a fault condition AND an undervoltage
detection occurs. Internal circuitry intelligently monitors
overcurrent, overtemperature and dropout conditions
and ORs these outputs together to indicate some fault
condition. The output of that OR gate is ANDed with an
output voltage monitor that detects an undervoltage
condition. The output drives an open-drain transistor to
indicate a fault. This prevents chattering or inadvertent
triggering of the reset. There set must be pulled up using
a resistor form the RESET pin to either the input or the
output.
Bypass Pin Input
A bypass capacitor is required from the noise bypass pin
to ground to reduce output voltage noise. The capacitor
bypasses the internal reference. A 0.01µF capacitor is
recommended for applications that require low-noise
outputs. The bypass capacitor can be increased, further
reducing noise and improving PSRR. Turn-on time
increases slightly with respect to bypass capacitance. A
unique quick-start circuit allows the MIC5249 to drive a
large capacitor on the bypass pin without significantly
slowing the turn-on time.
Active Shutdown
The MIC5249 also features an active shutdown clamp,
which is a N-Channel MOSFET that turns on when the
device is disabled. This allows the output capacitor and
load to discharge, de-energizing the load.
No-Load Stability
The MIC5249 will remain stable and in regulation with no
load unlike many other voltage regulators. This is
especially important in CMOS RAM keep-alive
application.
March 2012
5
M9999-030812